ALBERT

Description: Abstract High‐intensity precipitation represents a threat for several regions of the world because of the related risk of natural disasters (e.g., floods and landslides). This work focuses on low‐level precipitation enhancement that occurs in the cloud warm layer and has been observed in relation to collision‐coalescence (CC) leading to flash floods and extreme rainfall events in tropical and temperate latitudes. Specifically, signatures of precipitation enhancement (referred to as CC‐dominant precipitation) are investigated in the observations from the Global Precipitation Measurement (GPM) core mission Dual‐frequency Precipitation Radar (DPR) over the central/eastern Contiguous United States (CONUS) during June 2014 – May 2018. A classification scheme for CC‐dominant precipitation, developed for dual‐polarization S‐band radar measurements and applied in a previous work to X‐band radar observations in complex terrain, is used as a benchmark. The scheme is here applied to the GPM ground validation dataset that matches ground‐based radar observations across CONUS to space‐borne DPR retrievals. The occurrence of CC‐dominant precipitation is documented and the corresponding signatures of CC‐dominant precipitation at Ku‐ and Ka‐band are studied. CC‐dominant profiles show distinguishing features when compared to profiles not dominated by CC, e.g., characteristic vertical slopes of reflectivity at Ku‐ and Ka‐band in the liquid layer, lower freezing level height, and shallower ice layer, which are linked to environmental conditions driving the peculiar CC microphysics. This work aims at improving satellite quantitative precipitation estimation, particularly GPM retrievals, by targeting CC development in precipitation columns. This article is protected by copyright. All rights reserved.

Description: A climatology of the boundary‐layer wind‐turning angle over land is presented based on radiosonde observations from 800 stations in the Integrated Global Radiosonde Archive (IGRA). The dependence of the wind turning on a suite of parameters is analyzed. Results from previous studies indicating the importance of the planetary boundary layer (PBL) stratification for the angle of wind turning are confirmed here. A clear increase in the wind‐turning angle with wind speed, particularly for stratified conditions, is also evident. According to Rossby number similarity theory, the cross‐isobaric angle for a neutral and barotropic boundary layer decreases with the surface Rossby number, Ro. The IGRA observations indicate that this dependence on Ro might partly be linked to the dependence of the stratification on the wind speed, a dependence that seems to prevail even for the high wind speeds, a criteria that traditionally is used to approximate a neutral PBL. The vertical distribution of the turning of the wind is analyzed using the high resolution Stratospheric Processes And their Role in Climate (SPARC) data. For unstable cases, there is a maximum in the directional wind shear around the PBL top, whereas for the most stable class of cases there is a maximum near the surface. The wind‐turning angles from observations are also compared with values obtained from ERA‐Interim reanalysis fields, also presented over ocean. ERA‐Interim underestimates the magnitude of the wind‐turning angles as well as the range. Furtheremore, the midlatitude cross‐isobaric mass transport is estimated using the IGRA data. This transport is generally underestimated by ERA‐Interim, likely related to the too small wind‐turning angles. This article is protected by copyright. All rights reserved.

Description: A schematic illustration of the assimilation windows employed by the ECMWF Long Window Data Assimilation (LWDA) system. Black dots represent observations distributed quasi‐randomly in time throughout the window. This study quantifies the extent to which the ECMWF 4D‐Var displays differential (heightened) sensitivity to observations located near the end of the 12‐hr assimilation time window compared to observations located near the start of the window. Using dedicated satellite data denial experiments, it is shown that the lattermost 3 hr of observations are significantly more influential on the quality of the assimilation and forecasting system than the first 3 hr of data. Furthermore, it is found that the last 3 hr of data even outperforms the 6 hr of data (i.e. twice the number of observations) located in the first half of the window. The heightened importance of late window data is discussed in terms of these measurements being our most up‐to‐date information on the atmosphere, but also their ability to provide additional dynamical information to the assimilation system via feature advection wind tracing. The implications of this sensitivity are discussed. Firstly, it leads to the existence of influential (late window) satellite orbits, the location of which can have a strong bearing on the impact of observations from different satellites in different regions. Secondly, this sensitivity reinforces the need for data providers to minimize dissemination delays to ensure that crucial late window data reach users in time to be assimilated. Finally, numerical weather prediction (NWP) centres (who run 4D systems) must ensure that these lattermost observations are being captured and used effectively. Some suggestions for this are proposed.

Description: The Netherlands is characterized by highly variable land use within a small area, and a strong influence of the North Sea on national climate. Devoid of significant topography, it is an excellent location for assessing the relative influence of various factors on fog occurrence in the absence of terrain effects. Using observations from a dense network of weather stations throughout the country, the climatology of fog in the Netherlands is assessed over a period of 45 years. On a national scale, inter‐annual variability is linked to changes in synoptic pressure‐gradient forcing. Within the country, a comprehensive in‐depth analysis of regional differences between fog occurrence is made, together with an assessment of local physical factors that could bias fog formation in one location over another. Regional variability is shown to be strongly related to the mesoscale influences of urbanization and the North Sea. In fact, some locations experience over twice as much fog as others. From this finding, a simple index is presented, which combines the water and urban fraction surrounding a station. This “Regionally Weighted Index” (RWI) is able to accurately sort the stations according to their relative fogginess. Its practical use is encouraged for assessing a given site's climatological favourability, even when in situ meteorological observations are unavailable. This article is protected by copyright. All rights reserved.

Description: Atmospheric angular momentum (AAM) is a quantity related to the global distributions of surface pressure and zonal wind. We found that the AAM has increased in the 20th century, mainly due its zonal wind component. However, the AAM budget in the ERA‐20C reanalysis is not well‐closed, which can be largely attributed to the analysis increments. It is well known that global warming in the 20th century has influenced the global circulation of the atmosphere. Atmospheric angular momentum (AAM), a measure of the rotation of the atmosphere around the Earth's axis, is a useful quantity to investigate changes in the global atmospheric circulation. In this study, 20th century trends in the AAM budget are determined using the ERA‐20C reanalysis data of the European Centre for Medium‐Range Weather Forecasts (ECMWF). In addition, the closure of the AAM budget is determined to assess the ability of ERA‐20C to conserve angular momentum. The total AAM has increased in the 20th century, associated mainly with an increasing relative (zonal wind) AAM in most of the stratosphere and the tropical upper troposphere, and a poleward redistribution in the midlatitudes. These trends can be related to the warming in the troposphere and cooling in the lower stratosphere found in this study, likely caused by increasing atmospheric CO2 concentrations. The Ω‐AAM, representing the rotation of the atmosphere along with the Earth, shows no clear trend, but a spurious peak around 1920. This peak is caused by a global increase in surface pressure and is considered an artefact of changes in the amount of assimilated observations. It is also found that the AAM budget is not well closed in ERA‐20C, which is mainly the result of the assimilation of observations during production of the reanalysis. The trends in the AAM budget in ERA‐20C are likely affected by changes in the number of assimilated observations and should be validated with other reanalyses in further research.

Description: (a) Radar at [0800 UTC‐1100 UTC] on 30 October, (b) neighbourhood ensemble probability (NEP) of accumulated rainfall 0800‐1100 UTC on 30 October to exceed rain amounts of 6 mm over the 3 h for EC‐SINGV, and (c) UM‐SINGV. The simulations are initialized at 1500 UTC 29 October. A convective‐scale ensemble system was developed to predict the occurrence of heavy convective rainfall around Singapore with a focus on the prediction of high‐impact events. The new ensemble SINGV‐EPS has been nested within two global ensembles, MOGREPS‐G (UK Met Office) and EC‐ENS (ECMWF). Predicting the occurrence of convective rainfall in an area such as Singapore is challenging and this article discusses the use of the convection‐permitting ensemble to characterize the uncertainties in the prediction of such localized heavy rainfall. First, verification of wind, temperature, and precipitation is performed for a month‐long period to assess the relative performance of each ensemble. This reveals differences, but no robust signal to say one is better than the other. The results are not statistically significant and not all variables are consistently better with one ensemble or the other. Secondly, the precipitation characteristics of SINGV‐EPS are analysed from probabilities of precipitation and variability among the ensemble members. SINGV‐EPS is sensitive to the choice of the global ensemble providing the initial conditions and boundaries. The results suggest there is benefit, in some cases, from combining the two ensembles. Thirdly, the spread of the ensemble precipitation is analysed using the dispersion Fractions Skill Score (dFSS). We compare the impact of the initial perturbations and the perturbations in lateral boundary conditions in both nesting options. The initial perturbations dominate in the beginning of the forecasts, with influence up to T+24 h, and are associated with an upscale growth of the uncertainties. The impact of the parent ensemble and lateral boundary conditions dominate at the end of the forecast and tend to influence larger scales more.

Description: This study describes different processes leading to heat waves in Europe. Employing backward trajectories, three clusters with coherent thermodynamic characteristics and vertical motions are identified. In two of the three clusters, subsidence is of first‐order importance for high near‐surface temperatures, whereas the third cluster is primarily heated diabatically due to surface sensible heat fluxes. Western Russia, in particular, is largely affected by remote surface fluxes, whereas the British Isles are largely affected by subsidence and adiabatic warming. This study presents a comprehensive analysis of processes determining heat waves across different climates in Europe for the period 1979–2016. Heat waves are defined using a percentile‐based index and the main processes quantified along trajectories are adiabatic compression by subsidence and local and remote diabatic processes in the upper and lower troposphere. This Lagrangian analysis is complemented by an Eulerian calculation of horizontal temperature advection. During typical summers in Europe, one or two heat waves occur, with an average duration of five days. Whereas high near‐surface temperatures over Scandinavia are accompanied by omega‐like blocking structures at 500 hPa, heat waves over the Mediterranean are connected to comparably flat ridges. Tracing air masses backwards from the heat waves, we identify three trajectory clusters with coherent thermodynamic characteristics, vertical motions, and geographic origins. In all regions, horizontal temperature advection is almost negligible. In two of the three clusters, subsidence in the free atmosphere is very important in establishing high temperatures near the surface, while the air masses in the third cluster are warmed primarily due to diabatic heating near the surface. Large interregional differences occur between the British Isles and western Russia. Over the latter region, near‐surface transport and diabatic heating appear to be very important in determining the intensity of the heat waves, whereas subsidence and adiabatic warming are of first‐order importance for the British Isles. Although the large‐scale pattern is quasistationary during heat wave days, new air masses are entrained steadily into the lower troposphere during the life cycle of a heat wave. Overall, the results of the present study provide a guideline as to which processes and diagnostics weather and climate studies should focus on to understand the severity of heat waves.

Description: We investigate persistent low‐frequency variability of the stratospheric winter polar vortex in a rotating spherical shallow‐water model under the action of topographic wave‐forcing and radiative cooling to a simple time‐varying equilibrium state representative of the seasonal cycle in solar heating. A range of modes of variability is obtained, dependent on wave forcing amplitude and characterized by the distribution of quiescent and disturbed winters, defined as winters in which the vortex is either close to radiative equilibrium, with low planetary wave amplitude, or else strongly disturbed from equilibrium by the wave forcing. At low forcing amplitude the vortex is typically quiescent every year, while at higher amplitude it is typically disturbed; in both cases there is little year‐to‐year variation of the vortex state. For a range of intermediate forcing amplitudes, however, the vortex transitions between quiescent and disturbed states from one winter to the next with a persistent and well‐defined pattern of variability. To investigate the extent to which the low‐frequency variability found here may be explained in terms of a low‐latitude flywheel mechanism, we conduct additional experiments varying a linear drag on the zonal mean flow in the tropics and find that sufficiently strong drag can completely suppress the variability. The robustness of the variability is demonstrated by further experiments using a modified radiative equilibrium profile, associated with a tropical westerly flow: similar variability is obtained but the modified profile is less effective at constraining the tropical flow from a persistent easterly acceleration. This article is protected by copyright. All rights reserved.

Description: Two areas – one over the eastern part of the North Atlantic (a) and one over the Baltic Sea (b) – were identified as locations where blocking influences the occurrence of thunderstorms in parts of western and central Europe. Shown is the relative frequency of cases investigated in the study quantifying how often (a) blocking suppresses and (b) blocking supports thunderstorm days in Europe. A statistically significant link is presented between atmospheric blocking located over the eastern North Atlantic and northern Europe and warm‐season thunderstorm activity over western and central Europe. Lightning data from 2001 to 2014 were used to identify thunderstorm days and blocking events were extracted from the ERA‐Interim reanalysis using an objective identification algorithm. The statistical link between the two phenomena is established through odds ratio analysis. Two areas – one over the eastern part of the North Atlantic and one over the Baltic Sea – were identified as locations where blocking influences the occurrence of deep moist convection in parts of western and central Europe. Based on the mean ambient conditions on days with blocking in these two areas, well‐known dynamic and thermodynamic mechanisms supporting or suppressing the development of thunderstorms were confirmed. The anticyclonic circulation of a block over the eastern part of the North Atlantic leads to a northerly to northwesterly advection of dry and stable air masses into Europe on the eastern flank of the block. In addition, these environmental conditions are on average associated with large‐scale subsidence of air masses (convection‐inhibiting conditions). In contrast, the southerly to southwesterly advection of warm, moist and unstable air masses on the western flank of a block over the Baltic Sea results in convection‐favouring conditions over western and central Europe. Both blocking situations are on average associated with weak wind speeds at mid‐tropospheric levels and with weak wind shear. As a consequence, thunderstorms related to atmospheric blocking over the Baltic Sea tend to be on average less organised.

Description: Tropical convective systems are major sources of atmospheric gravity waves (GWs). These waves are a key driver of the global atmospheric circulation, especially in the middle and upper atmosphere. Tropical cyclones (TCs) such as hurricanes and typhoons are particularly dramatic examples of such systems, and are therefore potentially significant individual sources of GWs. To investigate this effect, I produce and analyse GW observations from three satellite limb‐sounders in the vicinity of TCs. By statistically combining 15 years of GW observations from 1 379 individual TCs represented in the International Best Track Archive for Climate Stewardship, I show that TCs are associated with a 15% increase over background GW amplitudes, and a 25% increase in measured momentum fluxes (MFs), primarily during the period immediately before the TC. I further show that this additional contribution is small relative to other GW‐generating processes, and thus that individual TCs do not have a large quantitative effect on the dynamics of the middle and upper atmosphere as a whole. Thus, I conclude that accurate modelling of TC‐generated short‐vertical‐wavelength GWs need not be a development priority for the next generation of weather and climate models. My results also demonstrate that stronger GW activity is associated with TCs which will later develop into hurricane‐intensity storms than those that will not, and thus that better space‐based monitoring of stratospheric GW activity could be a useful tool to help better forecast strong hurricane events in the presence of obscuring tropospheric cloud. Gravity waves, tropical cyclones, satellites, remote sensing, stratosphere, mesosphere This article is protected by copyright. All rights reserved.

Description: Abstract The data assimilation scheme used in the Met Office's OSTIA (Operational Sea Surface Temperature and Ice Analysis) system has been updated from an OI‐type scheme to a variational assimilation scheme, NEMOVAR. The updated system includes a dual length scale background error correlation operator, and a flow‐dependent component to adjust the length scale combination in favour of the short scale in regions of high sea surface temperature (SST) variability. The NEMOVAR assimilation scheme improves both the analysis performance and the representation of SST features in the OSTIA analysis compared to the OI scheme of the original system. The results of spectral analysis, assessment of horizontal SST gradients and the response of an atmospheric model to the OSTIA SST analysis as a boundary condition indicate that the flow‐dependent formulation successfully contributes to improvements in the feature resolution capability of the analysis. Overall, using a short length scale of 15 km and including a flow‐dependent adjustment component produces the best results compared to using either 40 km or the first Rossby radius of deformation as the short length scale. The new system successfully captures realistic ocean variability without introducing noise into the analysis, allowing the feature resolution capability of the new system to out‐perform that of other comparable SST analysis products. This article is protected by copyright. All rights reserved.

Description: The predictability of the atmosphere at short and long time scales, associated with the coupling to the ocean, is explored in a new version of the Modular Arbitrary‐Order Ocean‐Atmosphere Model (MAOOAM). This version features a new ocean basin geometry with periodic boundary conditions in the zonal direction. The analysis presented in this paper considers a low‐order version of the model with 40 dynamical variables. First the increase of surface friction (and the associated heat flux) with the ocean can either induce chaos when the aspect ratio between the meridional and zonal directions of the domain of integration is small, or suppress chaos when it is large. This reflects the potentially counter‐intuitive role that the ocean can play in the coupled dynamics. Second, and perhaps more importantly, the emergence of long‐term predictability within the atmosphere for specific values of the friction coefficient occurs through intermittent excursions in the vicinity of a (long‐period) unstable periodic solution. Once close to this solution the system is predictable for long times, i.e. a few years. The intermittent transition close to this orbit is, however, erratic and probably hard to predict. This new route to long‐term predictability contrasts with the one found in the closed ocean‐basin low‐order version of MAOOAM, in which the chaotic solution is permanently wandering in the vicinity of an unstable periodic orbit for specific values of the friction coefficient. The model solution is thus at any time influenced by the unstable periodic orbit and inherits from its long‐term predictability.

Description: Abstract Observations of the vertical and temporal structure of the nocturnal boundary layer before and after a transition from the weakly to the very stable regime have been presented in part 1. Here, similar transitions are investigated using a 1‐D second order closure numerical model, with an energy budget solved at the surface. The transition is driven by a decreasing mean wind at the top of the domain, and simulations with different cloud covers and surface thermal properties are considered. The time of the transition depends on the wind speed at the top of the domain and on the” coupling strength” between the surface and the atmosphere, which is affected by the cloud cover and surface thermal properties. The vertical profiles and temporal evolutions of the terms of the budgets of turbulent kinetic energy (TKE), heat flux and temperature variance are presented. Of these, only TKE budget presents the same dominant terms in both regimes. Absolute heat flux in the model is proportional to the cube of the wind speed in the very stable regime. This article is protected by copyright. All rights reserved.

Description: Abstract Continuous data assimilation (CDA) is successfully implemented for the first time for efficient dynamical downscaling of a global atmospheric reanalysis. A comparison of the performance of CDA with the standard grid and spectral nudging techniques for representing long‐ and short‐scale features in the downscaled fields using the Weather Research and Forecast (WRF) model is further presented and analyzed. The WRF model is configured at 0.25° × 0.25° horizontal resolution and is driven by 2.5° × 2.5° initial and boundary conditions from NCEP/NCAR reanalysis fields. Downscaling experiments are performed over a one‐month period in January, 2016. The similarity metric is used to evaluate the performance of the downscaling methods for large (2000 km) and small (300 km) scales. Similarity results are compared for the outputs of the WRF model with different downscaling techniques, NCEP/NCAR reanalysis, and NCEP Final Analysis (FNL, available at 0.25° × 0.25° horizontal resolution). Both spectral nudging and CDA describe better the small‐scale features compared to grid nudging. The choice of the wave number is critical in spectral nudging; increasing the number of retained frequencies generally produced better small‐scale features, but only up to a certain threshold after which its solution gradually became closer to grid nudging. CDA maintains the balance of the large‐ and small‐scale features similar to that of the best simulation achieved by the best spectral nudging configuration, without the need of a spectral decomposition. The different downscaled atmospheric variables, including rainfall distribution, with CDA is most consistent with the observations. The Brier skill score values further indicate that the added value of CDA is distributed over the entire model domain. The overall results clearly suggest that CDA provides an efficient new approach for dynamical downscaling by maintaining better balance between the global model and the downscaled fields. This article is protected by copyright. All rights reserved.

Description: Abstract This study analyses the mechanisms involved in the formation of a moist meso‐vortex associated with an extreme rain event that occurred in Burkina Faso on 1 September 2009, on the basis of high resolution convection‐permitting simulations. After an evaluation of the 6‐day simulation skill to capture the main characteristics of this event, budgets of heat, moisture and relative vorticity are calculated. Results allow to propose the following scenario for the occurrence of this extreme rain event, completing the large‐scale analysis of the same event performed in the companion paper Lafore et al. (2017): The arrival of a large‐scale wet spell is a key factor, creating a quite unusual wet environment over Sahel, resulting in weak rain evaporation. As over oceanic regions, it favours intense heating at low levels and thus strong mean ascent and convergence. The combination with the arrival of the trough of an African easterly wave provides a second key factor. The associated relative vorticity maximum on the southern flank of the African easterly jet core, is boosted by the strong low‐level convergence through the stretching term. The vortex deepening is favoured by the tilting and by eddies at upper‐levels. In turn the quasi‐balanced vortex circulation brings warm and moist air from the north, favouring the convection triggering to the west of the vortex in the down‐shear direction, in agreement with Raymond and Jiang (1990) theory for long‐lived meso‐scale convective systems. This scenario exhibits a positive feedback that could explain its extreme character. This article is protected by copyright. All rights reserved.

Description: This paper derives unified equations of A. Arakawa and C.S. Konor (2009) rigorously formulated in a suitable mass‐based sigma‐coordinate and develops an efficient semi‐implicit integration scheme. The unified equations accurately capture the non‐hydrostatic small‐scale effects and retain the hydrostatic compressibility of the flow at large scales. As with the classical quasi‐hydrostatic equations, the underlying approximations filter vertically‐propagating acoustic waves. In contrast to the quasi‐hydrostatic equations though, the filtering property of the unified equations imposes that the wind field satisfies a divergence constraint similar to anelastic and pseudo‐incompressible (small‐scale limit) soundproof systems. An efficient semi‐implicit integration scheme for the unified equation system is achieved by combining a constant‐coefficient linear partitioning approach with an iterative implicit treatment of the non‐linear residuals arising from the soundproof divergence constraint. The resulting linear implicit problem to be solved at each iteration may be reduced to a single Helmholtz equation with horizontally‐homogeneous coefficients, which is akin to the one typically solved in the semi‐implicit integration of the quasi‐hydrostatic equations. The stability and accuracy of the developed semi‐implicit scheme for the unified equations in the mass‐based coordinate is numerically assessed by means of standard vertical plane test cases in linear and nonlinear atmospheric flow regimes. Moreover, in order to ascertain the convergence of the iterative semi‐implicit scheme, the test cases also include a large‐scale three‐dimensional configuration that resembles the stiffness typically encountered in global atmospheric models. This article is protected by copyright. All rights reserved.

Description: Abstract The INCOMPASS field campaign combines airborne and ground measurements of the 2016 Indian monsoon, towards the ultimate goal of better predicting monsoon rainfall. The monsoon supplies the majority of water in South Asia, but forecasting from days to the season ahead is limited by large, rapidly developing errors in model parametrizations. The lack of detailed observations prevents thorough understanding of the monsoon circulation and its interaction with the land surface: a process governed by boundary‐layer and convective‐cloud dynamics. INCOMPASS used the UK Facility for Airborne Atmospheric Measurements (FAAM) BAe‐146 aircraft for the first project of this scale in India, to accrue almost 100 hours of observations in June and July 2016. Flights from Lucknow in the northern plains sampled the dramatic contrast in surface and boundary layer structures between dry desert air in the west and the humid environment over the northern Bay of Bengal. These flights were repeated in pre‐monsoon and monsoon conditions. Flights from a second base at Bengaluru in southern India measured atmospheric contrasts from the Arabian Sea, over the Western Ghats mountains, to the rain shadow of southeast India and the south Bay of Bengal. Flight planning was aided by forecasts from bespoke 4km convection‐permitting limited‐area models at the Met Office and India's NCMRWF. On the ground, INCOMPASS installed eddy‐covariance flux towers on a range of surface types, to provide detailed measurements of surface fluxes and their modulation by diurnal and seasonal cycles. These data will be used to better quantify the impacts of the atmosphere on the land surface, and vice versa. INCOMPASS also installed ground instrumentation supersites at Kanpur and Bhubaneswar. Here we motivate and describe the INCOMPASS field campaign. We use examples from two flights to illustrate contrasts in atmospheric structure, in particular the retreating mid‐level dry intrusion during the monsoon onset. This article is protected by copyright. All rights reserved.

Description: Ensemble weather forecasts often under‐represent uncertainty, leading to over‐confidence in their predictions. Multi‐model forecasts combining several individual ensembles have been shown to display greater skill than single‐ensemble forecasts in predicting temperatures, but tend to retain some bias in their joint predictions. Established postprocessing techniques are able to correct bias and calibration issues in univariate forecasts, but are generally not designed to handle multivariate forecasts (of several variables or at several locations, say). We propose a flexible multivariate Bayesian postprocessing framework, based on a directed acyclic graph representing the relationships between the ensembles and the observed weather. The posterior forecast is inferred from available ensemble forecasts and an estimate of the shared discrepancy, obtained from a collection of past forecast‐observation pairs. We also propose a novel approach to selecting an appropriate training set for estimation of the required correction, using synoptic‐scale analogues to obtain a regime‐dependent estimate of the adjustment. The proposed technique is applied to forecasts of surface temperature over the UK during the winter period from 2007‐2013. Although the resulting parametric multivariate‐normal probabilistic forecasts are marginally less sharp than those of the leading competitor, they capture the spatial structure of the observations better than a correlation structure based on either the ensembles or climatology alone, and are robust to changes in the variables and spatial domain of the forecast, at a greatly reduced computational cost. This article is protected by copyright. All rights reserved.

Description: Seasonal predictions of wintertime climate in the Northern Hemisphere mid‐latitudes, while showing clear correlation skill, suffer from anomalously low signal‐to‐noise ratio. The low signal‐to‐noise ratio means that forecasts need to be made with large ensemble sizes and require significant post‐processing to correct the forecast distribution. In this study, a recently introduced statistical model of seasonal climate predictability is adapted so that it can be used to examine the signal‐to‐noise ratio in two versions of the ECMWF seasonal forecast system. Three novel features of the low signal‐to‐noise ratio are revealed. The low signal‐to‐noise ratio is present only for forecasts initialised on November 1st and not for forecasts initialised on December 1st. The low signal‐to‐noise ratio is predominantly a feature of the lower and middle troposphere and is not present in the stratosphere. The low signal‐to‐noise ratio is linked to low signal amplitude of the forecast systems in early winter. Future studies attempting to examine the signal‐to‐noise ratio should focus on the extent to which this early winter variability is predictable. This article is protected by copyright. All rights reserved.

Description: Abstract This paper evaluates the prospects for increasing the horizontal resolution of the Aeolus horizontal line‐of‐sight (HLOS) wind profiles at the expense of their accuracy. The evaluation is performed by combining a 10‐day atmosphere simulation by the ECMWF model at T3999 horizontal resolution with the CALIPSO observations of atmospheric composition as inputs to the Aeolus simulator. The validation shows that the ECMWF model represents the location and the vertical structure of the observed cloud systems well. At the nominal accumulation length of L≈90 km (from the Aeolus measurement scale of ∼3 km), the Mie‐cloudy retrieval provides 1‐4 times fewer observations than Rayleigh‐clear but the Mie‐cloudy HLOS winds have the highest quality with estimated error standard deviation of about 1 m s−1 in the troposphere and no bias. The experiments with reduced L reveal that neither the observation error standard deviation nor bias of the Mie‐cloudy winds are significantly affected when the accumulation length L varies in the range between 100 and 10 km. At the same time, the number of observations significantly increases as L reduces. This suggests that mesoscale NWP may profit from the Aeolus Mie‐cloudy HLOS profiles with the accumulation lengths as small as 10 km. This article is protected by copyright. All rights reserved.

Description: Abstract Extreme precipitation events (EPEs) are meteorological phenomena of major concern for society. They can have different characteristics depending on the physical mechanisms responsible for their generation, which in turn depend on the large and mesoscale conditions. This work provides a systematic classification of EPEs over Northern‐Central Italy, one of the regions in Europe with the highest frequency of these events. The EPE statistics have been deduced using the new high‐resolution precipitation dataset ArCIS (Climatological Archive for Central Northern Italy), that gathers together a very high number of daily, quality controlled and homogenized, observations from different networks of 11 Italian regions. Gridded precipitation is aggregated over Italian operational warning‐area units (WA). EPEs are defined as events in which daily average precipitation in at least one of the 94 WAs exceeds the 99th percentile with respect to the climate reference 1979‐2015. A list of 887 events is compiled, significantly enlarging the database compared to any previous study of EPEs. EPEs are separated in three different dynamical classes: Cat1, events mainly attributable to frontal/orographic uplift; Cat2, events due to frontal uplift with (equilibrium) deep convection embedded; Cat3, events mainly generated by non‐equilibrium deep convection. A preliminary version of this classification is based on fixed thresholds of environmental parameters, but the final version is obtained using a more robust machine learning unsupervised K‐means clustering and random forest algorithm. All events are characterized by anomalously high‐integrated water vapour transport (IVT). This confirms IVT as an important large‐scale predictor, especially for Cat2 events, which is shown to be the most important category in terms of impacts and EPE area extension. Large IVT values are caused by upper‐level waves associated with remotely triggered Rossby Wave Packets, as shown for two example Cat2 events. This article is protected by copyright. All rights reserved.

Description: This paper presents an assessment of the usage of Arctic atmospheric observations in the Numerical Weather Prediction (NWP) system of the European Centre for Medium‐Range Weather Forecasts, and of their impact on the quality of short to medium range forecasts. The Arctic has low coverage of conventional data north of 70° N but one of the highest coverages of satellite sounding data on the globe. The impact of Arctic observations on forecast skill was assessed by performing Observing System Experiments (OSEs) in which different observation types were removed from the full observing system. This assessment was complemented by an analysis of Forecast Sensitivity to Observation Impact diagnostics. To our knowledge it is the first time that comprehensive numerical experimentation has been carried out to explore the role of different Arctic observations in a state‐of‐the‐art global operational NWP system. All Arctic observations were found to have a positive impact on forecast skill in the Arctic region with the largest tropospheric impacts on both short and medium range forecasts due to microwave sounding, conventional and infrared sounding observations. Results indicate the high importance of microwave sounding data and conventional data, which are found to be the key observing systems in the summer and winter seasons respectively. These observations were found to have a positive and statistically significant impact on forecasts not only in the Arctic but also in the mid‐latitude regions at longer lead times. Differences between the seasons are most likely due to problems assimilating microwave sounding observations over snow and sea‐ice leading to a reduced impact in winter, but there is also a suggestion of an increased importance of conventional data in winter, and other factors may also play a role. This article is protected by copyright. All rights reserved.

Description: Abstract The present paper introduces a new sea surface salinity (SSS) retrieval technique from Soil Moisture Active Passive (SMAP) L–band (1.4 GHz) microwave radiometer. Ocean surface wind speed (WS) correction, which is very critical in any of the SSS retrieval techniques is accounted for with a different approach. A model function between ΔSSS (the difference between SSS of flat and rough ocean surfaces) and ocean surface WS is developed and used to correct the impact of ocean surface roughness due to the winds on SSS. In addition to WS correction, the retrieval technique appropriately accounts for the effects of atmospheric attenuation on L‐band frequency due to water vapor and oxygen and ocean surface temperature on SSS. The performance of the estimations is statistically evaluated by comparing the estimations with the in situ Argo, buoy SSS and SMAP version 3, 70 km resolution SSS observations in global oceans during 2016. The retrieved salinity well captured the global distribution patterns of SSS, including low and high values. Comparison of retrieved salinity with Argo and SMAP products show a root mean square error (RMSE) of 0.32 psu and 0.39 psu, a Pearson correlation coefficient of 0.94 and 0.93 and a scatter index of 0.0093 and 0.011 respectively on monthly timescale. In addition, retrieved SSS promisingly captured day to day variability in SSS observed by tropical moored buoys array's SSS with RMSE of 0.36 psu and a Pearson correlation coefficient of 0.94. The validation confirms the potential ability of the new retrieval technique in capturing SSS changes on daily and monthly timescales. This article is protected by copyright. All rights reserved.

Description: In the final stage of growth, settling cloud drops become drizzle by collision and coalescence with the smaller droplets in their path. We study the impact of droplet clustering on the growth of settling collector drops by modelling it as an exponentially decaying correlation. Enhancement in collector drop growth is measured by the reduction in time needed for drizzle formation in the last stage. Effect of clustering is characterised by two dimensionless parameters C = δλL and K=(λ0 L) ‐1, where δλ is the enhancement in the collision rate inside the clusters, L is the cluster size perceived by the settling collector drop and λ0 is the initial collision rate. Increasing C or K reduces the time needed for drizzle formation compared to the unclustered case. We find that for K ≥ 5, C ≤ 2 even a small increase in droplet concentration inside clusters can cause a significant enhancement in the collector drop growth. Further even for low turbulence intensities (low K) significant reduction in the time for drizzle formation is achieved for moderate values of C. These findings are relevant to the debate on the intensity of clustering needed to explain rapid drizzle formation. This article is protected by copyright. All rights reserved.

Description: The consequences of regularizing the Sawyer‐Eliassen equation to calculate the streamfunction for the axisymmetric secondary circulation of a tropical cyclone are explored. Regularization is an ad hoc procedure in which the coefficients of the equation are suitably modified to replace negative values of the discriminant by small positive values, thereby ensuring that the equation is globally elliptic. The consequences of the procedure may be understood in terms of the analog behaviour of a stretched membrane subject to a particular force distribution. Several regularization procedures are assessed by comparing the azimuthally‐averaged radial flow from a three‐dimensional numerical simulation of a tropical cyclone with that from an axisymmetric balance calculation of the Sawyer‐Eliassen equation, forced by diabatic and frictional terms diagnosed from the simulation. The comparison shows that the largest challenge for regularization occurs in regions of inertial instability, especially when the diagnosed forcing overlaps with such regions. In the example shown, the diagnosed balanced flow is sensitive to the particular regularization procedure and none of the procedures examined gave a flow that was structurally and quantitatively close to that obtained from the numerical solution in and near the region of regularization. The flow in regions of large vertical shear that are common in the lower part of the boundary layer is less sensitive to the regularization procedure, even though such regions are ones in which there is (frictional) forcing. Nevertheless, there are comparatively large differences between the low‐level inflow in the azimuthally‐averaged numerical solution and the axisymmetric balance solution. These differences can be attributed to the intrinsic lack of balance in the boundary layer. This finding, together with the issues associated with regularization, is further confirmation that balance dynamics is unable to adequately capture the flow in the boundary layer, contrary to recent claims. This article is protected by copyright. All rights reserved.

Description: Abstract An appropriate parameterisation of turbulence is fundamental both in meteorological and atmospheric dispersion models, in order to properly simulate the dynamics of the flow and of the pollutants transported by it. The turbulence closure adopted in the meteorological model MOLOCH is presented, its reliability and effectiveness is evaluated based on turbulent kinetic energy data measured at two suburban sites in North and South Italy. Very different topographical situations characterise the two sites, an Alpine and a coastal region, respectively. Particular attention is paid in assessing the low‐wind speed regime, a critical condition for air pollution, associated to turbulent features not yet fully understood. The analysis has proved that the turbulence closure in the MOLOCH model is able to capture and effectively reproduce the observed low‐level atmospheric turbulence processes, in all stability conditions. This article is protected by copyright. All rights reserved.

Description: ABSTRACT Motions on planetary spatial scales in the atmosphere are governed by the planetary geostrophic equations. However, not much attention has been paid to the interaction between the baroclinic and barotropic flow within the planetary geostrophic scaling. This is the focus of the present study by utilizing planetary geostrophic equations for a Boussinesq fluid supplemented by a novel evolution equation for the barotropic flow. The latter is effected by meridional momentum flux due to baroclinic flow and drag by the surface wind. The barotropic wind on the other hand affects the baroclinic flow through buoyancy advection. By relaxing towards a prescribed buoyancy profile the model produces realistic major features of the zonally symmetric wind and temperature fields. We show that there is considerable cancelation between the barotropic and the baroclinic surface zonal mean zonal wind. The linear and nonlinear model response to steady diabatic zonally asymmetric forcing is investigated. The arising stationary waves are interpreted in terms of analytical solutions. We also study the problem of baroclinic instability on the sphere within the present model. This article is protected by copyright. All rights reserved.

Description: Abstract In recent years, the mid‐latitudes are characterized by more intense heat waves in summer and sometimes severe cold spells in winter that might emanate from changes in atmospheric circulation, including synoptic‐scale and planetary wave activity in the mid‐latitudes. In this study, we investigate the heat and momentum exchange between the mean flow and atmospheric waves in the North Atlantic sector and adjacent continents by means of the physically consistent Eliassen Palm flux diagnostics applied to reanalysis and forced climate model data. In the long‐term mean, momentum is transferred from the mean flow to atmospheric waves in the Northwest Atlantic region, where cyclogenesis prevails. Further downstream over Europe eddy fluxes return momentum to the mean flow, sustaining the jet stream against friction. A global climate model is able to reproduce this pattern with high accuracy. Atmospheric variability related to atmospheric wave activity is much more expressed at the intraseasonal rather than the interannual time scale. Over the last 40 years, reanalyses reveal a northward shift of the jet stream and a weakening of intraseasonal weather variability related to synoptic‐scale and planetary wave activity. This pertains to the winter and summer seasons, especially over Central Europe, and correlates with changes in the North Atlantic Oscillation as well as regional temperature and precipitation. A very similar phenomenon is found in a climate model simulation with business‐as‐usual scenario, suggesting an anthropogenic trigger in the weakening of intraseasonal weather variability in the mid‐latitudes. This article is protected by copyright. All rights reserved.

Description: The sedimentation process of columnar ice crystals was evaluated using data obtained by Lattice Boltzmann Method. The data used correspond to columnar ice crystals with maximum dimension less than 100μm and aspect ratios between 1 and 3. The terminal velocity was computed for different ice crystal bulk densities and for three falling orientations. The analysis corresponds to ice crystals falling in viscous flow regimes where theoretical formulations overestimate the terminal velocity. Different characteristic lengths of the columnar ice crystals and different theoretical proposals for the sedimentation process were tested in order to find the best representation of the data. Characteristic lengths reported in the literature do not represent the sedimentation process for all the falling orientations used in this study. Thus, it was not possible to obtain a unique relation between the Best and Reynolds numbers. Particularly, columnar ice crystals falling with their longer dimension parallel to the vertical direction show a large dispersion that does not seem possible to reduce. The theoretical and semi‐empirical formulations of the terminal velocity evaluated show large deviations on the computed velocity with a strong dependence with ice‐crystals aspect ratio. The dispersion observed seems to be intrinsically related to the dimensionless variables used to parameterize the terminal velocity. To derive a unique scale law which could represent the sedimentation process of ice crystals, geometric, kinematic and dynamic similarities are required. However, these conditions are not fulfilled in the sedimentation process. This article is protected by copyright. All rights reserved.

Description: Abstract The distribution of the cloud‐base mass flux is long thought to be independent of the large‐scale forcing. However, recent idealized modelling studies reveal its dependence on some of the large‐scale conditions. Such a dependence makes it possible to isolate the observed large‐scale conditions similar to those in the large‐eddy simulations (LES) in order to compare the observed and modelled mass flux distributions. In this study, we derive for the first time the distribution of the cloud‐base mass flux among individual shallow cumuli from the ground‐based observations at the Barbados Cloud Observatory (BCO) and compare it with the Rain In Cumulus over the Ocean (RICO) LES case‐study. The procedure of cloud sampling in LES mimics the point‐wise measurement procedure at BCO to provide a mass‐flux metric that is directly comparable with observations. We find a difference between the mass flux distribution observed during the year 2017 at BCO and the distribution modelled by LES that is comparable to the seasonal changes in the observed distribution. This difference between the observed and modelled distributions is diminished and an extremely good match is found by sub‐sampling the measurements under the similar horizontal wind distribution and area‐averaged surface Bowen ratio to those modelled in LES. This provides confidence in our observational method and also shows that LES produces realistic clouds that can be observed in nature under the same large‐scale conditions as imposed in LES. We also confirm that in our case‐study the stronger horizontal winds and higher Bowen ratios shift the distributions to higher mass flux values, which is coincident with clouds of larger horizontal areas and not with stronger updrafts. This article is protected by copyright. All rights reserved.

Description: Abstract Though deep convective clouds are important to numerous aspects of the climate system, they remain a source of much uncertainty. The varying large scale dynamics, relatively short development time, range of spatial scales, and complex microphysics involved in deep convection lead to difficulty in both observation of these clouds and modeling their impacts on larger spatial and longer temporal scales. This study utilizes an ensemble of high resolution cloud resolving simulations of deep convection forming in a spectrum of different environments to explore the sensitivity of deep convection, and in particular convective mass flux to changes in the initial conditions. We find that convection strength is strongly sensitive to small perturbations in the environment, specifically the convective available potential energy (CAPE) and boundary layer humidity. That storm effects on the environment, such as mass transport and tropospheric moistening through detrainment, are sensitive to these initial conditions points to the importance of better representing such parameters in global models. This article is protected by copyright. All rights reserved.

Description: Abstract This study introduces a new flow‐dependent distribution sampling (FDDS) scheme for air‐sea coupling. The FDDS scheme is implemented in a climate model and used to improve the simulated mean and variability of atmospheric and oceanic surface fields and thus air‐sea fluxes. Most coupled circulation models use higher resolutions in the sea ice and ocean compared to the atmospheric model component, thereby explicitly simulating the atmospheric subgrid‐scale at the interface. However, the commonly applied averaging of surface fields and air‐sea fluxes tends to smooth fine scale structures, such as oceanic fronts. The stochastic FDDS scheme samples the resolved spatial ocean (and sea ice) subgrid distribution that is usually not visible to a coarser‐resolution atmospheric model. Randomly drawn nodal ocean values are passed to the corresponding atmospheric boxes for the calculation of surface fluxes, aiming to enhance surface flux variability. The resulting surface field perturbations of the FDDS scheme are based on resolved dynamics, displaying pronounced seasonality with realistic magnitude. The AWI Climate Model is used to test the scheme on interannual timescales. Our setup features a high ocean‐to‐atmosphere resolution ratio in the tropics, with grid point ratios of about 60:1. Compared to the default deterministic averaging, changes are largest in the tropics leading to an improved spatial distribution of precipitation with bias reductions of up to 50%. Enhanced sea surface temperature variability in boreal winter further improves the seasonal phase locking of temperature anomalies associated with the El Niño‐Southern Oscillation. Mean 2 m temperature, sea ice thickness and concentration react with a contrasting dipole pattern between hemispheres but a joint increase of monthly and interannual variability. This first approach to implement a flow‐dependent stochastic coupling scheme shows considerable benefits for simulations of global climate and various extensions and modifications of the scheme are possible. This article is protected by copyright. All rights reserved.

Description: Quarterly Journal of the Royal Meteorological Society, Volume 145, Issue 719, Page i-iv, January 2019 Part B.

Description: Challenges related to the design and use of a convection‐permitting ensemble (CPEPS) are discussed. In particular the scale dependent predictability of precipitation and the use of a CPEPS as well as its potential added value over global ensemble prediction systems (EPS) are investigated. Forecasts of precipitation from the operational CPEPS in Finland, Norway and Sweden (MEPS) are used for the investigations. It is found that predictability for scales smaller than ~60 km is lost rapidly within the first 6 hours of the forecast with the smallest predictable scale growing more slowly to ~100 km over the following 18 ‐ 24 hours. However, there is large case to case variability and the ensemble perturbations fail to become fully saturated, especially in winter, suggesting a weakness in the design of the ensemble. The added value of CPEPS over deterministic forecasts and coarser resolution EPSs is discussed with summary statistics and case studies. It is shown that the added value varies between seasons and lead times. For precipitation there is an added value for both severe precipitation events and for precipitation/no precipitation decisions. The added value is higher in summer compared to winter and for shorter lead times compared to longer lead times.

Description: Fog plays a major role in many ecological aspects and it influences human life in various ways. In this study, a temporally highly resolved and spatially explicit analysis of variations in fog occurrence was conducted for Europe and links to general weather conditions were investigated. To this end, a high resolution fog product based on Meteosat Second Generation data was conducted. Characteristic fog distributions were identified by applying a Self Organizing Map approach onto the data set. It was found that the resulting fog patterns are primarily determined by terrain characteristics. Simultaneous occurrences between these patterns and the predominant general weather situations were analyzed. The results show that the general weather situations can be categorized into three main groups, each responsible for the formation of a different group of fog patterns. Additionally, distinct regional differences could be identified in the diurnal and annual fog frequency cycles and the derived region‐specific frequency variations were used to draw conclusions about the fog types prevailing in these regions.

Description: The computational cost as well as the probabilistic skill of ensemble forecasts depends on the spatial resolution of the numerical weather prediction model and the ensemble size. Periodically, e.g. when more computational resources become available, it is appropriate to reassess the balance between resolution and ensemble size. Recently, it has been proposed to investigate this balance in the context of dual‐resolution ensembles, which use members with two different resolutions to make probabilistic forecasts. This study investigates whether statistical post‐processing of such dual‐resolution ensemble forecasts changes the conclusions regarding the optimal dual‐resolution configuration. Medium‐range dual‐resolution ensemble forecasts of 2‐metre temperature have been calibrated using ensemble model output statistics. The forecasts are produced with ECMWF’s Integrated Forecast System and have horizontal resolutions between 18 km and 45 km. The ensemble sizes range from 8 to 254 members. The forecasts are verified with SYNOP station data. Results show that score differences between various single and dual‐resolution configurations are strongly reduced by statistical post‐processing. Therefore, the benefit of some dual‐resolution configurations over single resolution configurations appears to be less pronounced than for raw forecasts. Moreover, the ranking of the ensemble configurations can be affected by the statistical post‐processing. This article is protected by copyright. All rights reserved.

Description: Regimes of interactions between motions on different time‐scales are investigated in the FLOSSII dataset for nocturnal near‐surface stable boundary layer (SBL) turbulence. The non‐stationary response of turbulent vertical velocity variance to non‐turbulent, sub‐mesoscale wind velocity variability is analysed using the bounded variation, finite element, vector autoregressive factor models (FEM‐BV‐VARX) clustering method. Several locally stationary flow regimes are identified with different influences of sub‐meso wind velocity on the turbulent vertical velocity variance. In each flow regime, we analyse multiple scale interactions and quantify the amount of turbulent variability which can be statistically explained by external forcing by the sub‐meso wind velocity. The state of anisotropy of the Reynolds stress tensor in the different flow regimes is shown to relate to these different signatures of scale interactions. In flow regimes under considerable influence of the sub‐mesoscale wind variability, the Reynolds stresses show a clear preference for strongly anisotropic, one‐component states. These periods additionally show stronger persistence in their dynamics, compared to periods of more isotropic stresses. The analyses give insights on how the different topologies relate to non‐stationary turbulence triggering by sub‐mesoscale motions.

Description: This paper discusses methodological aspects of a monitoring process which focuses simultaneously on ensemble forecasts, surface variables, and high‐impact events. Which score(s) is (are) suitable for this task is a central question but not the only one to be answered. Here, we investigate the properties of the Brier score, logarithmic score, and diagonal elementary score in the context of forecast performance monitoring as well as the impact of methodological choices such as the event threshold definition, the reference forecast, and the role assigned to representativeness errors. A consistent picture of the verification process is eventually drawn where the design of the event climatology plays a key role. This study is illustrated by verification results for three surface variables (24 h precipitation, 10 m wind speed, and 2 m temperature) over 15 years of operational ECMWF ensemble forecasting activities. Results are also compared with a current ECMWF headline score: the relative operating characteristic skill score for the Extreme Forecast Index.

Description: Global climate models have circulation biases that the community aims to reduce, for instance through high‐resolution dynamical downscaling. We used WRF to downscale both ERA‐Interim and a bias‐corrected version of the Norwegian climate model NorESM1‐M on a high‐resolution grid. By varying the domain size, we investigated the influence of the driving data and highly resolved topography on the North Atlantic storm track and the precipitation in its exit region. In our largest domains, we found large‐scale circulation and storm track biases similar to those seen in global models and with spatial patterns independent of the driving data. The biases in the smaller domains were more dependent on the quality of the driving data. Nevertheless, the biases had little effect on the simulated precipitation in Norway. Although the added value of downscaling was clear with respect to the global climate models, all the downscaled simulations showed similar precipitation frequencies and intensities. We posit that, because the precipitation is so strongly governed by the local topographic forcing, a correct storm track is less critical for the precipitation distribution. This article is protected by copyright. All rights reserved.

Description: This study makes the case that monsoon depressions over South Asia can form from a variant of moist barotropic instability. Using an idealized numerical framework in which the atmosphere is partitioned into a basic state and a perturbation, we simulate vortices resembling monsoon depressions that draw energy from the meridional shear of the monsoon trough and amplify when they interact with precipitating ascent. The influence of the basic vertical shear on the vortex induces upward velocity which couples precipitation with a Rossby wave‐like mode arising from dry barotropic growth, allowing the vortex to intensify. Sensitivity experiments reveal that both the sheared basic state and latent heating are necessary to achieve positive growth rates and that this process requires a sufficiently large initial perturbation. Trajectory analyses suggest that the combined flow of the vortex and the large‐scale monsoon transport diabatically generated potential vorticity from southwest of the vortex into the vortex center, thus enabling growth. In contrast with tropical cyclones, this mechanism does not require a feedback between surface wind speed and surface heat and moisture fluxes, though their presence does ultimately result in a slightly stronger vortex. This article is protected by copyright. All rights reserved.

Description: Abstract This study investigates the Holton–Tan (HT) relationship based on re‐analyses and coordinated experiments for present‐day and warming scenarios within the SPARC Quasi‐Biennial Oscillation initiative (QBOi) using Meteorological Research Institute Earth System Model version 2.0 (MRI‐ESM2.0). The aim is to explore to what extent global warming modulates the HT mechanism in the effect of the QBO on the polar vortex. The simulations of the present‐day conditions reproduce the zonal wind composite differences in the re‐analysis; however, there are some discrepancies. In regions near the QBO (i.e. 20–40°N), there is good agreement between the simulations and the re‐analysis. Examples include the mid‐latitude (~30°N) lower (middle) stratosphere Eliassen–Palm (EP) flux convergence (divergence) anomalies between the easterly phase and the westerly phase of the QBO, which are common in the re‐analysis and in the present‐day and warming simulations. The influence of the QBO on the mid‐latitude lower stratosphere is associated with an effective waveguide for enhanced upward‐propagating Rossby waves near the subtropical jet. At high latitudes, the significant (less significant) convergence anomalies of significant horizontal EP fluxes in the middle stratosphere are dominant in the re‐analysis (the present‐day simulations). In the warming scenarios, the QBO signatures of the zonal wind are strengthened with the significant EP flux convergence anomalies of less significant upward‐propagating waves inside the polar vortex above 20 hPa. These results suggest that fundamental mechanisms behind the HT effect in regions near the QBO can be explained by enhanced upward‐propagating waves associated with an effective waveguide (by inhibiting equatorward‐refracting upward waves) in the lower‐to‐mid stratosphere. In the warming scenarios, however, HT mechanisms, especially inside the polar vortex, may have another pathway, different from that of the present‐day climate. This article is protected by copyright. All rights reserved.

Description: Abstract We use a simple, semi‐analytic, column model to better understand the meridional structure of tropopause height and the future changes in its height and temperature associated with global warming. The model allows us to separate the effects of tropospheric lapse rate, optical depth, outgoing longwave radiation (OLR) and stratospheric cooling on the tropopause height. When applied locally at each latitudinal band the model predicts the overall meridional structure of the tropopause height, with a tropical tropopause substantially higher than that in higher latitudes and with a sharp transition at the edge of the extratropics. The large optical depth of the tropics, due mainly to the large water vapour path, is the dominant tropospheric effect producing the higher tropical tropopause, whereas the larger tropical lapse rate actually acts to lower the tropopause height. The dynamical cooling induced by the stratospheric circulation further lifts the thermal tropopause in the tropics resulting in it being significantly cooler and higher than in mid‐ and high latitudes. The model quantifies the causes of the tropopause height increase with global warming that is robustly found in climate integrations from the fifth Coupled Model Intercomparison Project (CMIP5). The large spread in the increase rate of tropopause height in the CMIP5 model is captured by the simple model, which attributes the dominant contributions to changes in water vapour path and lapse rate, with changes in CO 2 concentration and OLR having much smaller direct effects. The CMIP5 models also show a small but robust increase in the tropopause temperature in low latitudes, with a much smaller increase in higher latitudes. We suggest that the tropical increase may at least in part be caused by non‐grey effects in the radiative transfer associated with the higher levels of water vapour in the tropics, with near constant tropopause temperatures predicted otherwise. This article is protected by copyright. All rights reserved.

Description: Previous observations suggested connection between the Madden‐Julian Oscillation (MJO) and the North Atlantic Oscillation (NAO), yet few of them contrasted the influences between the MJO and the NAO on extreme weather and climate events. In this study, it is found that the impacts of the MJO and the NAO on winter cold wave amplitude (CWA) over China can be distinctive. Time‐lagged correlation analysis indicates that the response of the CWA to the MJO is characterized by significant anomalous center over the Tibetan Plateau and its adjacent region, whereas that to the NAO is mainly over western and northeastern China. The ECMWF model from the sub‐seasonal to seasonal (S2S) project can approximately reproduce the relationship between CWA and these indices at intra‐seasonal time scale. The MJO associated convective activities along the equator can modulate the local Hadley circulation. When the convection center of the MJO is located over the Maritime Continent, it can trigger subsidence over the Tibetan Plateau and its adjacent region, warm the surface and consequently decrease the CWA. When descending flows associated with the MJO prevail over the Maritime Continent due to the eastward propagation of the MJO, the situations tend to be opposite. The intraseasonal variations of the NAO will induce eastward‐propagating Rossby wave trains, which usually favor the significantly positive (negative) pressure, temperature and consequently negative (positive) CWA anomalies over western and northeastern China. Thus, the MJO and the NAO might provide two critical predictability sources for subseasonal forecast of the extreme temperature events.

Description: Idealised 2‐dimensional simulations have been used to examine the dependence of valley cold air pool (CAP) behaviour, morphology and duration (persistence) on terrain scale. The simulations use terrain of constant aspect ratio but ranging from modest to Alpine scale, under mid‐latitude winter conditions with a finite background flow and incorporating a diurnal cycle. CAPs in smaller valleys are found to be more morphologically constrained, whereas larger valleys provide scope for more complex flow dynamics and more dramatic interplay of the external flow with the valley, including gravity wave breaking. Simulations of a valley within a plateau show that as valley depth exceeds the depth scale of the nocturnal stable boundary layer, the CAP becomes increasingly confined and sheltered from the background flow (reduced mixing), becoming harder to remove by processes related to daytime insolation, and can eventually become persistent (never removed). Both the surrounding terrain and gravity wave dynamics are found to play a part in the enhancement of sheltering. Therefore background atmospheric stability and wind are also crucial, summarised in a non‐dimensional valley depth parameter which largely determines CAP duration. More stable conditions, representative of for instance large scale subsidence or warm advection aloft, result in longer durations and a greater likelihood of persistence. Stronger nocturnal radiative cooling results in stronger CAPs, which are harder again to remove. Several mechanisms appear to act in concert to give rise to these behaviours and the dependence on non‐dimensional valley depth. Replacing the valley‐in‐plateau with a double hill enhances wave breaking, which complicates the relationship to non‐dimensional valley depth, reducing CAP duration significantly when subsidence‐/warm advection‐like effects are absent.

Description: Abstract Bay of Bengal (BoB) exhibits notable seasonal variations in tropical cyclone heat potential (TCHP), barrier layer thickness (BLT) and sea surface temperature (SST). These parameters also undergo profound changes in the presence of tropical cyclones (TCs). The composite structures of these ocean parameters as a function of the season of TC formation, intensity, and translation speed are unknown and are developed in the present study. Composite structures are examined based on 1,222 instantaneous samples from 83 TCs during 2003–2016 using INCOIS‐GODAS analyses. A BLT of 10‐30 m and TCHP of 40–80 kJ cm−2 favors the TC intensification in the central BoB. The multivariate regression of BLT and TCHP appears to be better for TC intensity up to 64 knots and is highly underestimated for the stronger TCs (〉64 knots). The TC right‐rear sector experiences significant changes in TCHP anomaly (TCHPA) as the intensity increases. The TCHPA ranges ~10–15 kJ cm−2, ~20–25 kJ cm−2, and ~25–30 kJ cm−2 when a TC is at Cyclone Storm (CS), Severe Cyclonic Storm (SCS) and Very SCS (VSCS) stages respectively. The maximum TCHPA is generally aligned along the TC track during the post‐monsoon season. Slow‐moving TCs produce maximum TCHPA cooling of ~20 kJ cm−2 within 250 km storm radius in the rear sector, while it is less and away from the storm center for normal and fast‐movers. The seasonal changes showed opposite relations between BLT and TCHP from pre‐ to post‐monsoon seasons during the TC intensification. TC induced SST cooling is maximum (~0.5–1.2°C) in the inner‐core for the strong (VSCS and above) and slow‐moving TCs. The cooling decreases with an increase in the translation speed and is more pronounced in the pre‐monsoon season. This study provides a baseline to verify and understand the limitations of the models, and also develop a climatological perspective of BoB TCs. This article is protected by copyright. All rights reserved.

Description: ABSTRACT During the period from 30th June to 6th July 2016, a heavy rainfall event affected the middle and lower reaches of the Yangtze River valley in eastern China. The event was characterized by high‐intensity, long‐duration (lasted more than 6 days) precipitation and huge amounts (over 600.0 mm) of rainfall. The rainfall moved eastward from the Sichuan basin to the middle Yangtze River valley during the first two days, then Mei‐yu front formed and circulations became more “quasi‐stationary”. During the second‐phase, successive heavy rainfall systems occurred repeatedly over the same areas along the front, leading to widespread and catastrophic flooding. In this study, limited‐area convection‐permitting models (CPMs) covering all of eastern China, and global‐model simulations from the Met Office Unified Model are compared to investigate the added values of CPMs on the veracity of short‐range predictions of the heavy rainfall event. The results show that all the models can successfully simulate the accumulated amount and the evolution of this heavy rainfall event. However, the global model produces too much light rainfall (10.0 mm day−1), fails to simulate the small‐scale features of both atmospheric circulations and precipitation, and tends to generate steady heavy rainfall over mountainous region. Afternoon precipitation is also excessively suppressed in global model. By comparison, the CPMs add some value in reproducing the spatial distribution of precipitation, the smaller‐scale disturbances within the rain‐bands, the diurnal cycle of precipitation and also reduce the spurious topographical rainfall, although there is a tendency for heavy rainfall to be too intense in CPMs. This article is protected by copyright. All rights reserved.

Description: SUMMARY To deal with the land surface physics uncertainties, a stochastic scheme based on stochastic perturbation of physics tendencies is implemented and tested. The impact of land surface physics uncertainties and its relative importance to land surface initial uncertainties are investigated in the regional ensemble forecasting system ALADIN‐LAEF (Aire Limitée Adaptation Dynamique Développement InterNational – Limited Area Ensemble Forecasting). The land surface initial perturbation is generated by using an ensemble of land surface data assimilation; and the land surface physics uncertainties by applying the idea of stochastically perturbed parameterization tendencies (SPPT) scheme. Three experiments are conducted and compared with the reference ensemble over a two month periods. The results show the introduction of land surface stochastic physics increases the ensemble spread, reduces the ensemble bias, and keeps neutral in deterministic forecast skill of the ensemble, its impact strongly depends on the quality of ensemble initial conditions. The ensemble land surface data assimilation has stronger positive impact on the ALADIN‐LAEF than the land surface stochastic physics for screen level temperature and humidity. There isn't much impact on 10 m wind and precipitation. Best results are obtained when both the ensemble land surface data assimilation and land surface stochastic physics are used simultaneously; it gives a more reliable and statistically consistent forecast, which is contributed mainly by ensemble land surface data assimilation in the first forecast hours and largely by land surface stochastic physics in the later forecast hours. This article is protected by copyright. All rights reserved.

Description: Abstract Climate modes as simulated by global climate models are often found to be considerably weaker than are observed. One possibility is that coarse resolution climate models do not capture turbulent air‐sea fluxes sufficiently. Ensemble experiments with the same atmospheric configuration of the Met Office Hadley Centre climate model, forced with observed sea surface temperatures and at three different horizontal model resolutions (approximately 130, 60 and 25 km), are used to test the sensitivity of air‐sea surface heat and moisture fluxes. We find that, although global mean budgets at the three resolutions are very similar, substantial differences appear in regional air‐sea flux patterns. Increased model resolution consistently enhances zonal‐mean air‐sea fluxes in the mid‐high latitudes while suppressing heat fluxes in the low latitudes and weakening the Hadley Circulation. In the North Atlantic, annual mean surface heat fluxes into the atmosphere along the Gulf Stream/North Atlantic Current and over the sub‐polar gyre can increase by up to 10 Wm‐2 when atmospheric model resolution increases from 130km to 60km. In the Pacific, increased model resolution tends to weaken the Walker Circulation with increased heat fluxes from the east but markedly reduced fluxes from the western Pacific, leading to significantly improved precipitation over the tropical western Pacific and the Maritime Continent. Changes in air‐sea heat fluxes come about mainly as a result of changed near surface ventilation. Generally, increasing resolution strengthens surface winds and reduces specific humidity in the mid‐high latitudes while weakening surface winds over the tropics and subtropics. This article is protected by copyright. All rights reserved.

Description: Abstract Previous investigations of urban heat island (UHI) are primarily focused either on the canopy heat island intensity (aUHII) derived from weather stations, or on the surface urban heat island intensity (sUHII) derived from satellite instruments. Research of the relationship between sUHII and aUHII (the sUHII − aUHII relationship) is limited and this study attempts to further progress this possibility by examining the night‐time sUHII − aUHII relationship for three factors: season, wind speed, and basic landuse categories modified from local climate zones (urban / suburban), in Birmingham, UK. Using high resolution datasets of canopy air temperature from Birmingham Urban Climate Laboratory and land surface temperature from the MODIS instrument aboard the Terra and Aqua satellites, with a unique methodology of regression analysis, confidence ellipse analysis of covariance (ANCOVA), and 2‐D Kolmogorov‐Smirnov (K‐S) tests, statistical evidence is provided to present the varying patterns and magnitudes between sUHII and aUHII. The significance of the impact of the three considered factors is clearly supported by the statistical tests. The results indicate that satellite data can be used to infer aUHII with a higher confidence for low wind speed conditions. Results also demonstrate better confidence in the approach for summer and spring seasons, and for more urbanised sites. Indeed, the analysis potentially indicates that wind advection is a key factor for the investigation of the sUHII‐aUHII relationship. Overall, the methods used here are transferrable to other cities and/or can be used to guide further research to explore the sUHII − aUHII relationship under other environmental conditions. This article is protected by copyright. All rights reserved.

Description: Ensemble forecasts are run operationally to determine forecast uncertainty arising from initial condition, model physics and boundary condition uncertainty. However, global configuration ensembles, which use a convection parametrization scheme, may miss uncertainty because of the misrepresentation of intense convection by such schemes. Here the impacts of the misrepresentation of Mesoscale Convective Systems (MCSs) on downstream ensemble forecast skill and evolution are determined for a case study. MCS perturbations (calculated from the difference between output from convection‐parametrizing and convection‐permitting Met Office model configurations) are added to six members of a global configuration ensemble created by downscaling forecasts from the global version of the Met Office Global and Regional Ensemble Prediction System. For the first 36 h differences grow on the convective scale related to the MCSs leading to systematic deepening of a developing UK cyclone, although there is damping of the perturbations found in root mean square difference calculations between the forecasts with and without the perturbations (particularly in mean sea level pressure). Subsequently, differences grow rapidly onto the synoptic scale and by five days impact the entire northern hemisphere. The MCS perturbations can have systematic effects on the ensemble forecasts (e.g. a systematic displacement of a downstream cyclone is found), but for this case, there is no discernible change in forecast skill as measured by root mean square error of the ensemble means and the effects of the MCS perturbations are smaller than those generated by the initial condition perturbations. The spread of the combined ensemble (the two ensembles with and without the MCS perturbations) is larger than that of the individual ensembles. Thus, perturbing convection‐parametrizing models to include PV anomalies associated with MCSs represented in convection‐permitting forecasts, or idealised representations of them, produces alternative realisations to those generated by initial condition perturbations and has the potential to be useful operationally.

Description: Abstract The nocturnal boundary layer regime transition from weakly stable (strong wind) to very stable (weak wind) is analyzed using 10 levels of turbulence observations made at a 140‐m micrometeorological mast near the southeastern Brazilian coast. The combination of synoptic and local flow favors the systematic occurrence of such a transition, typically 5 to 7 hours after sunset. The regime transition is marked by decreases in temperature, wind speed, turbulent kinetic energy and absolute heat flux. The decrease in temperature is often abrupt and the inflection point in the temperature series marks the regime transition. Absolute heat flux peaks before the transition during the weakly stable period, while temperature variance peaks near the transition. Composites from 36 cases when the cooling rate exceeded 2oC h‐1 are used to describe the vertical structure of the stable boundary layer (SBL) in both regimes. For these abrupt transitions, dimensionless variables that relate thermal and mechanical properties of the flow are compared as indicators of the SBL regime, and the gradient Richardson number is found to be better for that purpose. The absolute heat flux is shown to be proportional to the cube of the wind speed only in the strong wind limit of the very stable regime. Simulations of similar transitions using a second order model are described in part 2. This article is protected by copyright. All rights reserved.

Description: Abstract We present a characterization of the variability of clouds over the South‐West Indian Ocean between 2007 and 2010. Cloud occurrence is derived from the DARDAR (raDAR/liDAR) mask, a synergistic product based on CALIPSO (Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation) and CloudSat measurements. It provides a target classification for hydrometeors. We demonstrate that this product is suitable for studying the vertical, spatial and seasonal cloud distribution in the South‐West Indian Ocean. The variability of cloud occurrence increases approaching the tropics: the average maximum amplitude of monthly occurrence is ∼7% between 30°S and 60°S and ∼14% between 10°S and 30°S. The 10‐30°S latitudinal band exhibits the largest contrasts: summer (winter) total cloud occurrence is mainly driven by high‐ (low‐) level clouds. The vertical distribution of clouds differs on either side of 55°E and this is related to a land‐ocean contrast and to large‐scale influence. In winter, east of 55°E, the maximum of cloud occurrence corresponds to warm and mixed‐phased precipitating clouds associated with the Mascarene High. In summer, west of 55°E, the vertical distribution of cloud occurrence is driven by deep convection associated with the Inter Tropical Convergence Zone and the Near Equatorial Trough. The vertical distribution of cloud occurrence shows an interannual variability that is related to El Niño events. The influence of other oscillation modes should be further investigated.

Description: Abstract A number of realisations of one or more numerical weather prediction (NWP) models, initialised at a variety of initial conditions, compose an ensemble forecast. These forecasts exhibit systematic errors and biases that can be corrected by statistical post‐processing. Post‐processing yields calibrated forecasts by analysing the statistical relationship between historical forecasts and their corresponding observations. This paper aims to extend post‐processing methodology to incorporate atmospheric circulation. The circulation, or flow, is largely responsible for the weather that we experience and it is hypothesised here that relationships between the NWP model and the atmosphere depend upon the prevailing flow. Numerous studies have focussed on the tendency of this flow to reduce to a set of recognisable arrangements, known as regimes, which recur and persist at fixed geographical locations. This dynamical phenomenon allows the circulation to be categorised into a small number of regime states. In a highly idealised model of the atmosphere, the Lorenz ’96 system, ensemble forecasts are subjected to well‐known post‐processing techniques conditional on the system's underlying regime. Two different variables, one of the state variables and one related to the energy of the system, are forecasted and considerable improvements in forecast skill upon standard post‐processing are seen when the distribution of the predictand varies depending on the regime. Advantages of this approach and its inherent challenges are discussed, along with potential extensions for operational forecasters. This article is protected by copyright. All rights reserved.

Description: Abstract Experimental evidence has demonstrated that temperature on the surface of anice particle growing by accretion is not uniform. This non‐uniformity is relevant because the ice particle surface temperature is an important microphysical parameter that influences the sign and magnitude of the charge transfer during ice particles collisions. In particular, when high values of ambient temperature and liquid water content are reached, the ice particle surface temperature is expected to reach 0°C, which is known as wet growth regime. Previous experimental results have shown that wet growth is not uniformly reached on the ice particle surface and a surface temperature distribution is developed. In order to know the surface temperature distribution of a fixed ice particlegrowing by accretion of supercooled water, numerical calculations were carried out. It was found that the surface temperature distribution has a strong dependence on liquid water content, ambient temperature, airflow velocity and water droplet size. The stagnation point always reaches higher temperatures and, in many cases, its temperature is near 0°C. For some values of the liquid water content, ambient temperature and airflow velocity, it was possible to determine temperature differences up to 7°C between the stagnation point and the equator. This variation in the surface temperature implies that the region near the stagnation point would experience wet growth, while the rest of the ice particle surface would remain in dry growth regime, supporting the partial wet growth hypothesis. This could also explain the charge transfer reported during ice particles collisions under wet growth conditions. This article is protected by copyright. All rights reserved.

Description: Abstract Statistical post‐processing approaches are widely employed to construct improved probabilistic meteorological forecasts from Numerical Weather Prediction. However, generating calibrated and sharp probabilistic forecasts is challenging. In this paper, a post‐processing approach, Mixture Probabilistic Model (MPM), is proposed to calibrate probabilistic ensemble forecasts subject to sharpness. In particular, the proposed MPM is applied to precipitation forecasting. First, the Censored and Shifted Gamma (CSG0) distribution is considered as the Probability Density Function (PDF) for precipitation. Then, the predictive PDF of MPM is mixed by the individual PDFs which are fitted from raw ensemble members. Finally, to estimate optimal weight parameters for the mixture of individual PDFs, the Dirichlet distribution is utilized and the skills of the mixture model and individuals are both taken into consideration. The proposed MPM was tested using Innsbruck ensemble precipitation data and 6 h accumulated precipitation ensemble forecast data in East China from August to November in 2017. Compared with raw forecasts and three state‐of‐the‐art post‐processing approaches, MPM showed improved performance for all verification scores. The quantitative and qualitative analyses of results in both cases indicate the potential and effectiveness of MPM for precipitation ensemble forecasting. This article is protected by copyright. All rights reserved.

Description: Abstract The widely‐adopted discretisation of the horizontal pressure gradient term formulated by Simmons and Burridge (1981) for atmospheric models on σ‐p hybrid vertical coordinate is found to incur spectral blocking for rotational wind components at high vertical levels when used in a spectral semi‐Lagrangian model run on a linear grid. A remedy to this issue is proposed and tested using a spectral semi‐implicit semi‐Lagrangian hydrostatic primitive equations model. The proposed method removes aliasing errors at high wavenumbers by ensuring that the rotation‐free property of the pressure gradient term on isobaric surface, a feature possessed by the continuous system, is preserved in the discretised system, which highlights the significance of mimetic discretisation within the context of numerical weather prediction models. This article is protected by copyright. All rights reserved.

Description: In March 1972, Thule Air Base in northwest Greenland experienced an extreme downslope windstorm with observed gusts up to 93 m s−1. Trapped lee waves and rotors have been previously declared responsible for the severity of the storm. In this study, high‐resolution numerical simulations reveal that trapped lee waves and rotors indeed occurred, but were not responsible for the peak intensity. Instead, the highest gusts occurred during a period of wind pulsations resulting from Kelvin–Helmholtz instability, as illustrated in the figure. Thule Air Base in northwest Greenland experienced an extreme windstorm during the night of 8/9 March 1972. The event is not among official WMO records because the anemometer broke after recording the highest gust of 93 m s−1. A recent study re‐examined the event based on coarse‐resolution reanalyses and observations which, however, did not fully resolve the proposed storm processes. This is the first study that uses high‐resolution numerical simulations to investigate the processes associated with this windstorm. A cold‐frontal inversion and strong flow across the mountain ridge upstream of Thule Air Base (associated with a passing low pressure system) are shown to be key factors for the severe downslope windstorm in the lee of the ridge. It is shown that trapped lee waves occurred during the initial phase of the storm, but did not contribute to the highest wind speeds as proposed in the previous study. It is confirmed that rotor circulations occurred which possibly contributed to the large wind variability and, hence, large differences between individual observation sites. However, no rotors were present at the time of the highest simulated wind speed. Instead, wave breaking above the lee slope is found to indirectly contribute to the wind maxima by facilitating Kelvin–Helmholtz instability at the top of the shooting flow that caused intense wind speed pulsations near the surface. In agreement with the previous study, a corner jet was simulated, however it was not responsible for the strong winds in the vicinity of the air base. Sensitivity experiments showed that the flow field was considerably influenced by the high topography downstream of the air base and that simulations with very thin or no sea ice cover over Baffin Bay resulted in a weaker frontal inversion and up to about 30% lower maximum wind speeds.

Description: This article verifies 1 to 10‐day probabilistic precipitation forecasts in June, July, and August 2016 from an experimental dual‐resolution version of the European Centre for Medium‐Range Weather Forecasts (ECMWF) ensemble prediction system. Five different ensemble combinations were tested. These comprised subsets of the 51‐member operational ECMWF configuration (18 km grid) and an experimental 201‐member lower‐resolution configuration (29 km grid). The motivation of the dual‐resolution ensemble forecast is to trade some higher resolution members against a larger number of lower resolution members to increase the overall ensemble size at constant overall computational cost. Forecasts were verified against precipitation analyses over Europe. Given substantial systematic errors of precipitation forecasts, both raw and post‐processed dual‐resolution ensemble predictions were evaluated. Post‐processing consisted of quantile mapping, tested with and without an objective weighting of sorted ensemble members using closest‐member histogram statistics. Reforecasts and retrospective precipitation analyses were used as training data. However, the reforecast ensemble size and the dual‐resolution ensemble sizes differed, which motivated the development of a novel approach for developing closest‐member histogram statistics for the larger real‐time ensemble from the smaller reforecast ensemble. Results show that the most skillful combination was generally 40 ensemble members from the operational configuration and 40 from the lower‐resolution ensemble, evaluated by continuous ranked probability score, Brier Scores at various thresholds, and reliability diagrams. This conclusion was generally valid with and without post‐processing. Reliability was improved by post‐processing, though the improvement of resolution component is not so clear. The advantages of many members at higher resolution was diminished at longer lead times; predictability of smaller scale features was lost, and there is more benefit from increasing ensemble size to reduce sampling uncertainty. This paper evaluates only one aspect in deciding on any future ensemble configuration and other skill related considerations need to be taken into account. This article is protected by copyright. All rights reserved.

Description: The representation of deep convective clouds by convection‐permitting models could be improved by better parameterising the subgrid turbulent fluxes. Following Verrelle et al. (2017), a large eddy simulation (LES) of a population of convective clouds at a 50‐m grid spacing was explored during the cloud life cycle to characterize the second‐order moment turbulent fluxes. The reference turbulence fields were deduced by coarse‐graining the LES outputs at horizontal grid resolutions of 500 m, 1 km and 2 km. The ability of three parameterizations, the traditional K‐gradient Cuxart et al. (2000) (CBR) and Smagorinsky (1963) formulations and the Moeng (2014) (MOENG) approach based on the horizontal gradients of the resolved variables, to reproduce the thermodynamical and dynamical fluxes was assessed in an offline configuration via a comparison to the reference fields. MOENG was the most appropriate scheme to represent the vertical and horizontal heat fluxes at all grid spacings in the clouds and their environment over the entire cloud life cycle including the appropriate representation of countergradient areas. It also represented the vertical and horizontal moisture fluxes at the 1km and 500‐m grid spacings the best, while its advantage is reduced at a grid spacing of 2 km as CBR performed slightly better for the statistical scores. MOENG was also able to represent the dynamical covariances well at grid spacings of 1 km and 500 m, even though the statistical scores were not as good as those obtained for the thermodynamical fluxes. The dynamical variances were well represented by CBR; however, for this offline evaluation, the subgrid TKE, present in CBR formulations, is directly computed from the LES outputs giving CBR an advantage over the two other diagnosed parameterizations. Also, the reference fluxes revealed an anisotropic deformation of the turbulence throughout the troposphere, which was only captured by MOENG. This article is protected by copyright. All rights reserved.

Description: Abstract Since global reanalysis datasets first appeared in the 1990s, they have become an essential tool to understand the climate of the past. The wind power industry extensively uses those products for wind resource assessment, while several climate services for energy rely on them as well. Nowadays various datasets coexist, which complicates the selection of the most suitable source for each purpose. In an effort to identify the products that better represent the wind speed features at turbine hub heights, five state‐of‐the‐art global reanalyses have been analyzed: ERA5, ERA‐Interim, the Japanese 55‐year Reanalysis (JRA55), the Modern Era Retrospective Analysis for Research and Applications‐2 (MERRA2) and the NCEP/NCAR Reanalysis 1 (R1). A multi‐reanalysis ensemble approach is used to explore the main differences amongst these datasets in terms of surface wind characteristics. Then, the quality of the surface and near surface winds is evaluated with a set of 77 instrumented tall towers. Results reveal that important discrepancies exist in terms of boreal winter seasonal means, interannual variability (IAV) and decadal linear trends. The differences in the computation of these parameters, which are mainly concentrated inland, reach up to the order of magnitude of the parameters themselves. The comparison with in‐situ observations shows that the ERA5 surface winds offer the best agreement, correlating and reproducing the observed variability better than a multi‐reanalysis mean in 35.1% of the tall tower sites at a daily time scale. However, none of the reanalyses stands out from the others when comparing seasonal mean winds. Regarding the IAV, near surface100 m winds from ERA5 offer the closest values to the observed IAV. This article is protected by copyright. All rights reserved.

Description: The linear theory of vertically sheared convection is extended to deep‐atmosphere flows with arbitrary wind, stability, and diffusion profiles. Consistent with previous findings, reference single‐layer vertical channel flows show fastest growth for shear‐parallel roll circulations and much weaker growth for shear‐transverse circulations, causing the former to dominate. In a more realistic three‐layer setting, where the cloud layer lies between a mixed layer below and a stable free troposphere aloft, shear‐parallel rolls also dominate. However, shear‐transverse rolls grow much faster than before, which degrades the convective organization. An analysis focused on the vertical perturbation phase tilt leads to a novel interpretation of these results. Vertical shear imparts a downshear tilt, which acts to weaken the convective growth driven by dynamic and nonhydrostatic buoyant vertical perturbation pressure gradients (VPPGs). Whether these VPPGs can maintain growth in face of the shear depends largely on the Richardson number (Ri), with Ri≲ ‐ 2 becoming a necessary condition for (inviscid) growth of shear‐perpendicular rolls in the short‐wave limit. In deeper, three‐layer atmospheres, longer vertical wavelengths are admitted, which fosters less tilted and faster growing perturbations. This effect, however, is partially offset by differential tilting between kinematic and thermal anomalies. Numerical simulations are used to verify the linear results and to explore the evolution of the convection into the nonlinear regime. As the nonlinearities grow, an initial preference for smaller‐scale, shear‐parallel circulations is ultimately overwhelmed by larger‐scale perturbations with no preferred orientation. Thus, the linear findings are most applicable to the early evolution of cloud layers undergoing turbulent transition. This article is protected by copyright. All rights reserved.

Description: Case studies remain an important method for meteorological parameter sensitivity process studies. However, these types of study often use just a few case studies (typically up to three) and are not tested for statistical significance. This approach can be problematic at the convective scales, since uncertainty in the representation of an event increases, and the variability in the atmosphere arising from convective‐scale noise is not routinely taken into account. Here we propose a simple ensemble method for performing more robust sensitivity analysis without the need for an operational‐style ensemble prediction system and demonstrate it using a case study from the 2005 Convective Storm Initiation Project. Boundary layer stochastic potential temperature perturbations with Gaussian spatial structure are used to create small ensembles to examine the impact of increasing cloud droplet number concentration (CDNC) on precipitation. Whilst there is a systematic difference between the experiments, such that increasing the CDNC reduces the precipitation, there is also an overlap between the different ensembles implying that convective‐scale variability should be taken into account in case study process‐based sensitivity studies. This article is protected by copyright. All rights reserved.

Description: The relative contribution of soil moisture heterogeneities, a stochastic boundary layer perturbation scheme and varied aerosol concentrations representing microphysical uncertainties on the diurnal cycle of convective precipitation and its spatial variability is examined conditional to the prevailing weather regime. To achieve this, separate perturbed‐parameter ensemble simulations are performed with the Consortium for Small‐scale Modeling (COSMO) model at convection‐permitting horizontal grid spacing for 10 days during the high impact weather episode 2016 in Central Europe. We consider hourly precipitation amounts and their spatial distribution, focus on ensemble mean and spread aggregated over strong and weak forcing conditions and employ spatial evaluation techniques. The convective adjustment timescale diagnostic is used to distinguish the different precipitation regimes. While the total amount of daily precipitation is hardly changed by the different perturbation approaches (less than 5%) the spatial variability of precipitation exhibits clear differences. Soil moisture heterogeneity primarily introduces variability during convection initiation causing a steeper increase in normalized rainfall spread prior to the onset of afternoon precipitation. The stochastic boundary layer perturbations lead to the largest spatial variability impacting precipitation from initial time onwards with an amplitude comparable to the operational ensemble spread. Likewise, perturbed aerosol concentrations impact spatial precipitation variability from model start onwards but to a smaller degree. Soil moisture heterogeneity shows the strongest weather regime dependence with the gravest impact on convection during weak synoptic forcing. All types of perturbations increase dispersion of precipitation while maintaining the domain‐averaged precipitation rates. This article is protected by copyright. All rights reserved.

Description: Polar lows are intense mesoscale cyclones occurring during winter over open sea areas in certain polar sub‐regions. Due to their small size they are not explicitly represented in present global climate models or Earth system models. In this study 18 members of the CESM Large Ensemble were dynamically downscaled to 12km horizontal mesh width using the quasi‐hydrostatic ALARO model within the HARMONIE script system in climate mode (HCLIM‐ALARO). The domain covers the Nordic and Barents Seas. One historical and two future time‐periods were selected. For validation, the ERA‐Interim reanalysis was also downscaled. A cyclone tracking algorithm was used to identify tracks of individual polar lows. Their frequency of occurrence, lifetime, and maximum relative vorticity were estimated. Relative to ERA‐Interim, the historical frequency of occurrence of polar lows was slightly overestimated in the Nordic Seas and underestimated in the Barents Sea, which is likely due to positive biases in sea‐surface temperature and sea‐ice concentration. For the future climate projections, the regions of polar low genesis are diagnosed to move northwards in accordance with the sea‐ice retreat. In the Nordic Seas, the number of polar lows decreases in the beginning of the season, while there is an increase in March. In the Barents Sea, a February–April increase in the occurrence of polar lows is seen. This article is protected by copyright. All rights reserved.

Description: Abstract The summer of 2010 was characterized by weather and climate extremes such as the western Russia heat wave and the Pakistan floods. A recent study found that summer was dominated by a particular 200‐hPa geopotential height pattern featuring an anomalous Rossby wave train with ridges centered over Greenland, Europe, and Russia. The daily frequency of this pattern has dramatically increased recently and closely resembles the mean‐state difference in 200‐hPa geopotential height fields between 1998–2014 (P2) and 1979–1997 (P1). Because anomalous wave trains are often driven by localized diabatic heating, it is tested in this study whether the P2 minus P1 pattern is caused by diabatic heating anomalies near Greenland. While it is found that sea ice concentrations declined and sea surface temperatures rose over Baffin Bay to the west of Greenland during P2, surface latent heat fluxes actually increased downward, indicating that surface processes were likely not the source of diabatic heating. Rather, an increase in vertically integrated horizontal latent heat flux convergence over Baffin Bay was observed in P2, which led to the condensation of water vapor and latent heating. Thus, the mid‐tropospheric circulation established the diabatic heating. A set of initial‐value calculations with idealized heating over Baffin Bay show solutions that remarkably resemble the P2 minus P1 pattern and provide a plausible explanation as to why the pattern has been occurring more frequently. This study demonstrates that changes in the Arctic can arise from moisture transport from the mid‐latitudes, and, in turn, these changes can induce weather and climate extremes in distant mid‐latitude regions. This article is protected by copyright. All rights reserved.

Description: The Stochastically Perturbed Parametrization Tendencies (SPPT) scheme is used in ensemble forecast systems to represent uncertainties due to the atmospheric physics parametrizations. The scheme works by introducing random perturbations to the tendencies from the physics schemes. The size of the perturbations is modulated by the size of the net tendencies from the full physics suite. Recent work has identified an inconsistency in this approach to attributing model uncertainty: greater uncertainty is attributed to the night‐time sky than to the daytime sky in clear‐sky conditions, which is not deemed physically realistic. This paper documents the details of a revised configuration of SPPT, which became active in the latest operational version of the ensemble forecast system (“CY45R1”) at the European Centre for Medium‐Range Weather Forecasts (ECMWF) — released in June 2018. In this work, a modification to SPPT is outlined such that the clear‐sky contribution from the radiation scheme is subtracted from the physics tendencies that are acted on by SPPT. Inspection of individual vertical profiles of physics tendencies provides a greater understanding of the interactions between processes, and helps demonstrate that the revision gives a more realistic description of the associated uncertainties. Forecast experiments with the revised version of SPPT demonstrate neutral to positive impacts on the skill of the ensemble forecasts in the medium‐range; and in the extended‐range, a significantly improved match between the spread and error of the principal components describing the Madden‐Julian Oscillation. In addition, it is shown that including the revisions in the ensemble of data assimilations (EDA) leads to increases in spread in the boundary layer. This article is protected by copyright. All rights reserved.

Description: The spatiotemporal evolution of the atmospheric boundary layer (ABL), in a narrow, highly complex terrain located in the Colombian Andes, is studied using radiosondes and remote sensing equipment. Different techniques are implemented to automatically estimate the ABL height using ceilometer backscattering profiles and a combination of a radar wind profiler and microwave radiometer retrievals. The large aerosol load from anthropogenic emissions within the valley allows the use of ceilometer‐based ABL height detection methods, especially under stable atmospheric conditions. Convective atmospheres, however, favor aerosol dispersion, increasing the uncertainty associated with the estimation of the convective boundary layer using ceilometers. In contrast, the multisensor technique is more robust, performing better in stable and unstable conditions. All ceilometer‐based methods and the multisensor scheme capture the observed ABL height diurnal cycle. The main difference among ABL height retrievals occurs in the afternoon and during the night when Richardson number estimates tend to detect the top‐down contraction of the residual layer, while ceilometer‐based estimates detect the sudden bottom‐up onset of the nocturnal stable layer. The results also show that intra‐annual and annual variations of cloudiness strongly condition the ABL expansion, leading to a modulation of the ABL height diurnal cycle. The amount of aerosol particles near the surface is influenced by the evolution of the ABL, modifying the available control volume for the pollutants to interact and disperse. The evolution of ABL over the slopes and at the valley floor differs as a result of the local thickening associated with upslope winds. Weather Research and Forecasting model simulations, from a climatological point of view, skillfully simulate the observed ABL height concerning both the diurnal and annual cycles; the model skill is higher over the valley floor than over the slopes. This article is protected by copyright. All rights reserved.

Description: We derive and compare the properties of an adiabatic, rising parcel in the weak‐ and strong‐friction limits. Quantities such as cloud‐top height and maximum vertical velocity are compared in both limits. The two limits can be use for parcels that are either within a cloud or at sub‐cloud heights. A time‐dependent stochastic parcel model is proposed, which provides an approximation for the mean convective initiation time in analytical form. For moist convection, models of individual parcel dynamics are valuable for their simple formulation and predictions of cloud properties. Here, two limiting idealized cases of parcel theory are investigated: the weak‐ and strong‐friction limits. The weak‐friction limit is a traditional limit with no momentum drag and the dynamics are a Hamiltonian system for the parcel's height and vertical velocity. A strong‐friction limit is derived and studied here and its limiting form involves a balance between frictional drag and buoyancy, which provides a differential equation for parcel height as a function of time. In the two limiting regimes, analytical formulas are presented and compared for quantities such as maximum vertical velocity and cloud‐top height. For example, in the strong‐friction limit, the cloud‐top height coincides with the level of neutral buoyancy (LNB), whereas in the weak‐friction limit the cloud‐top height is far above the LNB. This comparison suggests that the strong‐friction limit may provide more realistic predictions of some averaged cloud properties. In general, since frictional effects and individual parcel properties can vary even within a single cloud, the predictions of the weak‐ and strong‐friction limits can be viewed as upper and lower bounds for the behavior of more realistic finite‐friction scenarios. Finally, in a stochastic version of the parcel model, in the strong‐friction limit, analytical formulas are derived for convective initiation time. Applications to convective parametrizations are discussed; for example, the formulas for convective initiation time could be applied as stochastic convective triggers.

Description: A sub‐grid parameterization scheme representing the enhancement of precipitation due to sub‐grid orography via the seeder‐feeder effect has been modified to account for flow blocking in small Froude number situations. The scheme was validated in a set of Limited Area Model simulations with a 1.5~km grid spacing, in which the orography was degraded by varying amounts. For simulations in which the largest orographic scales are still fairly well represented, the Seeder Feeder scheme was able to reduce the precipitation deficit by 30% to 70%. For simulations where the hills were completely sub‐grid, the Seeder Feeder scheme was still able to reduce the precipitation deficit by 10% to 30%. As well as increasing the integrated precipitation in global simulations with various grid spacings, some of the precipitation production was shifted from the convection scheme, with its very simple microphysical representation, to the microphysics scheme. In a long‐duration climate simulation, Seeder Feeder scheme enhancements of orographic precipitation perturb the large‐scale hydrological cycle, as evidenced by the far‐field changes in both microphysical and convective precipitation. Changes over major mountain ranges were similar to those described for the case studies, so long as the upstream precipitation impinging on the mountains was not reduced by these changes in the global hydrological cycle. Increased atmospheric drying reduces column‐integrated resolved cloud water mixing ratios over mountains, reducing a positive bias in the amount of optically thick cloud with low to mid‐level tops compared to ISCCP satellite observations. The associated reductions in cloud albedo slightly reduced the RMS error in the TOA outgoing shortwave radiative fluxes over mountains compared to CERES satellite observations.

Description: The NOAA National Severe Storm Laboratory's Warn‐on‐Forecast (WoF) project is developing a storm‐scale ensemble data assimilation and prediction system to provide short‐term probabilistic forecasts of high‐impact weather. WoF probabilistic guidance will enhance the ability of operational forecasters to provide the public with more accurate and longer warning lead times for severe thunderstorms, heavy rainfall, flash floods, tornadoes, and other hazardous weather. This study explores the application of an experimental WoF system in 0–6‐hr probabilistic forecasts of flash‐flood‐producing heavy rainfall events. Abstract The vision of the National Oceanic and Atmospheric Administration's (NOAA) Warn‐on‐Forecast (WoF) research and development project is to provide very short‐term probabilistic model guidance products that will aid the ability of National Weather Service forecasters to issue probabilistic warnings of severe convective hazards with higher accuracy and longer lead times. The experimental Warn‐on‐Forecast System (WoFS), which is under development, is a frequently cycled, regional, convective‐scale, on‐demand, ensemble data assimilation and prediction system. The system assimilates thunderstorm observations to forecast the life cycle and associated hazards of individual convective storms. Most of the initial research effort, since the beginning of the WoF project in 2009, has been focused on tornadic events, which constitute one of the most violent and difficult to predict weather threats. However, the system emerging from this project has significant potential to help with prediction of other hazards as well: in particular, flash‐flood‐producing, convectively driven intense rainfall events. This study focuses on the application of WoFS in 0–6‐hr probabilistic rainfall forecasts of several flash‐flood‐producing heavy rainfall events during the spring and summer of 2015 and 2016. Results indicate that WoFS successfully initializes the convective storms in the ensemble. Visual inspections and probabilistic verification metrics show that WoFS predicts the intense rainfall that often leads to flash flooding at the correct location with higher accuracy in areal coverage and amount during the 0–3 hr forecast period than during the later 3–6 hr period. Overall results indicate that the frequently updated 0–6‐hr ensemble forecast from the WoFS has the potential to highlight areas where intense rainfall can result in flash flooding and increase near‐term situational awareness of flash flood threats.

Description: The Weather Research and Forecasting (WRF) model is run over the Atacama Desert for two week‐long periods in the austral winter season coincident with surface temperature and relative humidity in‐situ observations at three sites. Satellite‐derived land surface temperatures are also used for evaluation. It is concluded that there is a need for ground‐based observational networks in remote regions such as the Atacama Desert where satellite‐derived and model products may not be very accurate. Abstract Good knowledge of the environmental conditions of deserts on Earth is relevant for climate studies. The Atacama Desert is of particular interest as it is considered to be the driest region on Earth. We have performed simulations using the Weather Research and Forecasting (WRF) model over the Atacama Desert for two week‐long periods in the austral winter season coincident with surface temperature and relative humidity in‐situ observations at three sites. We found that the WRF model generally overestimates the daytime surface temperature, with biases of up to 11°C, despite giving a good simulation of the relative humidity. In order to improve the agreement with observed measurements, we conducted sensitivity experiments in which the surface albedo, soil moisture content and five tuneable parameters in the Noah Land Surface Model (namely soil porosity, soil suction, saturated soil hydraulic conductivity, the b parameter used in hydraulic functions and the quartz fraction) are perturbed. We concluded that an accurate simulation is not possible, most likely because the Noah Land Surface Model does not have a groundwater table that may be shallow in desert regions. The WRF‐predicted land surface temperature is also evaluated against that estimated from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. While at night the satellite‐derived and ground‐based measurements are generally in agreement, during the day MODIS estimates are typically lower by as much as 17°C. This is attributed to the large uncertainty in the MODIS‐estimated land surface temperatures in arid and semi‐arid regions. The findings of this work highlight the need for ground‐based observational networks in remote regions such as the Atacama Desert where satellite‐derived and model products may not be very accurate.

Description: In this article, we demonstrate that predicted rainfall is much more sensitive to the assumed drop size distribution (DSD) width than the assumed collection efficiencies within microphysical parametrizations. This figure shows 5 min average rainfall rate (left) and total accumulated rainfall (right) for eight high‐resolution simulations that vary the DSD width (solid lines indicate a broader DSD and dashed lines indicate a narrower DSD) and the collision efficiency dataset used (colours). Abstract Rainfall prediction by weather forecasting models is strongly dependent on the microphysical parametrization being utilized within the model. As forecasting models have become more advanced, they are more commonly using double‐moment bulk microphysical parametrizations. While these double‐moment schemes are more sophisticated and require fewer a priori parameters than single‐moment parametrizations, a number of parameter values must still be fixed for quantities that are not Prognosed or diagnosed. Two such parameters, the width of the rain drop size distribution and the choice of collection efficiencies between liquid hydrometeors, are examined here. Simulations of deep convective storms were performed in which the collection efficiency dataset and the a priori width of the rain drop size distribution (RSD) were individually and simultaneously modified. Analysis of the results show that the a priori width of the RSD was a larger control on the total accumulated precipitation (a change of up to 75% over the typical values tested in this article) than the choice of collection efficiency dataset used (a change of up to 10%). Changing the collection efficiency dataset produces most of the impacts on precipitation rates through changes in the warm rain process rates. On the other hand, the decrease in precipitation with narrowing RSDs occurs in association with the following processes: (a) decreased rain production due to increased evaporation, (b) decreased rain production due to decreased ice melting, and (c) slower raindrop fall speed which leads to longer residency times and changes in rain self‐collection. These results add to the growing body of work showing that the representation of hydrometeor size distributions is critically important, and suggests that more work should be done to better represent the width of the RSD in models, including further development of triple‐moment and bin schemes.

Description: A sketch of the simulation set‐up. This is stretched in the vertical direction from the real aspect ratio for clarity. Z is a displaced coordinate to measure vertical distance with respect to the topography. The significance of air flow within dense canopies situated on hilly terrain is not in dispute given its relevance to a plethora of applications in meteorology, wind energy, air pollution, atmospheric chemistry and ecology. While the mathematical description of such flows is complex, progress has proceeded through an interplay between experiments, mathematical modelling, and more recently large‐eddy simulations (LESs). In this contribution, LES is used to investigate the topography‐induced changes in the flow field and how these changes propagate to scalar transport within the canopy. The LES runs are conducted for a neutral atmospheric boundary layer above a tall dense forested canopy situated on a train of two‐dimensional sinusoidal hills. The foliage distribution is specified using leaf area density measurements collected in an Amazon rain forest. A series of LES runs with increasing hill amplitude are conducted to disturb the flow from its flat‐terrain state. The LES runs successfully reproduce the recirculation region and the flow separation on the lee‐side of the hill within the canopy region in agreement with prior laboratory and LES studies. Simulation results show that air parcels released inside the canopy have two preferential pathways to escape the canopy region: a “local” pathway similar to that encountered in flat terrain and an “advective” pathway near the flow‐separation region. Further analysis shows that the preferential escape location over the flow‐separation region leads to a “chimney”‐like effect that becomes amplified for air parcel releases near the forest floor. The work here demonstrates that shear‐layer turbulence is the main mechanism exporting air parcels out the canopy for both pathways. However, compared to flat terrain, the mean updraught at the flow separation induced by topography significantly shortens the in‐canopy residence time for air parcels released in the lower canopy, thus enhancing the export fraction of reactive gases.

Description: Our coupled models outperform their counterpart atmosphere‐only global numerical weather prediction model for location predictions of 2016 tropical cyclones (TCs) in the western North Pacific (WNP). The improvement is associated with an enhanced Western North Pacific Subtropical High and an anticyclonic steering flow anomaly, which are caused mainly by colder initial sea‐surface temperatures (SSTs) in the WNP. TC intensity is generally less well predicted in the coupled models, due to colder initial SSTs and also air–sea coupling in the Tropics. Abstract We examine the merit of atmosphere–ocean coupled models for tropical cyclone (TC) predictions in the western North Pacific (WNP), where accurate TC predictions remain challenging. The UK Met Office operational atmospheric global numerical weather prediction (NWP) model is compared with two trial coupled configurations, in which the operational atmospheric model is coupled to a one‐dimensional mixed‐layer ocean model and a three‐dimensional dynamical ocean model. Reforecasts for the 2016 TC season show that the coupled models outperform the NWP model for TC location predictions, with a systematic improvement of 50–100 km over the seven‐day forecasts, but the coupled models amplify the underestimation of TC intensity in the NWP model. Nearly identical TC predictions (for both location and intensity) are found in the two coupled models, indicating the dominance of thermodynamic processes at the air–sea interface for TC predictions on these timescales. The improved prediction of the TC position in the coupled models is associated with an enhanced Western North Pacific Subtropical High (WNPSH), which introduces an anticyclonic steering flow anomaly that shifts TC tracks further west in the southern part of the region and further east in the northern part. Based on sensitivity experiments, we show that these improvements in the coupled models are due mainly to colder initial sea‐surface temperatures (SSTs). Air–sea feedbacks do not change the WNPSH or TC tracks noticeably. Apart from the effect of the initial SSTs, tropical ocean warming due to air–sea interaction in the coupled forecasts can also reduce the predicted TC intensity, presumably due to a stronger regional Hadley circulation with increased subtropical subsidence.

Description: Time‐averaged (a) diabatic PV tendency, (b) horizontal advection, (c) vertical advection, and (d) total PV tendency (all PVU/day) during the first 2 days of the axisymmetric 2D model in response to a combination of stratiform and deep convective heating. Diabatic PV tendency includes both stretching‐like and tilting‐like tendencies, with the tilting‐like tendency radially restricted to 10 km near the heating boundary. Negative PV tendencies are enclosed by a thin dotted line. Lowest panels depict the (e) PV (PVU) and (f) absolute vorticity (10−5s−1) on day 2. The vertical dotted line in all panels indicates the radius (r = 250 km) within which the heating is applied. Potential vorticity (PV) has been used to understand the intensification and motion of a variety of tropical vortices. Here, atmospheric reanalyses and idealized models are used to understand how the vertical structures of moist convective heating and adiabatic advection jointly shape the vertical structures of PV in tropical depressions. Observationally based estimates reveal a top‐heavy PV structure in tropical depressions, contrasting with bottom‐heavy structures of absolute vorticity and diabatic PV generation. These distinct vertical structures are reproduced in an axisymmetric model which employs the weak temperature gradient approximation for conceptual simplicity and is forced by stratiform and deep convective heating. When applied in isolation, the stratiform and deep convective heatings produce PV maxima at 500 hPa and near the surface, respectively. When these two heatings are applied simultaneously, interactions between the stratiform and deep convective modes enhance the adiabatic advective tendencies produced by the transverse circulation, making the PV distribution more top‐heavy. In the lower and middle troposphere, radial advection also greatly reduces the radius of the PV structure relative to that of the imposed heating, consistent with structures in observed tropical depressions; the implications of these differences in radial structures for using the flux form of the relevant conservation equations (e.g. for PV substance or absolute vorticity) are discussed.

Description: Cloud water path, cloud‐top altitude, and normalized nadir visible radiance for the boundary layer (504 × 504 = 254,016 columns with Δx = 0.04 km) and convective cloud (1,536 × 1,536 = 2,359,296 columns with Δx = 0.1 km) domains. For the visible images, the Sun had a zenith angle of 60° and azimuth of 45° counter‐clockwise from the top. Abstract This study presents a method for calculating all‐sky, broadband radiative fluxes for high‐resolution atmospheric model domains, such as limited‐area numerical weather prediction models and Cloud System‐Resolving Models, that have many columns (i.e. ≫ 105). The aim is to replace the Independent Column Approximation (ICA) which applies 1D radiative transfer models (RTMs) to all columns; but usually after skipping many dynamical time steps due to the ICA's computational burden. The proposed method, referred to as Partitioned Gauss–Legendre Quadrature (PGLQ), begins by partitioning a domain into sub‐domains. This study is concerned just with cloudy columns so partitioning was done according to cloud‐top altitudes. Then, for each sub‐domain, sort columns from smallest to largest cloud water path , assign each column a sequence number, identify nG GLQ points, and apply 1D RTMs to the nG columns with resulting flux profiles assigned to nearby columns in the sorted sequence. Last, reposition columns back to the full domain. Marine boundary‐layer and deep convective cloud fields were used to assess PGLQ. For ratios of RTM calculations between ICA and PGLQ of fRT = /(nG · )−1 ∈ [103, 104], MBEs and RMSEs for net short‐wave (SW) surface fluxes scale close to (nG · )−1, RMSEs being typically ∼200× smaller than domain‐average . Structure function analyses of indicate that PGLQ errors resemble white noise. Long‐wave (LW) fluxes perform less well due to sorting on but seem likely to be acceptable, RMSEs being ∼5–10× smaller than domain‐average . All MBEs are typically 〈±0.5 W/m2. Estimates of radiative heating rate profiles are unbiased, and while noisy at small scales, they are portrayed well at scales larger than typical individual clouds. Overall speed‐up of PGLQ relative to ICA, for serial computation as consider here, is expected to be 〉300×; likely to be comparable to parallel computation. This should allow application of RTMs at every dynamical time step.

Description: An intercomparison of seven gridded rainfall products incorporating satellite data is carried out over Central Africa, by evaluating them against observed, partly unpublished datasets. All the products fairly well reproduce the mean rainfall regimes, but their skills differ for the interannual variability of monthly rainfall and for daily rainfall occurrence and amounts. Abstract An intercomparison of seven gridded rainfall products incorporating satellite data (ARC, CHIRPS, CMORPH, PERSIANN, TAPEER, TARCAT, TMPA) is carried out over Central Africa, by evaluating them against three observed datasets: (a) the WaTFor database, consisting of 293 (monthly records) and 154 (daily records) rain‐gauge stations collected from global datasets, national meteorological services and monitoring projects, (b) the WorldClim v2 gridded database, and (c) a set of stations expanded from the FAOCLIM network, these two latter sets describing climate normals. All products fairly well reproduce the mean rainfall regimes and the spatial patterns of mean annual rainfall, although with some discrepancies in the east–west gradient. A systematic positive bias is found in the CMORPH product. Despite its lower spatial resolution, TAPEER shows reasonable skills. When considering daily rainfall amounts, TMPA shows best skills, followed by CMORPH, but over the central part of the Democratic Republic of the Congo, TARCAT is amongst the best products. Skills ranking is however different at the interannual time‐scale, with CHIRPS and TMPA performing best, though PERSIANN has comparable skills when only fully independent stations are used as reference. A preliminary study of Southern Hemisphere dry season variability, from the example of Kinshasa, shows that it is a difficult variable to capture with satellite‐based rainfall products. Users should still be careful when using any product in the most data‐sparse regions, especially for trend assessment.

Description: Spectral decompositons are employed to test nonorographic gravity wave drag parameterizations for consistency with the saturated universal vertical wavenumber spectrum. The figure shows eastward and westward components of spectral density of ⟨u′w′⟩ versus vertical wave number m at various altitudes for the Warner–McIntyre–Scinocca scheme with CIRA winter and summer zonal wind profiles. A column model based on CIRA wind and temperature profiles is employed to assess the characteristics of the Hines Doppler‐spread and Warner–McIntyre–Scinocca (WMS) non‐orographic drag parametrizations for internal gravity waves. The “Alexander–Dunkerton” variant of the WMS scheme is also briefly considered. This study goes into more detail than previous comparisons by performing a spectral analysis of the momentum deposition and drag, and by examining the ability of each scheme to reproduce a high vertical wavenumber tail consistent with atmospheric measurements. We find several undesirable characteristics in the drag produced by the Hines scheme. For typical midlatitude wind profiles, it produces an abrupt onset of large accelerations that vary strongly from one layer to the next. It is also unable to reproduce spectra consistent with observed wave saturation at high vertical vertical wavenumbers, even for the windless case. The WMS scheme has the ability to reproduce the observed spectral tail for the case of no background wind. In the presence of typical CIRA midlatitude background winds, however, it is demonstrated that the WMS saturation threshold generally does not follow the observed spectral behaviour at high vertical wavenumbers, except for one specific frequency dependence not considered in previous work. Doppler shifting is also found to interfere with the production of high‐wavenumber spectral tails consistent with observations.

Description: Sensitivity tests reveal that the simulations are especially sensitive to ice nuclei concentration when the Fletcher (1962) scheme is used. The Shen et al. (2014) scheme works better than the Zeng scheme by changing the radius of base ice crystal from 0 to 40 µm. When DeMott et al. (2010) ice crystal concentration parametrization is applied to the Zeng scheme, the anomalous rainfall is also significantly weakened. Time series of (a) hourly rainfall for the observation (black solid line), model runs (colour lines) from 1200 UTC 6 October 2013 on the 3 km mesh on land in the innermost domain of WRF modelling, (b) the best track (black line with blue dots) issued by Chinese Central Weather Bureau (CCWB) and the three‐hourly simulated tracks (colour lines with coloured dots), (c) time series of minimum sea‐level pressure (SLP; hPa) of the best‐track analysis (black solid line) and the simulated cyclone (colour lines). (a,b) use the same legend labels as (c). ⋆ denotes locations of the cities in Zhejiang and Fujian seriously affected by typhoon Fitow (2013). Abstract Uncertainty in the ice nucleation process is still large, yet the torrential precipitation associated with the landfall of a typhoon is strongly contributed by stratiform rainfall, mainly composed of ice‐phase clouds. This study investigates the ice growth parametrization proposed by Zeng et al. under the assumption that the ice crystal concentration is proportional to the mass of ice crystal. Sensitivity tests reveal that simulations with the Zeng scheme are especially sensitive to ice nuclei concentration. Zeng schemes with high ice crystal concentration simulate anomalously large rain rate, weak intensities and abnormal tracks, and this is directly related to the over‐production of ice nuclei concentration propagating into cloud ice and other microphysical fields mainly through the depositional growth of cloud ice from cloud water (PIDW). The Shen et al. scheme works better than the Zeng scheme by changing the radius of base ice crystal from 0 to 40 µm and reducing sensitivity of PIDW to the ice nuclei concentration. But Shen schemes simulate the number of ice crystals as anomalously large as those in Zeng schemes, since both Zeng and Shen schemes use the Fletcher equation in the parametrization of ice crystal concentration. When DeMott et al. ice crystal concentration parametrization is applied to the Zeng scheme, the anomalously large rainfall is significantly weakened, further indicating that Zeng schemes with high ice crystal concentration poorly simulate a typhoon, primarily due to the over‐nucleation of ice in the Fletcher parametrization.

Description: Correlation of errors between different experiments (see legend) for 500 hPa geopotential height over Europe (a) and contiguous US (b), including confidence intervals from 5th to 95th percentile based on bootstrap method for FV3gfs versus FV3ec (green shading). While the models with the same initial conditions have the highest correlations of errors over Europe, the correlations are higher for the experiment with a similar model over contiguous US. Understanding the root causes of forecast errors and occasional very poor forecasts is essential but difficult. In this paper we investigate the relative importance of initial conditions and model formulation for medium‐range errors in 500 hPa geopotential height. The question is addressed by comparing forecasts produced with ECMWF‐IFS and NCEP‐GFS forecasting systems, and with the GFDL‐fvGFS model initialized with the ECMWF and NCEP initial conditions. This gives two pairs of configurations that use the same initial conditions but different models, and one pair with the same model but different initial conditions. The first conclusion is that the initial conditions play the major role in differences between the configurations in terms of the average root‐mean‐square error for both Northern and Southern Hemispheres as well as Europe and the contiguous US (CONUS), while the model dominates the systematic errors. A similar conclusion is also found by verifying precipitation over low latitudes and the CONUS. The day‐to‐day variations of 500 hPa geopotential height scores are exemplified by one case of a forecast bust over Europe, where the error is found to be dominated by initial errors. The results are generalized by calculating correlations between errors integrated over Europe, CONUS and a region in the southeastern Pacific from the different configurations. For Europe and southeast Pacific, the correlations in the medium range are highest between the pairs that use the same initial conditions, while over CONUS they are highest for the pair with the same model. This suggests different mechanisms behind the day‐to‐day variability of the score for these regions. Over CONUS the link is made to the propagation of troughs over the Rockies, and the result suggests that the large differences in parametrizations of orographic drag between the models play a role.

Description: This paper presents an object‐based evaluation of precipitation forecasts from the French deterministic and ensemble high‐resolution models, using a novel object detection method. Object verification metrics are then used to objectively weight ensemble forecasts based on their performance at early forecast ranges. These weights are consistent with a subjective ranking and significantly improve forecast scores at very short ranges. Traditional pointwise verification scores are not always appropriate for the evaluation of high‐resolution precipitation forecasts because of double‐penalty problems. An alternative approach, based on the identification of homogeneous rainfall areas called “precipitating objects”, allows forecast evaluation at a larger and thus more predictable scale, and specific information about the nature of errors (e.g. location, size, intensity) can be obtained. A novel object detection method is first introduced and the object‐based verification of precipitation forecasts from the convective‐scale deterministic and ensemble models Arome and Arome‐EPS is then discussed, using several scores and diagnostics. Three types of precipitating objects characterizing total, moderate and heavy rainfall are considered. In the second part, object‐based metrics are used to compute objective weights for time‐lagged ensemble forecasts, based on their performance at early forecast ranges. The weights obtained clearly depend on the meteorological situation and on the precipitation type, reflecting for instance the lower predictability of moderate precipitation compared to total precipitation. There is also a dependence on the production time with, on average, slightly larger and more homogeneous weights associated with the most recent run. However, in some situations of moderate and heavy rainfall, a relevant signal can be extracted from older runs. It is finally shown that object‐based weights are better suited than classical quadratic weights to improve nowcasting performance.

Description: The Adriatic Sea and its coasts. The dots show the locations of the sea level stations. In this research we present a modelling study of storm surges influenced by pre‐existing seiches in the Adriatic Sea. The natural oscillation modes – seiches – of the Adriatic Sea can be easily excited by wind, due to the particular marine bathymetry and coastal orography. Moreover, such oscillations can interact with newer wind‐induced storm surges or with other sea‐level components. This paper first considers modelling issues in the seiche reproduction, using a simplified approach, and then analyses the reproduction with the model of two extreme storm surge events, influenced by pre‐existing seiches. In order to investigate the impact of assimilating sea level data, these events are simulated without and with a data assimilation system, based on an Ensemble Kalman Filter. Results show that, while the formulation of the bottom shear stress does not influence the seiche period, the seiche decay time is correctly estimated with a hybrid linear‐quadratic formulation. We found also that non‐linear interactions of bottom currents, caused by different sea level components, can modify the decay time. However, the two events are poorly reproduced by the model, even with a correct modelling set‐up, because of wind deficiencies. Therefore we considered the issue and found that the assimilation of tide‐gauge residual sea level has a strong positive impact, lasting several days, despite the errors in the atmospheric forcing. This is mainly due to the persistence of the seiche oscillations in the Adriatic Sea.

Description: We introduce a novel Bayesian approach based on hierarchical Kalman filters to infer stochastic parameters. The technique is proposed to be applied offline as part of an apriori optimization of the data assimilation system. Parameters that control the stochastic forcing variance and spatial covariances are successfully estimated. The identified stochastic parameters not only alleviate the analysis errors associated with unresolved processes, but they also optimize the ensemble spread. Stochastic parametrizations are increasingly used to represent the uncertainty associated with model errors in ensemble forecasting and data assimilation. One of the challenges associated with the use of these parametrizations is the characterization of the statistical properties of the stochastic processes within their formulation. In this work, a hierarchical Bayesian approach based on two nested ensemble Kalman filters is proposed for inferring parameters associated with stochastic parametrizations. The proposed technique is based on the Rao‐Blackwellization of the parameter estimation problem. It consists of an ensemble of ensemble Kalman filters, each of them using a different set of stochastic parameter values. We show the ability of the technique to infer parameters related to the covariance of stochastic representations of model error in the Lorenz‐96 dynamical system. The evaluation is conducted with stochastic twin experiments and with imperfect model experiments with unresolved physics in the forecast model. The technique performs successfully under different model error covariance structures. The technique is conceived to be applied offline as part of an apriori optimization of the data assimilation system and could, in principle, be extended to the estimation of other hyperparameters of the data assimilation system.

Description: In this study, we apply the heat budget approach to the urban boundary layer using WRF simulations and a variety of observational data. In doing so, the relative importance of surface sensible‐heat flux and transport processes in heating the urban boundary layer under heatwave conditions is quantified. The results have important implications for the application of one‐dimensional convective boundary‐layer models at the city scale. Previous studies on urban heatwaves (HWs) often focused on the surface and near‐surface thermal conditions, but the urban boundary layer under HW conditions is less studied. This study examines the urban boundary‐layer heat budget over the Phoenix metropolitan area throughout a HW event in June 2016 using the Weather Research and Forecasting (WRF) model. The simulations are first validated against a wide range of observational data, including radiosonde, aircraft, weather station, and flux tower measurements. Then the boundary‐layer heat budget integrated over the urban area is examined. It is found that the surface sensible‐heat flux plays a major role in heating the urban boundary layer during daytime. The warming rate of the urban boundary layer is modulated by the large‐scale subsidence and the horizontal advection throughout the HW period, which however largely cancel each other due to the mass balance constraint. Our findings suggest that the traditional one‐dimensional convective boundary‐layer model may be more applicable than its original derivation implies.

Description: Abstract A review of the progression of cloud physics from a subdiscipline of meteorology into the global science it is today is described. The discussion briefly touches on the important post‐war contributions of three key individuals who were instrumental in developing cloud physics into a global science. These contributions came on the heels of the post‐war weather modification efforts that influenced much of the early development of cloud physics. The review is centered on the properties of warm clouds primarily to limit the scope of the paper and the connection between the early contributions to cloud physics and the current vexing problem of aerosol effects on cloud albedo is underlined. Progress toward estimating cloud properties from space and insights on warm cloud processes are described. Measurements of selected cloud properties, such as cloud liquid water path are now mature enough that multi‐decadal time series of these properties exist and this climatology is used to compare to analogous low cloud properties taken from global climate models. The too wet (and thus too bright) and the too dreary biases of models are called out underscoring the challenges we still face in representing warm clouds in Earth system models. We also provide strategies for using observations to constrain the indirect radiative forcing of the climate system. This article is protected by copyright. All rights reserved.

Description: Recent research has shown that high resolution observations, such as Doppler radar radial winds, exhibit spatial correlations. High resolution observations are routinely assimilated into convection permitting numerical weather prediction models assuming their errors are uncorrelated. To avoid violating this assumption observation density is severely reduced. To improve the quantity of observations used and the impact that they have on the forecast requires the introduction of full, correlated, error statistics. Some operational centres have introduced satellite inter‐channel observation error correlations and obtained improved analysis’ accuracy and forecast skill scores. Here we present a strategy for implementing spatially correlated observation errors in an operational system. We then provide the first demonstration of the practical feasibility of incorporating spatially correlated Doppler radial wind error statistics in the Met Office numerical weather prediction system. Inclusion of correlated Doppler radial winds error statistics has little impact on the computation cost of the data assimilation system, even with a four‐fold increase in the number of Doppler radial winds observations assimilated. Using the correlated observation error statistics with denser observations produces increments with shorter length scales than the control. Initial forecast trials show a neutral to positive impact on forecast skill overall, notably for quantitative precipitation forecasts. There is potential to improve forecast skill by optimising the use of Doppler radial winds and applying the technique to other observation types. This article is protected by copyright. All rights reserved.

Description: We describe a variational bias correction system for satellite Sea Surface Temperature (SST) data that includes the use of ‘observations‐of‐bias’. The bias correction scheme is designed to work in the historical period, when good quality low‐bias reference data were scarce, but also take advantage of reference data when they are available. In testing with a simple Lorenz 63 model, our new scheme outperformed traditional variational bias correction. When compared to an off‐line bias correction method, the new scheme showed superior performance both when the bias was large, and when reference observations were sparse. The bias correction scheme has also been tested using a three year assimilative run (2008‐2010) of the NEMO ocean general circulation model, with reference data from the AATSR instrument withheld in 2009. In these tests the new scheme was found to be more robust to missing reference observations than an off‐line scheme. Against AATSR data, the new bias correction method had lower biases and RMS errors than an off‐line scheme, but was degraded relative to a pure variational technique. However, in comparisons with drifting buoys, the new scheme outperformed both off‐line and pure variational methods. This article is protected by copyright. All rights reserved.

Description: Abstract In this study, potential impacts of monsoon gyres (MG) on the development of tropical cyclone (TC) are investigated using idealized numerical simulations. When a TC is embedded in a MG, our results show that its development is slower, and its intensity is weaker compared with that evolved with no background flow. We identified three MG dynamic mechanisms that can affect TC development. First, the superposition of TC and MG circulations produce a larger outer size for the TC. A TC with a larger size develops slowly and has a reduced capacity to organize convection through vorticity segregation process. Deep convection is located mainly outside the radius of maximum wind (RMW), resulting in slower intensification rates. Secondly, when the TC is embedded on the eastern flank of the MG, its outer profile becomes barotropically unstable. The asymmetric perturbations exhibit an up‐shear titling, which extracts kinetic energy from the primary vortex and thus hinders TC intensification. In the third mechanism, the vertical baroclinic nature of the MG imposes an ambient vertical wind shear (VWS) on the TC, leading to a vertical tilting and increased asymmetric perturbation that hinders TC intensification. The MG relative humidity also impacts TC intensification, with negative impacts greater in drier environments. This article is protected by copyright. All rights reserved.

Description: ABSTRACT Current climate models still suffer from many biases which are partly due to excessive subgrid scale dissipation. Here we systematically develop energetically consistent stochastic energy backscatter (SEB) and deterministic energy backscatter (DEB) parameterization schemes. We implement our schemes in a simplified spectral atmospheric General Circulation Model (GCM). It is shown that the SEB scheme performs better than the DEB scheme at low horizontal resolutions (T21 and T31), whereas, the performance of both schemes becomes comparable as the resolution increases to T42 when comparing with our reference simulation at T127 resolution. The energy backscatter parameterization schemes improve eddy variability in low‐resolution models and correctly capture the dominant mode of zonal mean zonal wind variability. The autocorrelation time scale of low‐resolution models is also found to be more consistent with the reference simulation when applying the SEB and DEB parameterizations. Our schemes are scale adaptive and computationlly efficient. This article is protected by copyright. All rights reserved.

Description: The climate and circulation of a terrestrial planet are governed by, among other things, the distance to its host star, its size, rotation rate, obliquity, atmospheric composition and gravity. Here we explore the effects of the last of these, the Newtonian gravitational acceleration, on its atmosphere and climate. We first demonstrate that if the atmosphere obeys the hydrostatic primitive equations, which are a very good approximation for most terrestrial atmospheres, and if the radiative forcing is unaltered, changes in gravity have no effect at all on the circulation except for a vertical rescaling. That is to say, the effects of gravity may be completely scaled away and the circulation is unaltered. However, if the atmosphere contains a dilute condensible that is radiatively active, such as water or methane, then an increase in gravity will generally lead to a cooling of the planet because the total path length of the condensible will be reduced as gravity increases, leading to a reduction in the greenhouse effect. Furthermore, the specific humidity will decrease, leading to changes in the moist adiabatic lapse rate, in the equator‐to‐pole heat transport, and in the surface energy balance because of changes in the sensible and latent fluxes. These effects are all demonstrated both by theoretical arguments and by numerical simulations with moist and dry general circulation models.

Description: A new hierarchical “model of truth” with stochastic partial differential equations on two levels in the hierarchy is proposed. The model is linear and generates conditionally Gaussian random fields with a tunable degree of non‐stationarity in space and time. Non‐stationarity of prior covariances is found to have a crucial impact on the performance of the three covariance‐hybridization techniques in EnKF: blending ensemble covariances with static, time‐smoothed, and space‐smoothed covariances. Effects of non‐stationarity on the performance of hybrid ensemble filters is studied. (By hybrid filters we mean those which blend ensemble covariances with some other regularizing covariances.) To isolate effects of non‐stationarity from effects due to nonlinearity (and the non‐Gaussianity it causes), a new doubly stochastic advection‐diffusion‐decay model (DSADM) is proposed. The model is hierarchical: it is a linear stochastic partial differential equation whose coefficients are random fields defined through their own stochastic partial differential equations. DSADM generates conditionally Gaussian spatiotemporal random fields with a tunable degree of non‐stationarity in space and time. DSADM allows the use of the exact Kalman filter as a baseline benchmark. In numerical experiments with DSADM as the “model of truth”, the relative importance of the three kinds of covariance blending is studied: with static, time‐smoothed, and space‐smoothed covariances. It is shown that the stronger the non‐stationarity, the less useful the static covariance matrix becomes and the more beneficial the time‐smoothed covariances are. Time‐smoothing of background‐error covariances proved to be systematically more useful than their space‐smoothing. Under non‐stationarity, a filter that extends the (previously proposed by the authors) Hierarchical Bayes Ensemble Filter and accommodates the three covariance‐blending techniques is shown to outperform all other configurations of the filters tested.

Description: Abstract The city of Melbourne in southeast Australia is planning to substantially expand urban areas by the year 2050 and this expansion has the potential to alter the Urban Heat Island (UHI), i.e., higher temperatures in urban areas as compared to surrounding rural areas. Moreover, Melbourne has been experiencing more frequent heatwaves for last two decades, and the intensity and duration of heatwaves is expected to increase in the future, which could exacerbate the UHI. This study evaluates the potential impacts of future urban expansion on the urban meteorology in southeast Australia during four of the most severe heatwave events during the period of 2000 to 2009. Urban expansion is implemented as high density urban with a high urban fraction of 0.9 to investigate the maximum possible impacts. Simulations are carried out using the Weather Research and Forecasting model coupled with the Single Layer Urban Canopy Model with current land use and future urban expansion scenarios. Urban expansion increases the near‐surface (2 m) UHI (UHI2) by 0.75 to 2.80 °C and the skin‐surface UHI (UHIsk) by 1.9 to 5.4 °C over the expanded urban areas during the night, with no changes in existing urban areas. No substantial changes in the UHI2 and UHIsk occur during the day over both existing and expanded urban areas. This is largely driven by changes in the storage heat flux, with an increase in storage heat at night, and a decrease during the day, i.e., excess storage heat accumulated during the day is released at night, which causes slower decrease of near surface temperature and increase in the UHI. Urban expansion did not affect human thermal comfort (HTC) in existing urban areas and there were no marked differences in HTC between existing and expanded urban areas. This article is protected by copyright. All rights reserved.

Description: ABSTRACT This study investigates whether and how three synoptic‐scale teleconnections, i.e. East Asia/Pacific teleconnection (EAP), Silk‐road teleconnection (SR) and Eurasia teleconnection (EU), induce persistent precipitation extremes (PPEs) in the Yangtze River Valley. Results show that only the EAP teleconnection has the potential of independently incurring PPEs; while the other two teleconnections’ influences on PPEs need to be exerted via their liaison with the EAP pattern. Cases are accordingly grouped into two subsets, i.e. single EAP‐PPEs and three teleconnection‐PPEs. In both groups, the EAP teleconnection evolves following a similar pathway that poleward energy dispersion dynamically links a westward‐extended subtropical high, a deepened mid‐latitude trough and the Okhotsk blocking. EAP‐induced circulation anomalies enhance low‐level convergences and upper‐level divergences, convey exceptionally abundant moisture, and hence give birth to a quasi‐stationary front leading to PPEs in the Yangtze River Valley. Despite similarities, PPEs last noticeably longer in the three‐teleconnection context. The EU teleconnection‐related downstream energy dispersion at higher latitudes favors both earlier establishment and latter decay of the Okhotsk blocking. Constructed meridional flows at mid‐high latitudes continuously steer migratory disturbances southeastward into the EAP trough. The SR plays a bigger role during the latter half of PPE lifespan (3 days after onset) via stimulating new cyclonic disturbances that largely overlap with the EAP trough spatially. Moreover, EU‐ and SR‐excited disturbances could effectively extract baroclinic and barotropic energies from local mean flows to replenish components of the EAP tripole pattern, which therefore survives longer and prolongs PPEs by several days. This article is protected by copyright. All rights reserved.

Description: The assimilation of SMOS brightness temperature ( TB) data in Numerical Weather Prediction Systems influences the state of the soil, which in turn affects the exchange of energy and water fluxes between the soil and the near surface atmosphere, with potential implications in the prediction of atmospheric variables. In this paper, the impact of assimilating SMOS TB alone or in combination with screen level observations and ASCAT soil moisture retrievals is assessed. Independent quality controlled in situ soil moisture observations belonging to several networks, included in the International Soil Moisture Network, were used to validate the quality of both the new soil moisture analyses and the skill to predict soil moisture up to 5 days ahead. The impact on atmospheric variables is indirect and it was evaluated through computation of the forecast skill at different lead times. The analysis period was selected to be around the boreal summer, a period of the year when evaporatranspiration fluxes are stronger, and when it is therefore expected that the assimilation of remote sensing data provides the largest impact on the state of the soil. The results show that the soil moisture state benefits from the direct assimilation of SMOS TB, especially in better representing the temporal variations of soil moisture. The skill on atmospheric variables is mainly driven by the screen level observations. Despite the clear benefits to the soil state, remote sensing data needs to be used with screen level variables to add value to the state of the atmosphere, pointing to inconsistencies in the physical coupling between the land and near‐surface components of the ECMWF Earth system.

Description: The modification of potential vorticity (PV) by diabatic processes is assessed in an IFS simulation of a North Pacific cyclone. Hourly temperature and momentum tendencies from every parametrized physical process are used to compute their respective PV rates and subsequently traced in a Lagrangian framework. Condensation, sublimation and melting of snow, long‐wave radiative cooling, convection, and turbulence all significantly affect PV along the fronts and in the cyclone centre. Turbulent mixing of temperature generally mitigates the influence exerted by the other processes. Processes that do not conserve potential vorticity (PV) have a profound impact on the intensity, evolution, and mesoscale details of extratropical weather systems. This study aims at quantifying and improving the understanding of how and when physical processes modify PV in cyclones. To this end, a 6‐day forecast of a North Pacific cyclone is performed using a recent operational version of the ECMWF global numerical weather prediction model. Hourly instantaneous temperature and momentum tendencies of each parametrized process are used to compute the corresponding PV tendencies. By integrating these diabatic PV rates along backward trajectories, the relative contribution of individual processes for the PV budget can be assessed. The cold front is characterized by an elongated filament of increased PV, generated by latent heating due to condensation at the front as well as long‐wave radiative cooling at the surface. Turbulent mixing at the interface of the boundary layer decreases PV behind the cold front during the early stage of the cyclone, while sublimation of snow produces negative PV in the mature phase. A broad region of enhanced PV is found along the warm front, generated by condensation and turbulence at the front as well as long‐wave radiative cooling at the surface. The region of decreased PV north of the warm front is mainly modified by snow melting and sublimation. Finally, high values of PV along the bent‐back front and the cyclone centre are generated by condensation, convection, snow melting and sublimation. In general, turbulent mixing offsets intense PV modification induced by the other processes. This study highlights the relevance of condensation, melting and sublimation of snow, long‐wave radiative cooling, turbulence, and convection for the production of low‐level PV anomalies and underlines the importance of correctly representing these processes in weather prediction models.

Description: The three‐dimensional structure of the Red Sea Low‐Level Jet (RSLLJ) that occurs over the southern Red Sea during the winter season is examined using the WRF model. The jet is a terrain‐induced phenomenon that initiates as a gap flow as the jet forms north of the strait of Bab el‐Mandab as a result of a hydraulic effect. The jet intensity is enhanced by the presence of the Red Sea trough over the southern Red Sea. Abstract The three‐dimensional structure of the Red Sea Low‐Level Jet (RSLLJ) that occurs over the southern Red Sea during the winter season is examined using the Weather Research and Forecasting (WRF) model. The causes of the jet formation are investigated, and the simulations confirm that the jet is a terrain‐induced phenomenon. It initiates as a gap flow as the jet forms north of the strait of Bab el‐Mandab as a result of a hydraulic effect when a stably stratified cool layer is channeled through the mountainous gap at low levels over the strait into the southern Red Sea. The jet intensity is enhanced by the Red Sea trough over the southern Red Sea. The cool dense air mass south of the strait is supported by a generally persistent synoptic regime that includes a high‐pressure system over most of the eastern Arabian Peninsula and the Sudan low‐pressure system over Sudan. The jet extends more than 500 km northward to slightly north of 17°N from the strait around 12.5°N, with a width of 100–150 km over the southern Red Sea. The jet core lies at 500–600 m above sea level between 14°N and 16°N, with wind speeds exceeding 25 m/s. The north–south pressure gradient along the strait, as well as the depth of the stratified cool layer south of the strait, are directly responsible for the jet length and speed over the southern Red Sea, with stronger pressure gradients and deeper cool layers resulting in longer and stronger jets. Sea/land breezes also directly influence the spatial scale, intensity, and diurnal variation of the jet, with land breezes causing faster and more constricted jets at night and divergence due to sea breezes leading to relatively slower jets during the daytime.

Description: Abstract Most global models employ a vertical coordinate based on the moist hydrostatic pressure, and therefore do not conserve dry air mass. Such issue should be seriously taken into account, especially in developing global high‐resolution atmospheric model to explicitly address non‐hydrostatic motions. In this paper, we present a modified non‐hydrostatic moist global spectral dynamical core using a dry‐mass vertical coordinate, which conserves the mass of dry air naturally. In addition to the vertical coordinate, the modified dynamical core differs from the original Aladin‐NH like dynamical kernel in the employed state variables. Specifically, a new temperature variable is introduced to formulate the governing equations and the mass continuity equation is expressed in terms of the dry air density. To assess its performance, an idealized splitting supercell test is conducted. Simulation results from both the modified and original dynamical cores are presented and compared. The results indicate that only the modified dynamic core is able to simulate the splitting supercell with good accuracy comparable to reference solutions from the Model for Prediction Across Scales (MPAS).

Description: The vorticity field at time t = 1,000, resulting from a numerical integration of the reference model. (a) shows the field for the whole flow domain and (b) the upper right quadrant, both at the full resolution N = 170. (c) shows the same field as (b) but truncated to R = 85. A numerical model of a fluid dynamical system is necessarily limited to scales of motion down to a given minimum length‐scale. All interactions with smaller scales have to be dealt with in some approximate way by methods that are called parametrizations. The problem can be studied particularly well in a system that is geophysically relevant yet relatively simple and therefore amenable to very high‐resolution modelling: two‐dimensional turbulence. By using spectral models with high and low spatial resolutions and concentrating on the energy and enstrophy and their time‐derivatives as functions of wavenumber, we study the process of interaction between small and large scales. We show that this interaction can be modelled quite well by representing the small scales by means of a probability density function that is based on the principle of maximum entropy and on constraints on their energy and enstrophy as well as their time‐derivatives. Two sets of constraints are discussed and assessed in terms of the performance of their corresponding parametrizations.

Description: Abstract Seasonal forecasts of winter North Atlantic atmospheric variability have until recently shown little skill. Here we present a new technique for developing both linear and non‐linear statistical forecasts of the winter North Atlantic Oscillation (NAO) based on complex systems modelling, which has been widely used in a range of fields, but generally not in climate research. Our polynomial NARMAX models demonstrate considerable skill in out‐of‐sample forecasts and their performance is superior to that of linear models, albeit with small sample sizes. Predictors can be readily identified and this has the potential to inform the next generation of dynamical models and models allow for the incorporation of non‐linearities in interactions between predictors and atmospheric variability. In general there is more skill in forecasts developed over a shorter training period from 1980 compared with an equivalent forecast using training data from 1956. This latter point may relate to decreased inherent predictability in the period 1955‐1980, a wider range of available predictors since 1980 and/or reduced data quality in the earlier period and is consistent with previously identified decadal variability of the NAO. A number of predictors such as sea‐level pressure over the Barents Sea, and a clear tropical signal are commonly selected by both linear and polynomial NARMAX models. Tropical signals are modulated by higher latitude boundary conditions. Both approaches can be extended to developing probabilistic forecasts and to other seasons and indices of atmospheric variability such as the East Atlantic pattern and jet stream metrics. This article is protected by copyright. All rights reserved.

Description: We present an analytical model reproducing literature‐based numerical simulations of the Marine Atmospheric Boundary Layer (MABL) over a SST front, with wind blowing from the cold to the warm side. Turbulence is parameterised through a varying diffusion coefficient with two critical features: it is parabolic on the vertical and its mean value is decoupled from the MABL height (unlike an Ekman layer model). These two novel features are found essential to recover the internal structure of the MABL from numerical simulations. Different dynamical regimes are obtained and interpreted in terms of non‐dimensional numbers characterising the relative importance of terms driving the momentum equation. A closed‐form expression of the vertically integrated wind divergence in the MABL is then obtained. The resulting divergence is linearly linked to the SST Laplacian and to the downwind SST gradient. This shows that the response of the MABL wind divergence to a SST front is highly dependent on its spatial scale. The coupling coefficients vary with the ratio of MABL height to turbulence strength, i.e. the inverse Ekman number. We further show different regimes in the rate of variation of the coupling coefficients, depending on the Ekman number value. This can result in qualitatively different vertical winds, having potential implications for the coupling of the MABL with the free troposphere.