ALBERT

Description: Three storm-scale ensemble prediction models (AROME-EPS, COSMO-DE-EPS, and MOGREPS-UK) were combined over Western Europe to create two convection-allowing, multi-model ensemble prediction systems (EPSs) with the goal of improving ensemble spread-skill relationships and probabilistic forecasting ability. A shared, uniform grid of _2.2 km resolution was used, containing two regions where model domains overlapped (AROME-EPS/COSMODE-EPS and AROME-EPS/MOGREPS-UK).Verification was conducted over a fivemonth period spanning two years using near-surface observations of wind, temperature, relative humidity and precipitation. Bias correction was also applied to each model in order to remove systematic error and to better assess the added value of the multi-model ensembles. Analyses of EPS errors and ensemble scores are presented, including comparisons between individual EPS and multi-model scores. Verification results show that the multi-model ensembles exhibit generally lower RMSE, increased spread, and improved ROC, ROCA, and Brier scores than the individual EPSs. In addition, a case study was selected to highlight discrepancies in precipitation frequency bias between the AROME-EPS and COSMODE-EPS models, attributable to differences in distribution and intensity of precipitation. When combined as a multi-model EPS, the sixhour precipitation accumulation forecast for this case study matched the observations better than the individual EPS forecasts. Together, these results highlight the potential advantage of using multiple models with differing dynamics and physics parameterizations when developing a convection-allowing EPS.

Description: The impact of assimilating lower–tropospheric lidar temperature profiles into a numerical weather prediction (NWP) model was investigated. The profiles were measured with the Temperature Rotational Raman Lidar (TRRL) of the University of Hohenheim on 24 April 2013. The day showed the development of a typical daytime planetary boundary layer (PBL) with no optically thick clouds. The Weather Research and Forecasting model was operated with 57 vertical levels covering Central Europe with 3 km horizontal resolution. Three different experiments were carried out with a rapid update cycle with hourly three–dimensional variational data assimilation. The impact run (ALL_DA) was performed with the assimilation of conventional data and the additional assimilation of TRRL profiles between 0900 and 1800 UTC in a height range from about 500 m to 3000 m above ground level with a vertical resolution of about 100 m. In CONV_DA and NO_DA, only conventional data and no data were assimilated, respectively. To consider the representativeness of the TRRL profiles, an observation error of 0.7 K was used for all heights. The assimilation was performed using the radiosonde operator. The TRRL data assimilation corrected the temperature profiles towards the lidar data. In the mean, the boundary layer height was improved by 60 m in ALL_DA compared to the TRRL data and the temperature gradient in the entrainment layer by 0.19 K (100 m) − 1 . While ALL_DA showed a root mean square error (RMSE) of 0.6 K compared to the TRRL data, the RMSE of CONV_DA was twice as large. Compared to data from radiosondes launched at the TRRL site, ALL_DA showed a significantly smaller RMSE than CONV_DA in two out of four times radiosonde data were available. We conclude that the assimilation of TRRL data has great potential to close the critical gap of missing temperature observations in the lower troposphere.

Description: Oceanic and atmospheric global numerical models represent explicitly the large-scale dynamics while the smaller-scale processes are not resolved so that their effects in the large-scale dynamics are included through subgrid-scale parameterizations. These parameterizations represent small-scale effects as a function of the resolved variables. In this work, data assimilation principles are used not only to estimate the parameters of subgrid-scale parameterizations but also to uncover the functional dependencies of subgrid-scale processes as a function of large-scale variables. Two data assimilation methods based on the ensemble transform Kalman filter (ETKF) are evaluated in the two-scale Lorenz ’96 system scenario. The first method is an online estimation that uses the ETKF with an augmented space state composed of the model large-scale variables and a set of unknown global parameters from the parameterization. The second method is an offline estimation that uses the ETKF to estimate an augmented space state composed of the large-scale variables and by a space dependent model error term. Then a polynomial regression is used to fit the estimated model error as a function of the large-scale model variables in order to develop a parameterization of small-scale dynamics. The online estimation shows a good performance when the parameter-state relationship is assumed to be quadratic polynomial function. The offline estimation captures better some of the highly nonlinear functional dependencies found in the subgrid-scale processes. The nonlinear and nonlocal dependence found in an experiment with shear-generated small-scale dynamics is also recovered by the offline estimation method. Therefore, the combination of these two methods could be a useful tool for the estimation of the functional form of subgrid-scale parameterizations.

Description: Large-eddy simulation of a nocturnal stratocumulus-topped boundary layer in a continental mid-latitude environment has been performed to examine the sensitivity of the cloud to a number of different environmental parameters. The simulations showed that the stratocumulus cloud was strongly affected by the presence of an overlying free tropospheric cirrus cloud (FTC), in agreement with previous studies of marine nighttime stratocumulus. When introducing an FTC with an optical thickness of 2, stratocumulus liquid water path decreased by 30%. Enhancing the optical thickness of the FTC to 8 further decreased the liquid water path by almost 10%. The presence of an FTC decreased the cloud-top radiative cooling which decreased the turbulent mixing in the boundary layer so that the liquid water content and cloud depth were reduced. The sensitivity of the stratocumulus cloud to an overlying FTC was found to be affected by the moisture content in the free troposphere. When a clear positive or negative moisture gradient above the inversion was imposed, and an overlying FTC with an optical thickness of 8 was introduced, the stratocumulus cloud LWP decreased by more than 40%. Furthermore, the effect of changes in free tropospheric moisture content and an overlying FTC on the stratocumulus cloud properties was found to be non-linear, the combined response was in general weaker than the two responses added together. The modeled response to changes in cloud condensation nuclei (CCN) concentrations was found to be non-significant, unless the CCN concentrations were so low that drizzle was induced (r 50 cm −3 ).

Description: ABSTRACT The specification of model parameters in numerical weather prediction (NWP) models has great influence on model performance. However, how to specify model parameters properly is not a trivial task because a typical NWP model like the Weather Research and Forecasting (WRF) model contains many model parameters and many model outputs. This paper presents the results of an investigation into the sensitivities of different WRF model outputs to the specification of its model parameters. Using a global sensitivity analysis method, the sensitivities are evaluated for surface meteorological variables such as precipitation, surface air temperature, humidity and wind speed, as well as for atmospheric variables such as total precipitable water, cloud cover, boundary layer height and outgoing longwave radiation at the top of the atmosphere, all simulated by the WRF model using different model parameters. The goal of this study is to identify the parameters that exert most influence on the skill of short-range meteorological forecasts. The study was performed over the Greater Beijing Region of China. A total of 23 adjustable parameters from seven different physical parameterization schemes were considered. The results indicate that parameter sensitivities vary with different model outputs. However, some of the 23 model parameters considered are shown to be sensitive to all model outputs evaluated, while other parameters may be sensitive to a particular output. The sensitivity results from this research are a basis for further optimizations of the WRF model parameters.

Description: Using a Mesosphere Stratosphere Troposph e re (MST) radar operating at 53 MHz, the present work reports that during the development of conve c tion over the Indian tropical station of Gadanki at 16:38 – 17:30 UT (Universal time) on 06 June 2011, all these three source mechanisms played important roles in determining the characteristics of high-frequency internal-atmospheric gravity waves generated: (1) latent heating of convection, (2) mechanical oscillation of up and down drafts associated with convection and (3) obstacle effect of wind flows over convective towers near the tropopause level. In general, it is found that while the depth of latent heating determines the vertical wavelength of gravity waves, the oscillation frequency of up and down drafts determines the observed frequency of waves. From the study of vertical structure of vertical wind velocities and phases of three waves (~13, ~17 and ~26 min) generated during this event, it is observed that while the ~13 min and ~26 min oscillations are associated with mechanical oscillator mechanism, the ~17 min oscillation is associated with obstacle effect. Analyses (Fourier, Morlet-wavelet transforms and Maximum Entropy Method (MEM)) of all the three components of wind velocities, measured by the MST radar, show that there is a clear association of gravity waves generated with convection and the vertical propagation characteristics of the gravity waves are found to be in good agreement with theoretical expectations. With the back ground atmospheric information obtained by using the data of GPS radiosondes, ERA-Interim and NCEP-NCAR reanalyses, high-resolution WRF model simulations support the present observations that in turn will help in a large way to the progress of parameterization of convection-generated high-frequency gravity waves in general circulation models. The present work also finds that water molecules induced distinct atmospheric polarized-refractive-index-structures are in existence, which is in accordance with an earlier report on this subject.

Description: Aspects of a large-eddy simulation (LES) model performance are investigated in simulations of a moderately stable boundary layer. The LES utilizes the constant-coefficient Smagorinsky–Lilly subgrid-scale (SGS) closure. Three model parameters are considered: grid spacing, SGS model constant andorder of accuracy (resolving power) of the advection discretization. Second-, fourth- and sixth-orderfully-conservative non-dissipative advection schemes are examined. All three model parameters consid-ered significantly affect the LES results. Depending on the value of the model constant, two main errorproducing mechanisms are identified. For high values of the model constant spurious turbulence collapse,either during the short period of model spin-up, or for the entire simulation duration, is observed. Even though this spurious model characteristic was previously documented, and perhaps expected for low resolution simulations, it depends on the order of the advection discretization implying a significant dis- cretization and SGS closure interaction. For low values of the model constant, numerical discretization errors dominate, leading to accumulation of energy at small scales and over-prediction of the magni- tude of the surface heat flux. Differences in potential temperature profiles are well correlated with the surface heat flux. Overall, the fourth- and sixth-order schemes perform significantly better than the second-order scheme. The differences between the fourth- and sixth-order schemes are relatively small and the increased computational expense of the sixth-order scheme may not be effective in most ap-plications, at least for the low-order statistics considered in this study. Even though the results of the Smagorinsky–Lilly closure show persistent dependence on all model parameters examined, for several pa- rameter combinations the differences with respect to a reference simulation are small. Thus, in contrast to the conclusions of previous studies, the closure can accurately capture moderately stable flows.

Description: New techniques have recently been developed to quantify the location-dependent spatial agreement between ensemble members, and the spatial spread-skill relationship. In this paper a summer of convection permitting ensemble forecasts are analysed to better understand the factors influencing location-dependent spatial agreement of precipitation fields and the spatial spread-skill relationship over the UK. The aim is to further investigate the agreement scale method, and to highlight the information that could be extracted for a more long-term routine model evaluation. Overall, for summer 2013, the UK 2.2 km-resolution ensemble system was found to be reasonably well spread spatially, although there was a tendency for the ensemble to be over confident in the location of precipitation. For the forecast lead times considered (up to 36 hrs) a diurnal cycle was seen in the spatial agreement and in the spatial spread- skill relationship: the forecast spread and error did not increase noticeably with forecast lead time. Both the spatial agreement, and the spatial spread-skill, were dependent on the fractional coverage and average intensity of precipitation. A poor spread-skill relationship was associated with a low fractional coverage of rain and low average rain rates. The times with a smaller fractional coverage, or lower intensity, of precipitation were found to have lower spatial agreement. The spatial agreement was found to be location dependant, with higher confidence in the location of precipitation to the northwest of the UK.

Description: ABSTRACT Using the inner field spectral nudging (IFSN) and lateral boundary filtering (LBF) methods in the regional Weather Research and Forecasting model, version 3.4 (WRF3.4), the simulation of persistent severe rainfall (PSR) events over South China was investigated. The results showed: (1) Simulation using IFSN improves the forecasting of precipitation, especially for a lead time of greater than 3 days. Meanwhile, simulation with a combination of the IFSN and LBF (IFSN+LBF) methods provides better forecasts at a lead time of 5–7 days. (2) The improvement in precipitation forecasting derives mainly from the reasonable simulation of the large-scale circulation fields and water vapour flux convergence patterns. (3) In terms of large-scale circulation, the anomaly correlation coefficient can be significantly improved for lead times of 1–5 days (7–11 days) by adopting the IFSN (IFSN+LBF) method.

Description: Cyclones impacting the densely populated Mediterranean region have been a continuous research focus, mainly for investigating either the associated heavy precipitation or the damaging wind gusts. In this study we examine five Mediterranean cyclones with combined large-scale impact of strong 10-m gusts and heavy precipitation. The selected events occurred in (i) December 2003 in the north-eastern Mediterranean; (ii) October 2007 in the central Mediterranean; (iii) January 2009, known as storm ‘Klaus’, in the western Mediterranean; (iv) December 2010 in the eastern Mediterranean; and (v) October 2011 in the central-northern Mediterranean. European Centre for Medium-range Weather Forecasts (ECMWF) reanalyses and 7-km resolution regional model simulations (COSMO) are analysed for each event. A Lagrangian viewpoint is employed to focus on interacting mechanisms that contribute to the joint impact on different spatial and temporal scales. In all cases, widespread strong wind gusts occur in the southwestern parts of the cyclone, while the precipitation field has localized peaks, with variable distribution in the central, southern, eastern and northern parts of the cyclone. Convective precipitation, significant in the cases in 2007, 2010 and 2011, is limited to the southern areas. In all cases, non-convective precipitation is associated with ascent in a warm conveyor belt. Intense gusts are found within unstable air, below a low tropopause in a region with strong vertical wind shear, favouring downward momentum flux by turbulent mixing. Strongly descending dry intrusions are located coherently to the south and west of strong gusts. Much variability exists with regard to the emergence of convection, where strong winds and convective precipitation co-occur: In the 2007 case, the dry intrusion is central in producing shallow convection in the cold frontal region. In the 2010 and 2011 cases, convective activity at high topography and in coastal regions leads to co-location of both types of impact.

Description: Fengyun-2 (FY-2) is the first generation geostationary meteorological satellites (GEOMS) in China, two of which, FY-2E and FY-2G, are operating workforce to support the dual-satellite observation mode. The onboard blackbody preliminarily used for stability monitoring of instrument's radiometric response was equipped behind its fore-optics due to the technical limitations in 1990s, which was referred as internal-blackbody (IBB) and operated on the unique preset temperature. In this article, a novel IBB calibration (IBBC) approach is proposed and the radiometric contributions of the main optical elements as well as the IBB itself are modeled and extracted. This is the most distinguishing difference from the traditional full-path blackbody calibration. The IBBC software has been in service for more than two years. Compared with high spectral resolution reference instruments recommended by Global Space-based Inter-Calibration System, the monthly mean calibration biases of long wave infrared band (IR1:10.8 µm) and water vapor band (IR3:6.95 µm) are shown to be lower than 1.5K for FY-2E and lower than 1.0K for FY-2G (for of order 90% of cases). Particularly, the annual mean biases for FY-2G IR3 band can maintain in a good state of 0.4K, which is nearly identical to that of other high class in-orbit satellites, i.e. MTSAT-2 and MSG-1. Likely impacted by ice contamination, the calibration biases of long wave split-window infrared band (IR2:12 µm) of FY-2G could be as large as -2.3K and similar phenomena once occurred in GOES-13 and MSG-1. It is expected to improve this in the near future. In general, the calibration accuracies of FY-2G IR1 and IR3 bands have reached to be lower than 1K, which is significantly superior to those of other 1 st GEOMS, such as GMS-5, Meteosat-7 and GOES-4/7, whose calibration accuracies are around 2-3K. The quality of the data could be greatly enhanced so as to benefit the global observation system.

Description: The drag due to resolved and sub-grid flow blocking and gravity waves is examined in simulations of flow over mountainous islands at ‘grey zone’ resolutions, in which the processes are neither well resolved or fully sub-grid-scale. Simulations of flows over narrow (South Georgia) and broad (New Zealand) mountain barriers are used to determine the behaviour of resolved and parametrized pressure drag and gravity-wave momentum fluxes as a function of horizontal grid length. Analysis of the pressure drag in spectral space suggests that contributions from wavelengths shorter than 8–10 grid lengths are poorly resolved and that parametrization schemes should compensate for the missing drag from processes on these scales, rather than strictly the sub-grid scales. The model performance for South Georgia is such that the total (resolved plus parametrized) pressure drag is approximately invariant across a range of resolutions. The parametrized (resolved) drag increases (decreases) as the grid length increases and the resolved and parametrized drag become comparable when the characteristic island wavelength is approximately 8 times the grid length. This is not true when the same tuning of the drag parametrization is applied to New Zealand, in which case the parametrized drag increases too rapidly with increasing grid length. However, satisfactory results are obtained when the scheme is re-tuned and it is shown that the optimal tuning for the two islands is consistent with scaling the sub-grid orographic heights to be representative of the orography on longer length scales, of up to 10 grid lengths. The results suggest that drag schemes require orographic information on these longer scales, rather than only the sub-grid scale. At the coarsest resolutions, where the island wavelength and grid length are similar, the parametrized drag is too small because the grid boxes are increasingly treated as sea points. Possible solutions to this problem are discussed.

Description: This paper presents results of a novel methodology capable of simultaneously retrieving optical and microphysical properties of multi-level ice and liquid clouds. The method was introduced in Part I, which theoretically demonstrated its capabilities, and its results are here analysed and evaluated against A-Train operational products. In addition to being robust to multi-layer conditions, another advantage of the method is that rigorous uncertainties and analysis tools are attached to its retrievals. Also, the combined use of shortwave and thermal infrared channels provides a wide range of sensitivity from moderately thin to thick ice cloud layers. Finally, the method is also novel in that the ice water path (IWP) is directly retrieved. These new retrievals should therefore be useful in providing new data for evaluating climate model predictions of IWP. In this study, our methodology has been applied to one year of A-Train measurements, narrowed to daytime conditions over oceanic surfaces. The retrievals and their uncertainties are statistically analysed, after a thorough discussion of the filtering process. It appears that our method is sensitive to IWPs ranging between about 0.5 and 1000 g m − 2 , with uncertainties better than 25% between 5 to 500 g m − 2 . Retrievals of the optical depth and effective radius of liquid layers have uncertainties better than 20%. Our retrievals are then compared to five independent operational A-Train products. Very good agreements, well within a factor of 2, are found by comparisons to products from active and passive instruments. These results overall lead to the validation of our method. Additionally, the robustness of passive operational products to multi-layer conditions is discussed. Preliminary comparisons show a possible overestimation of retrievals obtained under the single-layer approximation. A thorough assessment of this problem will be addressed in a following study.

Description: The behaviour of the eddy-driven jet over the Atlantic sector during the winter season is analysed for the ERA-Interim reanalysis and the coupled and atmosphere-only configuration of HadGEM3-GC2 - the climate model in use at the Met Office. The tri-modal distribution that characterises the jet-stream structure in terms of its preferred locations is reproduced with good accuracy by the model, although a distinct bias towards the high-latitude position is observed. Two different scenarios are found to contribute to this bias. One occurs when the jet shifts from its southern regime, whereby it settles too far north and for too long compared to the reanalysis. The other is associated with the exit from the central latitude regime, with too many events shifting poleward rather than equatorward. Excessively large lower tropospheric eddy heat fluxes during these transitions may account for the jet errors, even though the heat fluxes do not exhibit a climatological bias. Interestingly, these biases are weaker when the atmosphere model is forced with observed SSTs, suggesting that either it is vital to have the correct SST distribution or that ocean-atmosphere coupling plays a key role in the biases . Additional analysis revealed that the Pacific jet exit is biased south in the coupled model and that this contributes to the Atlantic bias. Anomalously warm SSTs in the Gulf Stream region may be acting together with the Pacific bias in fostering the anomalous activity in the low level eddy heat fluxes.

Description: The 4DEnVar is a credible alternative to the 4D-Var formulation, especially for Numerical Weather Prediction centres that have invested a lot in this latter formulation during the last decades. First implementations of this technique however rely on a simplified form for the localisation of the 4D covariances inside the assimilation period. It is shown in this paper that the use of a unique localisation for all the cross-covariances between perturbations at different times can be a crude approximation especially in areas where the mean flow speed is large. To overcome this problem a Lagrangian advection of the localisation is proposed. It is first tested in the simplified Burgers’ model and then introduced in the real size system associated with the French global model ARPEGE. The test of this advection in both environments shows a significative positive impact in regions where the advection is large. The possibility to use such a Lagrangian advection to evolve the static initial covariance matrix in a flow-dependent way inside the assimilation period, in a hybrid 4DEnVar formulation, is also investigated. Copyright c 2015 Royal Meteorological Society

Description: Representing model uncertainty in convection-permitting ensemble prediction systems is a developing area of research. While methods for including variability to account for model uncertainty at the global scales are quite mature, it is not clear that these methods will necessarily be applicable at the convective-scale. One such method is the Random Parameter (RP) scheme, where parameters from the physics parametrizations are perturbed at regular intervals throughout the forecast. In this work, we adapt the RP scheme to represent model uncertainty in the Met Office's convection permitting ensemble prediction system for the UK (MOGREPS-UK). The revised version of the RP scheme is applied to a sub-set of model parameters, chosen to target specific physical processes relevant to the UK forecast. Objective verification scores from two one-month trials show particular improvements for visibility and surface temperature when the RP scheme is used. Application of the RP scheme results in a modest increase in the ensemble spread for all surface parameters. The results of low visibility case studies show that applying the RP scheme enables the ensemble to capture observed fog events otherwise missed by the forecast. Overall, the RP scheme has a positive effect on MOGREPS-UK, and demonstrates the benefit of schemes that target known areas of model uncertainty.

Description: Independent observations can be used to diagnose and tune a data assimilation (DA) system. Analysis increments generally improve the model state nearby assimilated observations but degrade it further away. High-resolution aircraft observations from Mode-S EHS are used as an independent data source to verify increment degradation as a function of distance from assimilated observations. An adaptation of the inherently imperfect gain matrix in DA is proposed such that resulting analyses better fit the independent data source and as such draw model simulations closer to the true atmospheric state. It is found that the structure functions of the background error covariance matrix of the experimental mesoscale HARMONIE model are appropriate but too much weight is given to observations relative to the model background. The ECMWF model is well tuned with a slight overestimation of temperature information in the upper troposphere. Finally, a caveat is highlighted when comparing model forecasts from different experiments against observations. It is common practice to use the same observing system both in the analysis and for forecast verification. However, forecast verification is prone to sampling errors yielding less favourable scores when using an independent data source. To avoid biased conclusions on the impact of observing systems, e.g., in observing system experiments (OSE), requires an independent data source (best practice) or a data source used in all experiments (best pragmatic practice) for verification of forecasts from different experiments.

Description: There is as yet no standard methodology for measuring wind gusts from a moving platform. To address this, we have developed a method to derive gusts from research aircraft data. First we evaluated four different approaches, including Taylor's hypothesis of frozen turbulence, to derive the gust length scales that correspond to the gust time scales, namely, the gust duration (seconds) and the sample period (typically 10 min). The novelty of our method is in using peak factors (deviation of the gust from the mean wind speed normalized by the local turbulence) to convert between the scales. After devising a way to derive the gust length scales, we calculated the gust factors from aircraft observations and tested them against those from four parameterizations originally developed for weather stations. Three of them performed well ( R 2  = 0.66 or higher), while the fourth overestimated the gust factors in unstable conditions ( R 2  = 0.52). The mean errors for all methods were low, from −0.02 to 0.05, indicating that wind gust factors can indeed be measured from research aircraft. Moreover, we showed that aircraft can provide gust measurements within the whole boundary layer, if horizontal legs are flown at multiple levels over the same track. This is a significant advance, as gust measurements are usually limited to heights reached by weather masts. In unstable conditions over the open ocean the gust factor was nearly constant with height throughout the boundary layer, the near-surface values only slightly exceeding those at upper levels. Furthermore, we found gust factors to be strongly dependent on surface roughness conditions, which differed between the open ocean and sea ice in the Arctic marine environment. The roughness effect on the gust factor was stronger than the effect of boundary-layer stability.

Description: Black Carbon (BC) induced indirect radiative forcing and cloud albedo effect has been studied for the first time over Northeast India. Measurements of BC and Cloud microphysical parameters were carried out during Phase-I of Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) over North East India (Guwahati) in 2009. Liquid water path (LWP) in the cloud layers coherent with BC on different experimental days was found to be 206 to 327 gm -2 over the region. Black carbon aerosol Indirect effect (BCIE) for fixed LWP is found to be 0.32 to 0.48 on different days of observations. The indirect forcing corresponding to this BCIE has been estimated using a radiative transfer model for fixed LWP by altering the derived BC-AOD (from measured BC profiles) and cloud effective radius (R e ) combinations. The estimated average BC induced indirect forcing (BCIF) was -24 to -37.1Wm -2 at the surface and +2.5 to +14.8 Wm -2 at Top Of the Atmosphere (TOA). The average albedo due to BCIF at TOA was 0.49 to 0.61. BCIF is found to reduce the cloud reflection by 1.5 to 2% over the region. The sensitivity of cloud parameters to BCIF and albedo effect are illustrated.

Description: Due to the emergence of new high resolution numerical weather prediction (NWP) models and the availability of new or more reliable remote sensing data, the importance of efficient spatial verification techniques is growing. Wavelet transforms offer an effective framework to decompose spatial data into separate (and possibly orthogonal) scales and directions. Most wavelet based spatial verification techniques have been developed or refined in the last decade and concentrate on assessing forecast performance (i.e. forecast skill or forecast error) on distinct physical scales. Particularly during the last five years, a significant growth in meteorological applications could be observed. However, a comparison with other scientific fields such as feature detection, image fusion, texture analysis, or facial and biometric recognition, shows that there is still a considerable, currently unused potential to derive useful diagnostic information. In order to tab the full potential of wavelet analysis, we revise the state-of-the art in one- and two-dimensional wavelet analysis and its application with emphasis on spatial verification. We further use a technique developed for texture analysis in the context of high-resolution quantitative precipitation forecasts, which is able to assess structural characteristics of the precipitation fields and allows efficient clustering of ensemble data.

Description: The present work proposes a novel method to derive vertical profiles of ozone from infrared measurements onboard geostationary sounders. The methodology is based on the principal component analysis (PCA) and has been applied over a large geographical region from 45°S to 45°N. It has been found that the first two principal components (PCs) together explain 71% of the total variance of the ozone profiles and are statistically significant and quite stable. Therefore, an algorithm for the retrieval of the first two PCs has been proposed. The magnitude of the retrieval error using PCA is significantly smaller than the variance of the dataset in the atmospheric layer from 200.0 hPa to 5.0 hPa indicating that the skill of the new algorithm is very high. The new algorithm was applied on the sounder observations from GOES-13 (Geostationary Operational Environmental Satellite) and INSAT-3D (Indian National Satellite) for the validation study. The validation of GOES-13 retrieved profiles with the ozonesonde have shown that the algorithm is working better in the stratosphere than in the troposphere. Furthermore, the GOES-13 retrieved ozone was also compared with the SBUV/2 (Solar Backscatter Ultraviolet) ozone. Overall the %RMSD (percentage root mean square deviation) values were within 30%. However, the values were within 10% above 20 hPa for most of the cases. The comparison of INSAT-3D retrieved profiles with SBUV/2 and OMPS profiles have shown results almost similar to GOES-13. The ozone retrieved by using the existing algorithms and the algorithm presented in this work has been compared. It is found that the ozone retrieval has improved significantly with the algorithm presented in this work.

Description: Column water vapor (CWV) in the tropics is well known to affect tropical deep convection. It has been established that the tropical deep convection required for tropical cyclone formation markedly increases after reaching a critical CWV threshold. Therefore, CWV values below this threshold represent an atmosphere that is too dry for tropical cyclone formation. The interannual variability of the occurrence of this dry air and its impacts on tropical cyclone activity over the Atlantic are examined. CWV empirical orthogonal function (EOF) analysis reveals a dominant mode of interannual variability of dry air events over the tropical north Atlantic, with strong variations over the central and eastern Atlantic between 15-30°N. In the positive (negative) phase of the EOF modes, tropical cyclone activity is reduced (increased) over the Atlantic basin, particularly over the main development region. On the origin of dry air, the Saharan Air Layer does not explain the CWV EOF pattern, but the pattern is shown to be related to large-scale subsidence in association with enhanced anticyclonic vorticity. The leading mode shows significant correlations to several large-scale climate oscillations, including the Atlantic Multi-decadal Oscillation, Pacific Decadal Oscillation, Atlantic Meridional Mode, Pacific North American Pattern, and others. The EOF mode also indicates a relationship to the large scale circulation via the Hadley cell. An evaluation of the steering flow for tropical cyclones from the modulation of the subtropical high and the values of deep layer vertical wind shear is consistent with the distribution of tropical cyclone activity in only certain limited areas, suggesting that the impacts of the dry air patterns are required to provide a more complete explanation of the activity.

Description: Idealized numerical model simulations are used to investigate the generation and evolution of vertical vorticity by deep convection in a warm-cored vortex of near tropical storm strength. Deep convective updraughts are initiated by thermal perturbations located at different radii from the vortex axis. It is found that, as the location of the thermal perturbation is moved away from the axis of rotation, the updraught that results becomes stronger, the cyclonic vorticity anomaly generated by the updraught becomes weaker, the structure of the vorticity anomaly changes and the depth of the anomaly increases. For an updraught along or near the vortex axis, the vorticity anomaly has the structure of a monopole and little or no anticyclonic vorticity is generated in the core. Vorticity dipoles are generated in updraughts near or beyond the radius of maximum tangential wind speed and this structure reverses in sign with height. In all cases the anomalies persist long after the initial updraught has decayed. Implications of the results for understanding the vorticity consolidation during tropical cyclogenesis are discussed. The effects of eddy momentum fluxes associated with a single updraught on the tangential-mean velocity tendency are investigated and a conceptual framework for the interpretation of these eddy fluxes is given. The simulations are used to appraise long standing ideas suggesting that latent heat release in deep convection occurring in the high inertial stability region of a vortex core is “more efficient" than deep convection outside the core in producing temperature rise in the updraught.

Description: The increasing number of wind farms in the North Sea planned for 2020 is likely to affect the local, regional, and perhaps global climate. To this effect, it is crucial to be able to adequately model wind farms on the regional scale. Wind farms have previously been implemented in regional climate models. However such models have rarely been thoroughly evaluated, due to a lack of observation. In this study, an idealised version of the COSMO-CLM regional climate model is used in order to evaluate a constant thrust version of the Fitch wind farm parameterisation using large eddy simulations. As a benchmark for the COSMO-CLM and LES comparison, a channel flow without wind farms is first evaluated. COSMO-CLM is able to reproduce this channel flow with an RMSE of 0.6 ms-1 (4%) and an Ekman flow with an RMSE of 0.05 ms-1 (2%). Next, the wind farm parameterisation is implemented and evaluated. The wind farm parameterisation yields a root mean squared error (RMSE) of 4 ms-1 (12%) in wind speed for the channel flow, and an RMSE of 0.2 ms-1 (3%) for the Ekman flow. Based on LES output a parameter was introduced to correct for the use of the grid box averaged velocity to calculate the thrust force in the wind farm parameterisation instead of the disk averaged wind speed. This correction had little impact on the RMSE, showing that the use of the grid box velocity in the wind farm parameterisation is adequate.

Description: With the increasing volume of satellite observations, dimension reduction techniques are more and more important for storage or transmission. Furthermore, they are essential for inversion schemes that, in practice, cannot handle the huge amount of information provided by modern instruments for near real time inversion. In this paper, we compare the theoretical advantages and limitations of the two general strategies: compression (i.e., feature extraction) and channel selection (i.e., feature selection). The statistical “Input Variable Selection" framework is adopted to revisit the basis of these remote sensing techniques. The flexibility of these methods to specific applications is demonstrated. Special emphasis is put on the optimization of the observation dimension reduction for the simultaneous retrieval of several variables (e.g., temperature and humidity). In addition to considering the signal-to-noise ratio for the variable to retrieve, we propose to account for the contamination by other unknown variables. We also introduce a new approach named “Bottleneck Channels" (BC) that combines compression and channel selection techniques and can therefore benefit from the advantages of both strategies. Various configurations of the BC approach can be considered: strict, linearly or non-linearly projected, each with advantages and drawbacks. In the companion the paper, experiments will be conducted on microwave data to illustrate the practical advantages of each approach. BCs are able to compress and suppress noise in a similar way as PCA and they can be interpreted as channels like in a channel selection.

Description: In the first companion paper, the two classical strategies to reduce the satellite data dimension (i.e., compression and channel selection) were presented together with the introduction of a new method, the so-called Bottleneck Channel (BC). BCs are a compromise between the two approaches and can benefit from the advantages of both of them. In this paper, the three methodologies are tested using experiments on a synthetic dataset corresponding to a hyper-spectral conceptual instrument in the microwave, for frequencies up to 500 GHz. As expected, Principal Component Analysis (PCA) based methods are best to compress data, but their components lacks on the physical interpretability of real channels. Channel selection methods preserve this physical meaning but require a much larger number of channels in order to use the redundancy of information to reduce instrumental noise. The new BC method appears to be a good compromise. It can be seen as a PCA compression method where the components are constrained to be instrument channels, facilitating their understanding, inversion or assimilation. BCs allow for an easy calibration of data based on radiative transfer simulations. They also alleviate the mixing problem of PCA technique where various physical variabilities (e.g., temperature, humidity, clouds) can be mixed in the same extracted components. Furthermore, BC compression rate is equivalent to PCA-based methods even with a limited number of BCs.

Description: The influence of microphysical processes on the upper level flow features associated with an extra-tropical cyclone is investigated with the ECMWF global atmospheric model. A control simulation with the model version operational at ECMWF during 2014/early 2015 and a simulation with new parametrisations of rain autoconversion/accretion, rain evaporation and snow riming operational since May 2015 are compared in detail. In order to investigate the impact of each microphysical process separately, the diabatic heating rate for each microphysical process and the associated change in potential vorticity (PV) is calculated and compared for both simulations. The influence on the upper-level ridge building and the downstream flow evolution is investigated. The changes in the microphysical parametrisation lead to differences in the position of the warm conveyor belt (WCB) outflow. The WCB reaches the upper troposphere with low PV values and shifts the location of the tropopause in a slightly different way in both simulations. Although these differences are relatively small at the beginning, they are advected downstream and amplify, leading to distinct differences in the upper level PV pattern. These results highlight the importance for a correct representation of microphysical processes for large-scale flow features. Additionally they emphasise the need for detailed microphysical measurements in extra-tropical cyclones in order to better understand and constrain the microphysical processes in NWP models.

Description: Temperature and humidity retrievals from an international network of ground-based microwave radiometers (MWR s ) have been collected to assess the potential of their assimilation into a convective-scale Numerical Weather Prediction (NWP) system. Thirteen stations over a domain encompassing the western Mediterranean basin were considered for a time period of forty-one days in autumn, when heavy-precipitation events most often plague this area. Prior to their assimilation, MWR data were compared to very-short-term forecasts. Observation-minus-background statistics revealed some biases, but standard deviations were comparable to that obtained with radiosondes. The MWR data were then assimilated in a three-dimensional variational data assimilation system through the use of a rapid update cycle. A first set of f our different experiments were designed to assess the impact of the assimilation of temperature and humidity profiles, both separately and jointly. This assessment was done through the use of a comprehensive dataset of upper-air and surface observations collected in the framework of the HyMeX programme. The results showed that the impact was generally very limited on all verified parameters, except for precipitation. T he impact was found to be generally beneficial in terms of most verification metrics for about 18 hours, especially for larger accumulations. Two additional data-denial experiments showed that even more positive impact could be obtained when MWR data were assimilated without other redundant observations. The conclusion of the study points to possible ways of enhancing the impact of the assimilation of MWR data in convective-scale NWP systems.

Description: Recently derived analytic wave solutions of the Shallow Water Equations (SWE) on the rotating spherical Earth are employed to construct a test case for hydrostatic dynamical cores of global-scale General Circulation Models (GCMs). The proposed test case is more relevant to the SWE than the frequently used Rossby-Haurwitz test case which is based on wave solutions of the non-divergent barotropic vorticity equation and not the SWE. The applicability of the proposed test case to operational GCMs is demonstrated by using the spectral Eulerian dynamical core of the atmospheric component of NCAR’s Community Earth System Model to simulate the analytic solutions. An initial slowly propagating Rossby wave and a fast eastward propagating Inertia Gravity wave are both accurately simulated for 100 wave periods. In order to quantify the accuracy of the simulations, two error-measures are suggested which complement the conservation of global energy and, unlike the frequently used L 2 error-measure, provide independent assessments of the errors in the phase speeds and the meridional structures of the simulated waves and are therefore more relevant to periodic wave solutions.

Description: Diurnal variations of the areas and temperatures in tropical cyclone convective cloud systems in the western North Pacific were estimated using pixel-resolution infrared (IR) brightness temperature (BT) and best-track data for 2000–2013. The mean areal extent of very cold cloud cover (IR BTs 〈208 K) reached a maximum in the early morning (0000–0300 local solar time (LST)), then decreased after sunrise. This was followed by increasing cloud cover between 208 K and 240 K, reaching its maximum areal extent in the afternoon (1500–1800 LST). The time at which cloud cover reached a maximum was sensitive to the temperature thresholds used over the ocean. IR BTs 〈240 K reached minima in the morning (0300–0600 LST), and IR BTs 〉240 K reached minima in the afternoon (1500–1800 LST). The out-of-phase relationships between IR BTs 〈240 K and IR BTs 〉240 K, and between the maximum coverage times of IR BTs 〈208 K and 208 K 〈IR BTs〈 240 K, can both lead to the radius-averaged IR temperature having two minima per day. The different diurnal evolutions under different cloud conditions suggest tropical cyclone convective cloud systems are best described in terms of both areal extent and cloud-top temperature. Maximum occurrence of clouds with IR BTs 〈208 K in the morning and maximum occurrence of clouds with 208 K 〈IR BTs〈 240 K in the afternoon suggest that two different mechanisms might be involved in causing diurnal variations under these two types of tropical cyclone cloud conditions.

Description: The extension of all-sky assimilation of SSMIS humidity sounding channels to land surfaces is investigated in this paper. The scattering index, which is able to discriminate cloudy and precipitating regions over land, is used as a predictor to develop a ‘symmetric’ model for observation error. This formulation is able to increase the observation error in those scenes which are more difficult to model because of radiative transfer and ‘mislocation’ errors. The implementation of an instantaneous emissivity retrieval from SSMIS observations is also presented. In clear-sky scenes, emissivity retrievals appear better at capturing daily differences in surface conditions, compared to emissivity atlas values. In the presence of clouds, retrievals have different behaviour. In the lower microwave frequencies (less than 50 GHz), emissivity estimates appear nearly as reliable as those in clear-skies, but at higher frequencies, as the magnitude of scattering increases, so does the error in the retrieval and the resultant emissivity estimate can be unphysically low or high. However, the retrieval still appears feasible at high frequencies in light cloud situations; the number of retrievals discarded due to these kinds of problems is around 10%. In these cases, an estimate from an emissivity atlas can be substituted instead. Together the new observation error model and the instantaneous emissivity retrievals were adopted for the assimilation of SSMIS 183 GHz channels over land in all-sky conditions. Assimilation experiments showed that the assimilation system is not degraded and the improvements on analysis and forecast scores are about the same as those which are obtained by the equivalent clear-sky approach. The developments described in this study were an essential first step to create framework to allow the all-sky assimilation over land of other microwave humidity sounders: this started operationally at ECMWF in 2015, covering both SSMIS and four MHS (Microwave Humidity Sounder) instruments.

Description: An analysis is presented here of intense convection affecting the Friuli Venezia Giulia region (FVG, northeastern Italy) during the Intensive Observation Period 2b (IOP2b) in the first Special Observation Period (SOP1) of the HyMeX (HYdrological cycle in Mediterranean EXperiment). The present study focuses on the first of three severe-convection episodes that affected FVG on the morning of 12 September 2012. In the first episode, a supercell, which produced hail and severe damage to trees and buildings, was observed on the plain of FVG. The available observations are analyzed together with a high-resolution mesoscale model, in order to identify the relevant mechanisms for the formation and development of the cell. Six different simulations were performed starting at three different initial times, using respectively two different analysis/forecasts as initial/boundary conditions. A large spread in forecast precipitation is found among the six simulations. Only a few of the simulations were able to reproduce intense rainfall on the plain of FVG during the morning, although with significant differences in the rainfall distribution among them. One of the six simulations well reproduces the observed elongated distribution of the intense rainfall maximum; the characteristics of the cell responsible for this distribution are consistent with those expected for a supercell and its simulated evolution near the Adriatic coast agrees well with the other observations. Some instability parameters over the FVG plain and offshore (over the northern Adriatic Sea) are analyzed every 5 minutes, showing that during this event the potential instability varies significantly over small space and time intervals and among the simulations. The best simulations have the best match to the observed potential instability calculated using the mean characteristics of the lowest 500 m layer.

Description: Nearly all large-scale cloud parameterisations require the specification of the critical relative humidity (RHcrit). This is the gridbox-mean relative humidity at which the subgrid fluctuations in temperature and water vapour are assumed to become so large that part of a subsaturated gridbox becomes saturated and cloud starts to form. Until recently, the lack of high-resolution observations of temperature and moisture variability has hindered a reasonable estimate of RHcrit. However, the advent of ground-based Raman lidar now allows the acquisition of long records of temperature and moisture with sub-minute sample rates. Lidar observations are inherently noisy and any analysis of higher-order moments will be dependent on the ability to quantify and remove this noise. We present an exploratory study aimed at understanding whether current noise levels of lidarretrieved temperature and water vapour are sufficiently low to obtain a reasonable estimate of RHcrit. We show that vertical profiles of RHcrit can be derived with an uncertainty of a few percent. RHcrit tends to be smallest near the boundary-layer top and seems to be insensitive to the horizontal grid spacing at the scales investigated here (30 - 120 km). However, larger sensitivity was found to the vertical grid spacing. RHcrit is observed to decrease by 10% as the vertical grid spacing quadruples. By way of example, the lidar-retrieved RHcrit profiles were used to evaluate a parameterisation that estimates RHcrit from variances diagnosed from the boundarylayer parameterisation. It is shown that this parameterisation overestimates RHcrit by up to 10 %, but captures the diurnal variability of RHcrit well, with lower values of RHcrit near the boundary-layer top. While we show that the uncertainties associated with the retrievals are large, lidar observations seem promising to diagnose and evaluate a very important parameter to predict cloud fraction in climate and numerical weather prediction models.

Description: This study reports new measurements of the charge separation during ice crystal–graupel collisions in the absence of supercooled water droplets. Experimental measurements of the average charge acquired by a simulated graupel, under condition of subsaturation with respect to water and supersaturation with respect to ice, were performed. The relative humidity of the air was controlled and measured during the experiments. The experiments were carried out in the temperature range between −7 °C and −21 °C, with a relative velocity of 3 m s −1 between the ice particles and the graupel. The aim of the measurements was to study the dependence of the non–inductive mechanism on the presence/absence of supercooled water droplets. Present results together with previous findings indicate that the sign of the graupel charging depends on the presence/absence of supercooled water droplets and on the relative humidity (subsaturation/supersaturation) of the environment in the case of absence of water droplets. This laboratory study is the first to map the non-riming conditions with low velocity impact and these new results confirm the negative charging of graupel under these conditions. It was found that the magnitude of the charge separated per collision in non-riming conditions is of the same order of magnitude as in riming conditions. This suggests that the non-inductive mechanism could be operating in the storms even in the absence of water droplets; emphasizing the importance of knowledge of the environmental thermodynamic conditions in order to know the charging behaviour of the graupel particles.

Description: ABSTRACT By using daily NCEP-NCAR reanalysis and observed rainfall data, we investigated the east–west shift of the South Asian High (SAH) on the subseasonal timescale and its relationship with the rainfall anomaly in China. It is found that the zonal shift of the SAH at 200 hPa displays a dominant periodicity of 10–50-day, with two preferable locations, one over the northeast Tibetan Plateau (eastern location) and the other over the northeast Iranian Plateau (western location). The zonal shift of the SAH is closely related to the southward movement of the intraseasonal perturbation originated from middle latitudes. A vorticity budget analysis reveals that the advection of mean vorticity by the perturbation of meridional wind primarily contributes to the vorticity tendency over both the eastern and western locations. The zonal shift of the SAH closely connects to the summer rainfall anomalies over China. An eastern (a western) location of the SAH corresponds to generally positive (negative) rainfall anomalies in northwestern China and negative (positive) anomalies in southern China. A further diagnosis reveals that the strengthening of the intraseasonal perturbation during the southward journey in the eastern location is attributed to a positive convection-circulation feedback. On the one hand, a positive geopotential height anomaly in upper troposphere and associated upper-level divergence induce anomalous ascending motion and condensational heating in middle troposphere. On the other hand, the positive heating anomaly increases mid-tropospheric temperature and strengthens upper-level geopotential height. The southward propagation leads to the zonal shift of the SAH.

Description: The use of kilometre-scale ensembles in operational forecasting provides new challenges for forecast interpretation and evaluation to account for uncertainty on the convective scale. A new neighbourhood based method is presented for evaluating and characterising the local predictability variations from convective scale ensembles. Spatial scales over which ensemble forecasts agree (agreement scales, S A ) are calculated at each grid point ij , providing a map of the spatial agreement between forecasts. By comparing the average agreement scale obtained from ensemble member pairs ( ), with that between members and radar observations ( ), this approach allows the location-dependent spatial spread-skill relationship of the ensemble to be assessed. The properties of the agreement scales are demonstrated using an idealised experiment. To demonstrate the methods in an operational context the and are calculated for six convective cases run with the Met Office UK Ensemble Prediction System. The highlight predictability differences between cases, which can be linked to physical processes. Maps of are found to summarise the spatial predictability in a compact and physically meaningful manner that is useful for forecasting and for model interpretation. Comparison of and demonstrates the case-by-case and temporal variability of the spatial spread-skill, which can again be linked to physical processes.

Description: With 69% of the world's population predicted to live in cities by 2050, modification to local climates, in particular Urban Heat Islands, have become a well studied phenomenon. However few studies have considered how horizontal winds modify the spatial pattern in a process named Urban Heat Advection (UHA) and this is most likely due to a lack of highly spatially resolved observational data. For the first time, this study separates the two-dimensional advection-induced UHI component, including its pattern and magnitude, from the locally-heated UHI component using a unique dataset of urban canopy temperatures from 29 weather stations (3 km resolution) recorded over 20 months in Birmingham, UK. The results show that the mean contribution of UHA to the warming of areas downwind of the city can be up to 1.2 °C. Using the inverse Normalized Difference Vegetation Index as a proxy for urban fraction, an upwind distance at which the urban fraction has the strongest correlation with UHA was demonstrated to be between 4–12 km. Overall, these findings suggest that urban planning and risk management needs to additionally consider UHA. However, more fundamentally, it highlights the importance of careful interpretation of long term meteorological records taken near cities when they are used to assess global warming.

Description: Interactions between motions on different timescales are investigated in the SnoHATS dataset of near-surface stable boundary layer turbulence. In an earlier study, the authors applied a data clustering methodology based on a bounded variation, finite element, vector autoregressive factor method (FEM-BV-VARX) to characterize the influence of non-turbulent, submesoscale motions on the turbulence in the SnoHATS dataset. Regimes were identified, two of them weakly stable and two very stable turbulence states. In each identified regime, the variability of turbulent momentum fluxes is characterized here using an extended multiresolution flux decomposition methodology. The transport properties in each regime of near-surface SBL turbulence are thereby assessed. The same methodology is used to investigate the scales of motion responsible for shear generation of turbulence.

Description: The Madden Julian oscillation (MJO) is the primary mode of intraseasonal variability in the tropics. The MJO often couples with convection as it moves over the Indian Ocean and decouples as it leaves the west Pacific Ocean. The association between convection coupled to the MJO and the midlatitude flow pattern has been the primary focus of numerous studies. In particular, it has been shown that there is a statistical relationship between the position and strength of the MJO and anticyclonic wavebreaking (AWB). This study expands on this statistical relationship by examining the two-way relationship between the MJO and AWB. Using a climatology of AWB during an extended boreal cold season, we demonstrate that there is enhanced AWB over the central North Pacific when active MJO convection is over the eastern Indian Ocean and Maritime Continent. Convection associated with the MJO can induce AWB when active MJO convection is over the Indian Ocean. Once the wave breaks over the central North Pacific, the residual cut-off low phases with the MJO circulation to enhance it as it propagates eastward, resulting in a higher amplitude MJO than when no AWB occurs.

Description: A numerical discretization of the 3-dimensional (3D) diffusion equation for the scalar case and the vector case (i.e. for the momentum equation) in terrain-following coordinates on the sphere is described. The discretization uses the HE-VI (horizontally explicit-vertically implicit) approach, which is often applied in atmospheric simulation models. Firstly, a spatially second order discretization is proposed which treats the metric terms of the terrain-following coordinates in a stable manner even for very steep terrain. A von-Neumann stability analysis calculates the maximum stable diffusion Courant number for different implicitness weights as a function of slope angle and grid anisotropy. Secondly, simple analytic solutions of the diffusion equation for both the scalar and the vector case are proposed for testing and validation purposes. The implementations in the two atmospheric model systems ICON (global) and COSMO (regional) are compared against these exact solutions.

Description: We investigate the response of moist convection to the spatial variation of surface sensible heat flux (SHF) in a mesoscale domain during the evolution of the afternoon convective boundary layer (CBL), using large eddy simulation. The surface SHF heterogeneity in the domain is analytically created as a function of the spectral slope in the wavelength range from a few tens of kilometers to a few hundreds of meters in the SHF spectrum on a log-log scale. Assuming surface energy balance and spatially uniform available energy, the prescribed SHF has a phase lag of 180° with respect to the latent heat flux (LHF) in the domain. Two sets of three simulations are forced by heterogeneous surface SHF fields, which are characterized by similar statistics. One set, however, is created with a spectral slope of κ − 3 (where κ is wavenumber) and the other with a slope of κ − 2 . All of the simulations are integrated with the same observation-based initial sounding favorable for moist convection. In all of the κ − 3 - slope cases, early non-precipitating shallow clouds further develop into deep thunderstorms. But in all of the κ − 2 slope cases, only shallow clouds develop. A key process in the transition to deep convection is the formation of a mesoscale pool of cool and moist air just above the top of the CBL. This high relative humidity (RH) pool is formed by repeated deep penetrations of turbulent plumes into the free atmosphere over a mesoscale surface of high SHF. These cross-scale fluxes, the vertical transports of mesoscale moisture and heat fluctuations by turbulent updrafts, are critical for the formation of the mesoscale pool of high RH. However, these cross-scale fluxes are canceled out in the process of averaging, and thus appear negligible in the vertical profiles of domain-averaged moisture and heat fluxes.

Description: Future extreme climate scenarios may harbor flows that have notably different intrinsic uncertainty as compared to those of present day. Here, ensemble perturbation analysis is used to explore this possibility for tropospheric flow on weather time scales for a climate scenario from the National Center for Atmospheric Research (NCAR) Community Climate Modeling System version 4 (CCSM4). Statistically significant changes are found in terms of basic uncertainty metrics including ensemble-average perturbation total energy, ensemble variance, and the growth rate of perturbation total energy and ensemble variance. Strong seasonal, geographic, and vertical variation in the changes is observed. For the winter season, the poles and the jet levels are regions that exhibit notable increases in ensemble uncertainty under the extreme scenario. These increases are accompanied in some cases by substantially larger peak growth rates. For the summer season, there are prominent increases in uncertainty under the extreme scenario in terms of moist metrics. Meanwhile, there are noteworthy reductions in uncertainty for many synoptic variables. These changes in uncertainty properties are accompanied by significant changes in measures of both baroclinic and convective flow stability. Collectively, the results suggest the changes in intrinsic uncertainty under the extreme climate scenario have a fairly nuanced character. Since the CCSM4 exhibits one of the lowest climate sensitivities of available climate models, the results define a conservative estimate of the changes to intrinsic uncertainty that might be expected from extreme climate change.

Description: It is argued that a simple explanation for the westward propagation of Indian Monsoon Depressions (IMDs) is the interaction of the depression vortex with the Himalayan “wall". This interaction can be modelled by simulating an IMD as a point vortex in a horizontal plane (at 850 hPa) and invoking image vortices behind the barrier. Solenoidal flows associated with the image vortices allow the boundary conditions at the Himalayas to be met, and cause the IMD vortex to propagate parallel to the barrier, toward the west. This simple model is tested against propagation speeds for observed IMDs. The histogram of observed propagation speed, normalised by the point-vortex model prediction, has a mean of 1.08 and standard deviation of 0.68. The model also explains the observed intensification of flow on the Himalayan side of the IMD which is a key process in enhancing rainfall to the Indo-Gangetic Plain in the monsoon season.

Description: A case study is presented of a tropical low that formed near Darwin, Australia, during the monsoon and subsequently intensified over land. The study is based on European Centre for Medium Range Weather Forecast (ECMWF) analyses. Interpretations of the formation over the sea are given in terms of vorticity dynamics. The thermodynamic support for the intensification and maintenance of the low over land is investigated also. The analyses indicate that the intensification of the low depends on repeated bursts of deep convection occurring near the centre of the circulation that promote the further concentration of vorticity near the centre. This concentration of vorticity increases the local circulation about the centre, which amounts to increasing the local tangential wind speed and, through approximate gradient wind balance above the boundary layer, to a lowering of the central pressure. It is found that the horizontal transport of moisture into a mesoscale column centred on the low is approximately equal to the moisture lost by precipitation so that total precipitable water levels are not rapidly depleted over land. While the contribution to the overall moisture budget by surface fluxes is comparatively small, these fluxes are necessary to maintain conditionally unstable conditions near the vortex centre so that deep convective bursts can continue to occur there, even when the system is located far inland.

Description: In numerical models of geophysical fluid systems parametrization schemes are needed to account for the effect of unresolved processes on processes that are resolved explicitly. Usually, these parametrization schemes require tuning of their parameters to achieve optimal performance. We propose a new type of parametrization that requires no tuning as it is based on an assumption that is not specific to any particular model. The assumption is that the unresolved processes can be represented by a probability density function that has maximum information entropy under the constraints of zero average time-derivatives of key integral quantities of the unresolved processes. In the context of a model of a simple fluid dynamical system it is shown that this approach leads to definite expressions of the mean effect that unresolved processes have on the processes that are resolved. The merits of the parametrization, regarding both short-range forecasting and long-term statistics, are demonstrated by numerical experiments in which a low-resolution version of the model is used to simulate the results of a high-resolution version of the model. For the fluid dynamical system that is studied, the proposed parametrization turns out to be related to the Anticipated Potential Vorticity Method with definite values of its parameters.

Description: This paper presents an algorithm for generating cloud-mask, over ocean, from multi-channel satellite data. The algorithm is based on the assumption that at any given instant of time, the observed radiances are a mixture of many Gaussians where each Gaussian mixture component is representative of a scene (clear or cloudy). The problem is attempted as one of the unsupervised clustering. The clusters in the data are separated using Gaussian mixture model. The proposed algorithm is applied on INSAT-3D imager data and its performance is assessed by comparing the cloud-mask generated from it against the Moderate Resolution Imaging Spectroradiometer (MODIS) cloud-mask for forty selected days in year 2014. The skill of the algorithm developed in this study is about 20 % higher than the algorithm currently operational for generating the INSAT-3D cloud mask. Due to the fact that currently geophysical parameters retrieval (e.g. SST) and data assimilation are performed in the clear sky regions, the developed algorithm will have large implications in retrieval and data assimilation studies.

Description: Stability analyses of baroclinic ocean eddies have previously demonstrated that eddies with no surface fronts are stable when the potential vorticity (PV) in the surrounding ocean is constant. This assumption of constant PV ocean which might explain the long life-span of observed ocean eddies is applied here in the stability analyses of two warm eddies that are characterized by surface fronts along the outcropping line of the interface that separates the eddy from the ocean. The first is an eddy in a solid body rotation and the second is a constant PV eddy, both of them in a two layer shallow water model. The prescription of the basic state of constant PV in the lower layer couples the mean fields in the eddy and the surrounding ocean so that the mean flow in the eddy changes with the ocean depth. The calculation of the growth rates of small-amplitude wavelike perturbations are done numerically using a shooting to fitting point method, which enables a consistent incorporation of the regularity conditions at the center, the surface front and infinity into the solution. In both eddies no unstable modes are found for wave numbers 2, 3 or 4. An analysis of the real phase speed shows that the coalescence of inertia-gravity waves in the two layers that generates instability in other settings is inhibited in the present model while Rossby waves are eliminated from the lower layer by the constant PV assumption. The elimination of Rossby waves in the lower layer by the constant PV assumption stabilizes the two eddies both of which were previously shown to be highly unstable when the underlying ocean is assumed motionless.

Description: Floods in mountainous regions like the Swiss Alps are still challenging to forecast because they result from very diverse hydrometeorological processes. A better understanding of large-scale flood precursors is therefore important. In this study the synoptic situations leading to 14 high-impact floods in Switzerland between 1987 and 2011 are analysed using the Era-Interim dataset. In a first step, the flood-related synoptic flow situations are classified into four categories: i) Atmospheric rivers (ARs) with NW flow associated with floods in NW Switzerland; ii) ARs with W-SW flow associated with floods in N Switzerland; iii) pivoting potential vorticity (PV) cut-offs associated with floods in NE Switzerland and iv) PV streamers associated with floods in S Switzerland. The strong link between ARs and floods in N Switzerland is demonstrated for the first time. The four synoptic categories are associated with very different flow patterns over Switzerland but all categories correspond to intense integrated vapour transport (IVT) directed perpendicular to orography. In a second step, the episodes of intense IVT related to the 14 floods are compared to the local climatology in terms of IVT amplitude, direction and duration. Ten of the 14 flood events correspond to exceptionally intense IVT perpendicular to orography. Despite the enormous complexity of the involved hydrometeorlogical processes, applying 4 simple thresholds of IVT toward orography at particular grid points of Era-Interim allows one to distinguish 10 flood events from all non-flood situations with only 6 non-events captured and 4 missed flood events. The close relationship between large-scale IVT and highly destructive local flood events shown here motivates the use of IVT information for medium-range flood warning systems.

Description: Sensitivity tests were performed on a mid-latitude continental case using a state-of-the-art aerosol-cloud model to determine the salient mechanisms of aerosol indirect effects (AIE) from solute aerosols. The simulations showed that increased solute aerosols doubled cloud droplet number concentrations, and hence reduced cloud particle sizes by about 20 % and consequently inhibited warm rain processes, thus, enhancing chances of homogeneous freezing of cloud droplets and aerosols. Cloud fractions and their optical thicknesses increased quite substantially with increasing solute aerosols. Although liquid mixing ratios were boosted, there was however a substantial reduction of ice mixing ratios in the upper troposphere owing to the increase in snow production aloft. The predicted total aerosol indirect effect was equal to −9.46 ± 1.4 Wm − 2 . The AIEs of glaciated clouds (−6.33 ± 0.95 Wm − 2 ) were greater than those of water-only clouds (−3.13 ± 0.47 Wm − 2 ) by a factor of two in this continental case. The higher radiative importance of glaciated clouds compared to water-only clouds emerged from their larger collective spatial extent and their existence above water-only clouds. In addition to the traditional AIEs (glaciation, riming and thermodynamic), sedimentation, aggregation and coalescence were new AIEs identified.

Description: In the framework of the Hydrological Cycle of the Mediterranean Experiment (HyMeX), the high precipitation event (HPE) of 23 October 2012 is investigated. In the morning, a highly localized convective system developed over the southeast of Corsica, close to Porto-Vecchio. Its precipitation maximum is estimated at over 100 mm within six hours. The Meso-NH model is used with a grid spacing of 2.5 km to simulate the HPE. Different input data sets and starting times are used to assess the predictability of the event. The large spread of this initial condition ensemble indicates low predictability. Among the nine ensemble members only one captures the timing and location of the event. The HPE shows some sensitivity to an increase in horizontal resolution. When the grid spacing is reduced to 500 m, precipitation is more widespread, maximum values are lower, and individual cells are smaller. Random perturbations to model physics are used to obtain a second ensemble, which shows less spread than the initial condition ensemble. The HPE over Porto-Vecchio is present in all members of the physics ensemble with varying intensity while precipitation away from orography is more sensitive. Flattening the Corsican orography removes the HPE while blocking the cold northerly inflow into the Mediterranean basin increases its duration and intensity. From the analyses and simulations it is shown that the HPE of 23 October 2012 was located over a convergence line resulting from flow splitting and lee side convergence of the northerly boundary layer flow impinging the Corsican mountains. Moist air located east of the island was fed into this convergence line by northerly low-level winds and southeasterly winds aloft advected the cells inland along the stationary convergence line, allowing large precipitation accumulations over a relatively small area.

Description: Kelvin-Helmholtz billows with horizontal scales of 3–4 km have been observed in midlatitude cyclones moving over the Italian Alps and the Oregon Cascades when the atmosphere was mostly statically stable with high amounts of shear and Ri 〈 0.25. In one case, data from a mobile radar located within a windward facing valley documented a layer in which the shear between down-valley flow below 1.2 km and strong upslope cross-barrier flow above was large. Several episodes of Kelvin-Helmholtz waves were observed within the shear layer. The occurrence of the waves appears to be related to the strength of the shear: When the shear attained large values, an episode of billows occurred, followed by a sharp decrease in the shear. The occurrence of large values of shear and Kelvin-Helmholtz billows over two different mountain ranges suggests that they may be important features occurring when extratropical cyclones with statically stable flow pass over mountain ranges.

Description: Owing to the recent, rapid development of computer technology, the resolution of atmospheric numerical models has increased substantially. With the use of next-generation supercomputers, atmospheric simulations using horizontal grid intervals of O (100) m or less will gain popularity. At such high resolution more of the steep gradients in mountainous terrain will be resolved, which may result in large truncation errors in those models using terrain-following coordinates. In this study, a new 3D Cartesian coordinate non-hydrostatic atmospheric model is developed. A cut-cell representation of topography based on finite-volume discretization is combined with a cell-merging approach, in which small cut-cells are merged with neighboring cells either vertically or horizontally. In addition, a block-structured mesh-refinement technique is introduced to achieve a variable resolution on the model grid with the finest resolution occurring close to the terrain surface. The model successfully reproduces a flow over a 3D bell-shaped hill that shows a good agreement with the flow predicted by the linear theory. The ability of the model to simulate flows over steep terrain is demonstrated using a hemisphere-shaped hill where the maximum slope angle is resolved at 71 ∘ . The advantage of a locally refined grid around a 3D hill, with cut-cells at the terrain surface, is also demonstrated using the hemisphere-shaped hill. The model reproduces smooth mountain waves propagating over varying grid resolution without introducing large errors associated with the change of mesh resolution. At the same time, the model shows a good scalability on a locally refined grid with the use of OpenMP.

Description: Precipitation is a critical aspect of climate. In Europe extreme precipitation events are costly and represent a threat to life. Evidence suggests the frequency and intensity of these events is increasing in Europe and therefore long-period datasets which can accurately represent such events are required. Precipitation is challenging to represent in gridded models and therefore is not well trusted in relatively low resolution global reanalyses. For the first time the European Reanalyses and Observations For Monitoring (EURO4M) project has produced high resolution atmospheric regional reanalyses of Europe which improve representation of precipitation. These are based on operational forecast systems at the Met Office and the Swedish Meteorological and Hydrological Institute (HIRLAM). The improvement of quality of precipitation in the regional reanalysis datasets over their parent global reanalysis is demonstrated here, together with a discussion on the difficulties of validation of gridded precipitation. It is shown that regional reanalyses show particular improvement in representing high threshold events. It is also shown that higher resolution, time-varying data assimilation and direct assimilation of precipitation all contribute to improve representation of precipitation. Resolution is of particular importance when representing extreme events.

Description: An adaptive observing system with terrestrial and space-based components has been explored, with the aim of improving Numerical Weather Prediction skill in the Mediterranean. Four Observing System Experiments based on the HIRLAM system have been conducted to test the influence of targeted observations on short-term forecasts of high-impact weather events over the first Special Observation Period of HyMeX international project. Extra radiosoundings and enhanced Advanced TIROS Operational Vertical Sounders (ATOVS) satellite observations are assimilated as targeted observations, and have been added to the baseline first separately and then jointly. In general, targeted observations have a positive but small impact on the short-term forecasts, noticeably at 700-500 hPa in all parameters and precipitation. Targeted radiosoundings produce a clear overall improvement of HIRLAM forecasts. Data targeting based only on satellite observations has a generally positive impact on precipitation, and in short-term forecasts of the rest of parameters. The assimilation of both types of extra observations produces the highest and most statistically significant improvements. The magnitude of the impact on the forecasts depends on the weather regime that determines the location of sensitive areas. According to the diagnostics obtained from the data assimilation cycle, the targeted observations had a still larger positive influence on the subsequent analyses. Extra radiosoundings and additional satellite radiances clearly improve the first guess quality over land and sea sensitive areas respectively.

Description: Southern Greenland is home to a number of weather systems characterized by high speed low-level winds that are the result of topographic flow distortion. These systems include tip jets, barrier winds and katabatic flows. Global atmospheric reanalyses have proven to be important tools in furthering our understanding of these systems and their role in the climate system. However, there is evidence that their mesoscale structure may be poorly resolved in these global products. Here output from the regional Arctic System Reanalysis (ASRv1–30 km and ASRv2–15 km grid resolutions) are compared to the global ERA-Interim Reanalysis (ERA-I–80 km grid resolution), focusing on their ability to represent winds in the vicinity of southern Greenland. Comparisons are made to observations from surface and upper-air stations, as well as from research aircraft flights during the Greenland Flow Distortion Experiment (GFDex). The ERA-I reanalysis has a tendency to underestimate high wind speeds and overestimate low wind speeds, which is reduced in ASRv1 and nearly eliminated in ASRv2. In addition, there is generally a systematic reduction in the root mean square error between the observed and the reanalysis wind speeds from ERA-I to ASRv1 to ASRv2, the exception being low-level marine winds where the correspondence is similar in all reanalyses. Case studies reveal that mesoscale spatial features of the wind field are better captured in ASRv2 as compared to the ERA-I or ASRv1. These results confirm that a horizontal grid size on the order of 15 km is needed to characterize the impact that Greenland's topography has on the regional wind field and climate. However even at this resolution, there are still features of the wind field that are under-resolved.

Description: The atmospheric northward energy transport plays a crucial role for the Arctic climate; this transport brings to the Arctic an amount of energy comparable to that provided directly by the sun. The transport is accomplished by atmospheric waves – for instance large-scale planetary waves and meso-scale cyclones – and the zonal-mean circulation. These different components of the energy transport impact the Arctic climate differently. A split of the transport into stationary and transient waves constitutes a traditional way of decomposing the transport. However this procedure does not take into account the transport accomplished separately by the planetary and synoptic-scale waves. Here a Fourier decomposition is applied, which decomposes the transport with respect to zonal wave numbers. Reanalysis and model data reveal that the planetary waves impact Arctic temperatures much more than do synoptic-scale waves. In addition the latent transport by these waves affects the Arctic climate more than does the dry-static part. Finally, the EC-Earth model suggests that changes of the energy transport over the 21 st century will contribute to Arctic warming, despite the fact that in this model the total energy transport to the Arctic will decrease. This apparent contradicting result is due to the cooling induced by a decrease of the dry-static transport by planetary waves being more than compensated for by a warming caused by the latent counterpart.

Description: This paper discusses different plausible mechanisms to account for the intrusion of dry ozone rich stratospheric air into the troposphere during the passage of the tropical cyclone Nilam-2012. The present study shows that the overshooting convection associated with the Nilam is found to be the prime candidate for the generation of turbulence in the vicinity of the tropopause (VOT). In adition, the presence of strong updarfts and downdrafts in the VOT weakened the stability of the tropopause which allows the dry ozone rich stratospheric air to penetrate downwards to the troposphere. Significant modulation of the tropopause structure is observed which is found to be accompanied by structures linked with shear instability. Concurrent ozone measurements also indicate the intrusion of stratospheric air mass to the upper and middle troposphere.

Description: In order to cope with small-scale unpredictable details of mesoscale structures in cloud-resolving models, it is suggested in this paper to process the model outputs following a fuzzy object-oriented approach to extract and track precipitating features (associated with a higher predictability than the direct model outputs). The present approach uses the particle filter method to recognize patterns based on predefined texture or spatial variability of the model output. This provides an ensemble of precipitating objects, which are then propagated in time using a stochastic advection-diffusion process. This method is applied to both deterministic and ensemble forecasts provided by the AROME-France convective-scale model . Specific case studies support the ability of the approach to handle precipitation of different types .

Description: In this paper we investigate the influence of the Gulf Stream SST front on the North Atlantic eddy-driven jet and its variability, by analysing the NCEP-CFSR dataset and a pair of AGCM simulations forced with realistic and smoothed Gulf Stream SST boundary conditions. The Gulf Stream SST front acts to generate stronger meridional eddy heat flux in the lower-troposphere and an eddy-driven jet over the eastern North Atlantic that is located further polewards, compared to the simulation with smoothed SST. The strong Gulf Stream SST gradient is found to be crucial in more accurately capturing the trimodal distribution of the eddy-driven jet latitude, with the more poleward climatological jet being the result of the jet occupying the northern jet position more frequently in the simulation forced with observed SSTs. The more frequent occurence of the northern jet location is associated with periods of high eddy heat flux over the Gulf Stream region. Composite analysis of high eddy heat flux events reveals that the significantly higher heat flux is followed by larger and more persistient poleward jet excursions in the simulations with realistic SSTs compared to the simulation with smoothed SSTs, with upper-tropospheric eddy momentum fluxes acting to maintain the more poleward eddy-driven jet. Periods of high eddy heat flux over the Gulf Stream region are also shown to be associated with increased blocking frequency over Europe, which are clearly distinct from periods with a northern jet position.

Description: A necessary condition for optimal use in decision making of probability forecasts in general, and ensemble forecasts in particular, is that they should be calibrated. Ensemble calibration implies that each observation is statistically indistinguishable from the members of its forecast ensemble. The dimensionality of multivariate forecast settings, where the observation is a vector of related quantities rather than a scalar, affords opportunities for a rich variety of types of ensemble miscalibration, and so requires more sophisticated metrics for detection and diagnosis. This paper investigates the performance in an artificial data setting of five multivariate ensemble calibration metrics that have been proposed in the literature. It is found that none of these dominates the others for detecting covariance miscalibration across a variety of miscalibration types, and that each exhibits weak performance in some instances, implying that several of the metrics should be used simultaneously when multivariate ensemble miscalibration is to be investigated. Multivariate biases were detected more strongly and diagnostically using univariate rank histograms for the individual forecast vector components.

Description: SUMMARY The main effect of gravity wave drag is generally considered to be an increased drag on upper tropospheric flow. However the nocturnal boundary layer overland is often extremely stable and in this case there can be considerable gravity wave drag on near-surface flow, affecting turbulent fluxes and screen temperatures. In order to determine whether this type of drag is correctly represented in numerical forecast models, a dataset of winds and temperatures output from a forecast model is compared with an archive of observations from two surface sites. Comparison with the output from a one dimensional model is used to differentiate gravity wave drag from form drag and flow blocking. The results indicate that a gravity wave drag component is probably present in the stable boundary layer flows of the forecast models. However the strength of the drag is not as great as that observed. This results in larger model turbulent fluxes and increased surface temperatures from those measured overnight.

Description: Skilful climate predictions of the winter North Atlantic Oscillation and Arctic Oscillation out to a few months ahead have recently been demonstrated, but the source of this predictability remains largely unknown. Here we investigate the role of the tropics in this predictability. We show high levels of skill in tropical rainfall predictions, particularly over the Pacific but also the Indian and Atlantic Ocean basins. Rainfall fluctuations in these regions are associated with clear signatures in tropical and extratropical atmospheric circulation that are approximately symmetric about the equator in boreal winter. We show how these patterns can be explained as steady poleward propagating linear Rossby waves emanating from just a few key source regions. These wave source “hotspots” become more or less active as tropical rainfall varies from winter to winter but they do not change position. Finally, we show that predicted tropical rainfall explains a highly significant fraction of the predicted year to year variation of the winter North Atlantic Oscillation.

Description: During March 2013 a series of polar lows originated in a high-vorticity (〉10 − 3 s − 1 ) shear zone that was associated with a prolonged marine cold-air outbreak over the Norwegian Sea. A detailed analysis of one shear-line polar low at the leading edge of the outbreak is presented using comprehensive observations from a well-instrumented aircraft, dropsondes, scatterometer and CloudSat data; and numerical modelling output from a convection-resolving configuration of the Met Office Unified Model. The maximum low-level wind gradient across the shear line was 25 m s − 1 over 50 km. High winds to the north and west were within the cold-air mass and were associated with large surface turbulent heat fluxes and convective clouds. Low wind speed to the south and east of the shear line were associated with low heat fluxes and a clear ‘eye’ in the polar low. Shear-line meso- γ -scale instabilities merging into the polar low appeared important to its structure and development. The model captured the shear line and the polar low structure very well — in particular the strength of the horizontal shear and the mesoscale thermodynamic fields. The spatial structure of convective cloud bands around the polar low was simulated reasonably well, but the model significantly underestimated the liquid water content and height of the cloud layers compared to the observations. Shear-line polar lows are relatively common, however this case is arguably the first to be examined with a wide range of in-situ and remote observations allied with numerical model output.

Description: Since radar observations are highly dense in spatial and temporal resolutions, they have been often used to improve the short-term numerical weather prediction (NWP) by means of detailed model verification and 3D radar data assimilation. However, the observed quantities are not directly comparable to the prognostic variables of NWP models (e.g., hydrometeor densities, wind vector, temperature, pressure, etc.), a common approach to facilitate this comparison is to derive synthetic radar observations from model variables, this is the so-called “radar forward operator”. In the present paper, a new Efficient Modular VOlume scanning RADar Operator (EMVORADO) for Doppler velocity and reflectivity is introduced. Although it has been developed in the COSMO-model framework, it can be also online coupled to any other NWP model. Comprehensive physical aspects of radar measurements (e.g., beam bending/broadening/shielding, Doppler velocity with fall speed and reflectivity weighting, attenuated reflectivity, detectable signal, etc.) have been implemented in a modular way, using state-of-the-art methods with different levels of approximations and numerical costs that can be optionally chosen. The reflectivity derivation from the prognostic model variables is as “model consistent” as possible and carefully honors the uncertainties associated with partially melted particles. Efficiency and applicability on supercomputers (MPI-parallelism) is a major design criterion, which allows to simulate entire networks of 3D volume scanning meteorological radars within one model run and makes EMVORADO well suited for operational applications. This paper is aimed to give a thorough description of the EMVORADO and to provide a first insight to the performance of different modules by some selected case studies.

Description: This study compares and evaluates two convection-permitting ensemble systems based on COSMO and AROME models in the HyMeX framework. The performance of both AROME-EPS and COSMO-H2-EPS are assessed over the whole HyMeX special observation SOP1 period. Afterwards, an analysis of the predictability of two heavy precipitation events observed during the intense observation period IOP16a of the first HyMeX special observation period SOP1 on 26 October 2012 are also undertaken. Ensemble discharge forecasts were carried out to reinforce the quantitative precipitation forecast evaluation. A probabilistic evaluation is conducted over a 53-day period of the HyMeX SOP1. AROME-EPS has a better discriminating behaviour than COSMO-H2-EPS, especially when comparing both ensembles over a verification domain including a strong variability in precipitation events. AROME-EPS still has a slightly better reliability but the statistical resolution are nearly the same for both CPEPS. For the specific case of heavy precipitation occurring over the Var region (Southeastern France), the fine-scale surface precipitation prediction is strongly sensitive to the good behaviour of a surface low pressure simulated by the ensembles, focusing strong low-level moisture towards Var. The convergence between strong moistened southerly low-level inflow and northerly cold air blowing from the Po Valley is a key factor controlling the predictability of the heavy precipitation episode over the Liguria region (Northwestern Italy). The two convection-permitting ensembles, though different in their characteristic, exhibit a good amount of probabilistic skill in forecasting heavy precipitation at a relatively high spatial and temporal resolution. Therefore they can be regarded as promising tools for operational forecast.

Description: Stratiform clouds constitute ~40% of global cloud cover and play a key role in determining the planetary radiation budget. Electrification remains one of the least understood effects on their microphysical processes. Droplet charging at the top and bottom edges of stratiform clouds arises from vertical current flow through clouds driven by the Global atmospheric Electric Circuit. In-cloud charge data are central in assessing the role of charge in droplet growth processes, which influence droplet size distributions and associated cloud radiative properties and precipitation. This study presents the first high vertical resolution electrical measurements made in multiple layer clouds. Of the 22 clouds sampled, all were charged at their edges, demonstrating unequivocally that all stratiform clouds can be expected to contain charge at their upper and lower boundaries to varying extent. Cloud base and cloud top are shown to charge asymmetrically, with mean cloud top space charge +32 pCm -3 and base space charge -24 pCm -3 . The larger cloud top charges are associated with strong temperature inversions and large vertical electrical conductivity gradients at the upper cloud boundary. Greater charging was observed in low altitude (〈2 km) clouds (20.2 pCm -3 ), compared to higher altitude (〉2 km) cloud layers (7.0 pCm -3 ), consistent with the smaller air conductivity at lower altitudes caused by reduced cosmic ray ionisation. Taken together, these measurements show that the greatest cloud droplet charges in extensive stratiform clouds occur at cloud tops for low altitude (〈2 km) clouds, when vertical mixing is suppressed by appreciable temperature inversions, confirming theoretical expectations. The influence of cloud dynamics on layer cloud edge charging reported here should inform modelling studies of cloud droplet charging effects on cloud microphysics.

Description: Mountain wave breaking, and the resulting potential for the generation of turbulence in the atmosphere, is investigated using numerical simulations of idealized, nearly hydrostatic atmospheric flows with directional wind shear over an axisymmetric isolated mountain. These simulations, which use the WRF-ARW model, differ in degree of flow non-linearity and shear intensity, quantified through the dimensionless mountain height and the Richardson number of the incoming flow, respectively. The aim is to diagnose wave breaking based on large-scale flow variables. The simulation results have been used to produce a regime diagram giving a description of the wave breaking behaviour in Richardson number–dimensionless mountain height parameter space. By selecting flow overturning occurrence as a discriminating factor, it was possible to split the regime diagram into sub-regions with and without wave breaking. When mountain waves break, the associated convective instability leads to turbulence generation (which is one of the known forms of Clear Air Turbulence, also known as CAT). Thus, regions within the simulation domain where wave breaking and the development of CAT are expected have been identified. The extent of these regions increases with terrain elevation and background wind shear intensity. Analysis of the model output, supported by theoretical arguments, suggest the existence of a link between wave breaking and the relative orientations of the incoming wind vector and the horizontal velocity perturbation vector. More specifically, in a wave breaking event, due to the effect of critical levels, the background wind vector and the wave-number vector of the dominant mountain waves are perpendicular. It is shown that, at least for the wind profile employed in the present study, this corresponds to a situation where the background wind vector and the velocity perturbation vector are also approximately perpendicular.

Description: ABSTRACT Hurricane forecasting skills may be improved by utilizing increased precipitation observations available from the Global Precipitation Mission (GPM). This study adds to the GSI capability to assimilate satellite retrieved hydrometeor profile data in the operational HWRF system. The newly developed Hurricane GPROF algorithm produces TRMM/GPM hydrometeor retrievals specifically for hurricanes. Two new observation operators are developed and implemented in GSI to assimilate Hurricane GPROF retrieved hydrometeors in HWRF. They are based on the assumption that all water vapor in excess of saturation with respect to ice or liquid is immediately condensed out. Two sets of single observation experiments that include assimilation of solid or liquid hydrometeor from Hurricane GPROF are performed. Results suggest that assimilating single retrieved solid or liquid hydrometeor information impacts the current set of control variables of GSI by adjusting the environment that includes temperature, pressure, and moisture fields toward saturation with respect to ice or liquid. These results are explained in a physically consistent manner, implying satisfactory observation operators and meaningful structure of background error covariance employed by GSI. Applying to two real hurricane cases, Leslie (2012) and Gonzalo (2014), the assimilation of the Hurricane GPROF data in the innermost domain of HWRF shows a physically reasonable adjustment and an improvement of the analysis compared to observations. However, the impact of assimilating the Hurricane GPROF retrieved hydrometeors on the subsequent HWRF forecasts, measured by hurricane tracks, intensities, sizes, satellite retrieved rain rates, and corresponding IR images, is inconclusive. Possible causes are discussed.

Description: Analytic solutions for the β -plume near a solid boundary with Rayleigh drag are presented and their vorticity budgets calculated. It is shown that when the boundary tilts into the westward plume of streamlines, the β -plume decays exponentially with a characteristic e -folding lengthscale that depends inversely as the cosine of the angle of the boundary. Most of the vorticity is extracted from the flow in a short range along the wall. By contrast, when the boundary tilts away from the westward plume of streamlines, the vorticity extraction is reduced by a factor that depends exponentially on the distance to the wall and the sine of the wall angle. Numerical solutions for horizontal viscous dissipation are also presented, and it is shown that they resemble the β -plume with Rayleigh drag. The plume tail decays as an exponential modulated by a harmonic oscillation, however. The characteristic e -folding decay scale is shorter than with Rayleigh drag by 1–5 times, depending on wall angle; the oscillation wavelength is about 3.4 times the e -folding scale. These results suggest the relevant parameters and scalings controlling bathymetric interactions in more complex situations, such as the real ocean.

Description: Initialisation shock is often discussed in the context of coupled atmosphere-ocean forecasting, but its detection has remained elusive. In this paper, the presence of initialisation shock in seasonal forecasts is clearly identified in the variability of the tropical thermocline. The specific source of shock studied here is the use of a bias correction procedure to account for errors in equatorial wind stress forcing during ocean initialisation. It is shown that the abrupt removal of the bias correction at the beginning of the forecast leads to rapid adjustments in the upper ocean, creating a shock that remains in the system for at least three months. By contrast, gradual removal of the correction term, over 20 days, greatly reduces the initialisation shock. Evidence is presented of substantial increases in sea surface temperature (SST) seasonal forecast skill, at around 3–7 months’ lead time, when the gradual removal approach is used. Gains in skill of up to 0.05, as measured by the anomaly correlation coefficient for SST in the Nino4 region, are found, using a modest hindcast set covering four seasonal start dates. The results show that improvements in coupled initialisation aimed at reducing shocks may considerably benefit seasonal forecasting.

Description: Mode Selective Enhanced Surveillance (Mode-S EHS) reports are aircraft-based observations that have value in numerical weather prediction (NWP). These reports contain the aircraft's state vector in terms of its speed, direction, altitude and Mach number. Using the state vector, meteorological observations of temperature and horizontal wind can be derived. However, Mode-S EHS processing reduces the precision of the state vector from 16-bit to 10-bit binary representation. We use full precision data from research grade instruments, on-board the United Kingdom's Facility for Atmospheric Airborne Measurements, to emulate Mode-S EHS reports and to compare with derived observations. We aim to understand the observation errors due to the reduced precision of Mode-S EHS reports. We derive error models to estimate these observation errors. The temperature error increases from 1.25 K to 2.5 K between an altitude of 10 km and the surface due to its dependency on Mach number and also Mode-S EHS precision. For the cases studied, the zonal wind error is around 0.50 ms − 1 and the meridional wind error is 0.25 ms − 1 . The wind is also subject to systematic errors that are directionally dependent. We conclude that Mode-S EHS derived horizontal winds are suitable for data assimilation in high-resolution NWP. Temperature reports may be usable when aggregated from multiple aircraft. While these reduced precision, high frequency data provide useful, albeit noisy, observations; direct reports of the higher precision data would be preferable.

Description: This study uses the ERA-Interm and Climate Forecast System reanalysis datasets to examine the relationship between the convective activity of the Madden-Julian Oscillation (MJO) and the normalized gross moist stability (NGMS) on various timescales. Previous studies based on proposed theories for the MJO and the assessments of general circulation models suggest that MJO convection tends to be more active where the NGMS is smaller or effectively negative. This relationship only appears using reanalysis data on some timescales and limited geographical regions. On seasonal timescales, MJO convective activity shifts with the regions of positive NGMS and net moisture import, indicating positive correlation between MJO convective activity and the NGMS. On interannual to decadal timescales, stronger MJO convective events tend to occur with a background state of smaller NGMS exclusively over the central Pacific basin, indicating negative correlation. When the environment switches between net moisture export to import, MJO convective activity tends follow the region of net moisture import, leading to its positive correlation with NGMS. However, when the environment already has net moisture import and positive NGMS, MJO convective activity is negatively correlated with the NGMS and stronger MJO convective events tend to occur with smaller positive NGMS. Therefore, the background NGMS is not a globally applicable metric for MJO convective activity and it needs to be used with caution over such environments where the NGMS can oscillate around zero. On intraseasonal timescales, as shown by previous modeling and observational studies, the NGMS becomes anomalously negative prior to the onset of MJO enhanced convection and becomes positive as the convection peaks over the Indo-Pacific basin. The results of this study demonstrate that the relationship between MJO convective activity and the NGMS in reanalysis data largely depends on timescales and geographical region.

Description: A large offshore observational data set from stations across the North and Baltic Sea is used to investigate the planetary boundary layer wind characteristics and their coherence, correlation and power spectra. The data of thirteen sites, with pairs of sites at a horizontal distance of 4 to 848 km, are analyzed for typical wind turbine nacelle heights. Mean wind characteristics, correlation and coherence are also calculated for analogous wind data from simulations with the Weather Research and Forecasting (WRF) model. Results indicate a general good agreement for the coherence calculated based on measurements and the WRF-derived time series. By normalising the frequency axes with the distance and mean wind speed it can be demonstrated that even for data with a wide range of distances, the coherence is a function of the frequency, mean wind and distance, which is consistent with earlier studies. The correlation coefficient as a function of the distance calculated from WRF is however higher than observed in the measurements. For the power spectra, wind speed and wind speed step changes distribution the results for all sites are quite similar. The land masses strongly influence the individual wind direction distribution of each site. The ability of the WRF model to reproduce the coherence of the measurements demonstrates that its output can be used to estimate the coherence of fluctuations for the integration of offshore energy. The power spectra of WRF time series underestimates the high frequency fluctuations. Due to the large number of measurement sites, the results can be used for further plausibility validation for mesoscale model runs over the sea.

Description: Seasonal forecasting models are increasingly being used to forecast application-relevant aspects of upcoming climatic conditions, often summarised by climate indices. Little is known, however, on how the predictive skill of such forecasts of climate indices relates to the predictive skill in forecasting seasonal mean conditions. Here we analyse forecasts of two generalised indices derived from daily minimum and maximum temperature: counts of events such as the number of frost days and accumulated threshold exceedances such as degree days. We find that the predictive skill of forecasts of these two types of indices is generally lower than the skill of seasonal mean daily minimum and maximum temperature. By use of a toy model we demonstrate that this reduction in skill is more pronounced for skilful forecasts and climate indices defined relative to a threshold at the tail of the distribution. Based on the toy model results we conclude that there is no indication of additional predictability in forecasts of these indices in excess to what is expected due to the predictability of the seasonal mean. To further support this hypothesis, we show that the skill in predicting climate indices can be statistically modelled successfully using the toy model and the skill in the seasonal mean.

Description: This study investigates the energetics of tropical cyclone intensification using the Available Potential Energy (APE) theory. While the idea that tropical cyclones (TCs) intensify as the result of the conversion into kinetic energy of the available potential energy (APE) generated by the release of latent heat extracted from the warm tropical ocean surface is now well accepted, its rigorous theoretical formalisation has remained elusive owing to the complexity of constructing a suitable reference state for defining and quantifying APE in a moist atmosphere. Yet, the construction of such a reference state is a key fundamental issue, because the magnitude of the APE reservoir and of its temporal evolution, as well as the values of the thermodynamic efficiencies controlling the rate at which diabatic processes generate or destroy APE, depend on its specification. This issue is illustrated in the idealised context of an axisymmetric TC model by comparing the energetics of TC intensification obtained by using two different sorting-based approaches to compute the reference state defining APE. It is found that the thermodynamic efficiency controlling the APE generation by surface latent heat fluxes is larger when the reference state is constructed using a ‘top-down’ sorting method, as the APE thus defined absorbs all the CAPE present in the system. However, because a large fraction of the overall CAPE is never released during the TC’s lifetime (e.g. in regions dominated by subsidence), there is a better agreement between the production of APE by surface fluxes and its subsequent conversion into kinetic energy when a ‘bottom-up’ reference state is used. These results suggest that contrary to what is usually assumed, the reference state in APE theory should be constructed to minimise , rather than maximise, the total APE, so that the introduction of dynamically inert APE is minimised.

Description: ABSTRACT The Coriolis mechanism advocated by Persson involves angular momentum, centrifugal force and Newtonian gravity. The derivation of the usual rotating frame momentum equation is here recalled, and an alternative form noted. Parcel-frame centrifugal force and Newtonian gravity appear; in the horizontal, coordinate-frame centrifugal force is a proxy for the latter. The meridional component equation displays part of the mechanism and suggests its close relation to that proposed by Ferrel in the 1850s. A shortcoming of Persson's treatment is corrected. The mechanism provides an illuminating deconstruction of the Coriolis force but emphasises the economy of the usual kinematic treatment.

Description: Ideally, the validation of weather and climate models requires that the predictions remain close to an exact solution of the governing equations. The complexity of weather and climate models means that it is not possible to compute exact solutions except in trivial cases. However, in the limit of small Rossby number, the exact solution of the Euler equations can be shown to be close to that of a semi-geostrophic model, which can be computed. Previous studies have used the small Rossby-number limit to validate numerical methods for a baroclinic wave without sub-grid physics. However, the method of coupling to the sub-grid physics plays an important role in the performance of weather and climate models. The aim of this paper is thus to extend the previous studies to include a boundary-layer parametrization. We use a balanced model that includes a known boundary-layer parametrization, the semi-geotriptic model. We then demonstrate that the semi-geotriptic model is the appropriate small Rossby-number limit of the solution of the Euler equations with the same boundary layer representation. The semi-geotriptic model is then used to expose weaknesses in the numerical methods for coupling the boundary layer to the rest of the model.

Description: The impact of uncertainties in surface layer physics on the atmospheric general circulation is comparatively unexplored. Here the sensitivity of the zonal-mean circulation to reduced air-sea momentum roughness ( Z 0 m ) at low flow speed is investigated with the Community Atmosphere Model (CAM3). In an aquaplanet framework with prescribed sea surface temperatures, the response to reduced Z 0 m resembles the La Niña minus El Niño response to El Niño Southern Oscillation variability with: i) a poleward shift of the mid-latitude westerlies extending all the way to the surface; ii) a weak poleward shift of the subtropical descent region; and iii) a weakening of the Hadley circulation, which is generally also accompanied by a poleward shift of the inter-tropical convergence zone (ITCZ) and the tropical surface easterlies. Mechanism-denial experiments show this response to be initiated by the reduction of tropical latent and sensible heat fluxes, effected by reducing Z 0 m . The circulation response is elucidated by considering the effect of the tropical energy fluxes on the Hadley circulation strength, the upper tropospheric critical layer latitudes, and the lower-tropospheric baroclinic eddy forcing. The ITCZ shift is understood via moist static energy budget analysis in the tropics. The circulation response to reduced Z 0 m carries over to more complex setups with seasonal cycle, full complexity of atmosphere-ice-land-ocean interaction, and a slab ocean lower boundary condition. Hence, relatively small changes in the surface parameterization parameters can lead to a significant circulation response.

Description: The Fractions Skill Score (FSS) is a spatial verification measure that is used for assessing the performance of precipitation forecasts from Numerical Weather Prediction models. Previous studies have shown that the FSS is able to give a direct measure of the error in the placement of the rain. This paper takes the approach further and derives analytical expressions and uses Monte-Carlo simulations for randomly positioned observed and forecast rainfall to reveal further characteristics of the FSS in both infinite and bounded domains. It reveals that the definition of an FSS value that determines the minimum scale at which a forecast should be deemed “useful” (useful forecast criteria) is a meaningful concept and shows how this value increases with increasing fractional rainfall coverage. A study of real forecast data is also presented using 8-years of European Centre for Medium-Range Weather Forecasts (ECMWF) model forecasts, out to a lead time of 9 days, over domains of differing sizes covering parts of Europe and North Africa. The FSS is examined using different strategies for dealing with the domain boundary and is compared with the analytical study. The findings give practical guidance on how to use the FSS. For most situations a FSS value of 〉0.5 serves as a good indicator of a useful forecast. The choice of domain size for rainfall forecast verification should consider the typical spatial errors of the forecast. For a domain that is large compared to the typical spatial error, the boundaries have little adverse affect, but this is not the case if the spatial errors start to become comparable to the size of the domain. The evaluation of ECMWF forecasts reveals the extent of the spatial errors that emerge for medium range forecasts and show the value of verifying those forecasts using the FSS over an appropriately-sized region.

Description: An issue in the determination of the direct component of solar radiation involving the circumsolar contribution in numerical weather prediction (NWP) and climate models is discussed. The direct solar radiation has multiple definitions in different applications, but this ambiguity has not always been clearly identified and has therefore sometimes caused confusion. The confusion arises from a fact that the photons that are scattered into the direct solar beam path by atmospheric particles are no different from the non-scattered photons travelling in the same direction. Therefore there is a question whether these scattered photons should be included in the direct flux calculation or whether they should be excluded. Different definitions of the direct solar radiation in wide range of applications have been discussed by Blanc et al., (2014). We further discuss it in terms of its relation to the NWP and climate models. The shortwave radiation schemes used in NWP and climate models can be classified into two groups, those that calculate the direct solar radiation without including scattering contributions and those which include the scattering contribution in terms of the delta-Eddington approximation. We assess the two treatments by comparing modelled direct solar radiation with observations. It is found that the use of delta-Eddington scaling results in positive errors in the direct solar flux at the surface while neglecting the delta-scaling leads to negative errors. The important result is that the positive error from using the delta-Eddington scaling is at least one order of magnitude larger than the absolute negative error due to neglecting it. In order to include the scattering contribution in the direct flux calculation properly, a simple parameterization for dust aerosol is developed which can be used to consider the scattering contribution due to the particular case of dust aerosol to the direct solar radiation within the circumsolar region.

Description: Two approaches to account for the radiative impacts of subgrid-scale variability of cloud in a General Circulation Model (GCM) are compared: i) deterministic reduction of cloud optical depth imbedded within the radiative transfer scheme; and ii) stochastic subcolumns employed with the Monte Carlo Independent Column Approximation (McICA). This article analyses impacts, as a function of cloud phase and cloud fraction, due to replacement of deterministic method (which is used currently in the GCM) with the McICA method, as well as the introduction of cloud water horizontal inhomogeneity and changes to the description of cloud vertical overlap. The largest radiative effects are produced by changing horizontal inhomogeneity of cloud water, which when enhanced generally decreases cloud albedo and emissivity; with an exception for some ice clouds whose albedo increases. Reducing the extent to which clouds overlap vertically has smaller, and opposite effects relative to increasing horizontal inhomogeneity, with the exception of some ice clouds where both effects have the same sign. These effects are, however, less pronounced than the increases to both cloud albedo and emissivity that stem from replacement of the deterministic optical depth reduction method by McICA. In essence, deterministic reduction of cloud optical depth has a more aggressive impact on radiative transfer than does McICA’s stochastic sampling of horizontal inhomogeneity. When the McICA methodology is applied interactively in the GCM, both cloud fraction and water content for low-level clouds are reduced relative to the deterministic method.

Description: The thermal environment of an urban street canyon in summer becomes a great concern for human health under rapid urbanization progress. For accurate prediction of the in-canyon thermal environment, it requires realistic representation of microscale physical processes within the canyon as well as multi-scale atmospheric interaction between the canopy air and the overlying urban boundary layer. To accomplish the capability, the Vegetated Urban Canopy Model (VUCM) that interactively parameterizes in-canyon radiative/dynamic/thermodynamic/hydrological processes based on a combined framework of the 2-dimensional single canyon and the single tree canopy is implemented into the Weather Research and Forecasting (WRF) model. Using the coupled WRF-VUCM model, a series of simulations is performed for a hot summer day with a finest grid resolution of 0.333 km to investigate the impacts of in-canyon vegetation ( f v ; permeable grass/soil surfaces and trees) (ranging f v = 0–0.4 which corresponds to 0–15% in urban patch area) and canyon aspect ratio ( h / w ; ranging h / w = 0.5–2) on the thermal environment of urban street canyons over the Seoul metropolitan area. The model simulation compares well with the measured 2-m temperatures (above zero-plane displacement height) and canopy air temperatures at 13 urban sites in Seoul, with a root mean square error of 1.0 °C and 0.96 °C, respectively. The increase of the in-canyon vegetation from 0% to 15% (at h / w = 1) leads to reduction of the canopy air temperature throughout the diurnal cycle, exhibiting relatively larger cooling effect during daytime (~1.1 °C on average) than at night (~0.8 °C on average) under a limited condition for evapotranspiration by the in-canyon vegetation. Provided that the soil moisture is enough for the hydrological effect, the cooling effects significantly increase by a factor of ~2.5 at both daytime and night. The increase of h / w from 0.5 to 2 (at f v = 0.2) reduces the daytime canopy air temperature (~1.3 °C on average) but increases the nocturnal canopy air temperature (~0.3 °C on average). It is also found that the existence of in-canyon vegetation at h / w 〉 1 has a synergic cooling benefit to the thermal environment of street canyons compared to the effects from no vegetation case. These results demonstrate the importance of interactive parameterization of the physical processes and the interplay of in-canyon vegetation and building density (via canyon aspect ratio) effects on accurate prediction of the thermal environment of urban street canyons.

Description: Part I presented a Monte Carlo Bayesian method for constraining a complex statistical model of GCM sub-gridcolumn moisture variability using high-resolution MODIS cloud data, thereby permitting parameter estimation and cloud data assimilation for large-scale models. This part performs some basic testing of this new approach, verifying that it does indeed significantly reduce mean and standard deviation biases with respect to the assimilated MODIS cloud optical depth, brightness temperature and cloud top pressure, and that it also improves the simulated rotational-Ramman scattering cloud optical centroid pressure (OCP) against independent (non-assimilated) retrievals from the OMI instrument. Of particular interest, the Monte Carlo method does show skill in the especially difficult case where the background state is clear but cloudy observations exist. In traditional linearized data assimilation methods, a subsaturated background cannot produce clouds via any infinitesimal equilibrium perturbation, but the Monte Carlo approach allows non-gradient-based jumps into regions of non-zero cloud probability. In the example provided, the method is able to restore marine stratocumulus near the Californian coast where the background state has a clear swath. This paper also examines a number of algorithmic and physical sensitivities of the new method and provides guidance for its cost-effective implementation. One obvious difficulty for the method, and other cloud data assimilation methods as well, is the lack of information content in passive-radiometer-retrieved cloud observables on cloud vertical structure, beyond cloud top pressure and optical thickness, thus necessitating strong dependence on the background vertical moisture structure. It is found that a simple flow-dependent correlation modification due to [Riishojgaard(1998)] provides some help in this respect, by better honoring inversion structures in the background state.

Description: A method is presented to constrain a statistical model of sub-gridcolumn moisture variability using high-resolution satellite cloud data. The method can be used for large-scale model parameter estimation or cloud data assimilation. The gridcolumn model includes assumed-PDF intra-layer horizontal variability and a copula-based inter-layer correlation model. The observables used in the current study are MODIS cloud-top pressure, brightness temperature and cloud optical thickness, but the method should be extensible to direct cloudy radiance assimilation for a small number of channels . The algorithm is a form of Bayesian inference with a Markov Chain Monte Carlo (MCMC) approach to characterizing the posteriori distribution. This approach is especially useful in cases where the background state is clear but cloudy observations exist. In traditional linearized data assimilation methods, a subsaturated background cannot produce clouds via any infinitesimal equilibrium perturbation, but the Monte Carlo approach is not gradient-based and allows jumps into regions of non-zero cloud probability. The current study uses a skewed-triangle distribution for layer moisture. The paper also includes discussion of the Metropolis and Multiple-try Metropolis versions of MCMC.

Description: ABSTRACT During Intensive Observation Period 13 (15–16 October 2012) of the first Special Observing Period of the Hydrological cycle in the Mediterranean Experiment (HyMeX), Southern Italy (SI) was affected by two consecutive heavy precipitation events (HPEs). Both HPEs were associated with multi-cell V-shaped retrograde regeneration mesoscale convective systems (MCSs). The life cycle of two MCSs in connection with their dynamic and thermodynamic environments were analysed using a combination of ground-based, airborne and spaceborne observations and numerical simulations. Rain gauges revealed that heavy precipitation occurred in two phases: the first one from 1300 to 1700 UTC (35 mm h −1 ) was caused by a V-shaped system initiating over the Tyrrhenian Sea in the early morning of 15 October. Convection was triggered by the low-level convergence between the south-westerlies ahead of an upper-level trough positioned over south-eastern France and very moist southerlies from the Strait of Sicily. The convection was favoured by high convective available potential energy (1500 J kg −1 ) resulting from warm and moist conditions at low levels associated with high sea surface temperatures in the Sicily Channel. In addition, humidity at mid-level was enriched by the presence of an elevated moisture plume from tropical Africa, favouring the efficiency of the convection to produce more precipitation. The second phase of heavy precipitation (2300 UTC on 15 October to 0200 UTC on 16 October, 34 mm h −1 ) was caused by a MCS initiating over Algeria around 1300 UTC, which subsequently traveled over the Strait of Sicily toward Sicily and SI. Convection was maintained by the combination of large low-level moisture contents and a marked convergence ahead of the cold front. Unlike other MCSs forming in the same region earlier on that day, this huge V-shaped system did affect SI because the strong upper-level flow progressively veered from southwesterly to south-southwesterly.

Description: Scatter plots of Ertel potential vorticity, Q , versus Bernoulli streamfunction, B , on potential-temperature surfaces, θ , are investigated for Mars using the global Mars Analysis Correction Data Assimilation (MACDA) reanalysis, which spans Mars Year (MY) 24.39 to 27.24. In mid-latitudes, Mars exhibits monotonic, function-like Q ( B ) correlations on θ surfaces similar to those observed for Earth. We quantify this with linear regressions of Q versus B over the vertical range θ  = 400 to 900 K (~30 to 60 km). In autumn, winter and spring, in both hemispheres, the nondimensionalized correlation generally lies between zero and unity and gradually decreases with height, whereas in northern summer, it swings negative. These characteristics match Earth's lower mesosphere ( θ  = 2000 to 3000 K; z  ≈ 48 to 62 k m ) during the same seasons. The exception is southern summer, when the correlation on Mars nearly vanishes. In time series, the transition into and out of northern summer is sinuous and centred just after solar longitude L s  = 90 ∘ , whereas in southern summer it is abrupt and spans ΔL s  ≈ 120 ∘ , which is 1/3 of a Mars year. A striking feature seen on Mars but not on Earth is a large range of Q over the narrow domain of B poleward of each winter polar jet, particularly in the north, which is consistent with the known annular structure of the Martian polar vortex. Froude number calculations suggest the existence of a planetary-scale hydraulic jump associated with the winter polar jet.

Description: The moist available potential energy (MAPE) of a domain of air is defined as the maximum amount of kinetic energy that can be released through reversible adiabatic motions of its air parcels. The MAPE can be calculated using a parcel-moving algorithm that finds the minimum enthalpy state for a given set of thermodynamic assumptions. However, the parcel-moving algorithms proposed previously do not always find the minimum enthalpy state. In this paper, we apply the Munkres algorithm to find the exact minimum enthalpy state, and we compare this exact algorithm with four inexact algorithms, including a new divide-and-conquer algorithm. The divide-and-conquer algorithm performs well in practice while being simpler and faster than the Munkres algorithm, and it is recommended for future calculation of MAPE when the exact result is not required.

Description: In this study, the relationship between the Madden-Julian Oscillation (MJO) and the North Atlantic Oscillation (NAO) is investigated by analyzing NAO events both when the MJO is active and inactive. The European Centre for Medium-Range Weather Forecasts Re-analysis (ERA-Interim) dataset is used, for the years 1979–2006 and the months of December-February. When the MJO is active, negative (positive) NAO events are preceded by enhanced MJO convection over the tropical western and central Pacific Ocean (tropical Indian Ocean and Maritime Continent). The accompanying sea level pressure (SLP) anomaly fields reveal patterns that closely resemble the Northern Annular Mode (NAM). The MJO-related negative NAO events are preceded by an increase in the vertical propagation of planetary-scale wave activity into the stratosphere, followed by a marked weakening of the zonally-symmetric component of the lower stratospheric zonal wind. The opposite behaviour is found for the MJO-related positive NAO events. The MJO-related NAO events are also long-lived, persisting for approximately 30 days. In contrast, when the MJO is inactive, there is little change to the zonally-symmetric component of the lower stratospheric zonal wind, these events are relatively short-lived, lasting for about 10 days, and the NAO corresponds to dipole that is confined to the North Atlantic.

Description: This study presents an analytical model for the amplitude of lee waves on the boundary-layer inversion in two-dimensional flow. Previous linear lee wave models, in which the amplitude depends on the power spectrum of topography, can be inaccurate if the amplitude is large. Our model incorporates nonlinear effects by assuming that lee waves originate at a region of transition between super- and subcritical flow (internal jump) downstream of topography. Energy flux convergence at this location is compensated by the radiation of laminar lee waves. The available energy is estimated using a hydraulic jump model and the resulting wave amplitude is determined from linear theory. According to this model, the amplitude of lee waves depends essentially on their wavelength and on the inversion height difference across the jump. The new amplitude model is verified against numerical simulations and water tank experiments. The agreement between the model and the numerical results is excellent, while the verification with water tank experiments reveals that the accuracy of the model is comparable to that of numerical simulations. Finally, we derive a nonlinearity parameter for interfacial lee waves and discuss the regime transition from lee waves to hydraulic jumps in terms of the Froude number and the nondimensional mountain and inversion heights.

Description: We use a high resolution numerical simulation of Atlantic Hurricane Earl (2010) to increase our understanding of Earl's intensification in relatively strong vertical shear in the context of a recent paradigm for tropical cyclone intensification. The integrity of the simulation is judged by comparing analyses thereof with those of the unprecedented observational data gathered in Earl. Consistent with the classical view of spin up, the amplification of the tangential wind field above the boundary layer is found to occur as the absolute angular momentumsurfaces are drawn inwards by the aggregate heating of the rotating convective clouds in the interior of the vortex. In addition to this classical pathway, spin up occurs within the inner-core boundary layer, where the maximum tangential winds occur. The latter is another element of the new paradigm. Despite the detrimental influence of the shear on the vortex alignment and on depressing the pseudo-equivalent potential temperature outside the developing eyewall, the combined eddy processes associated with the vortical plume structures in and around the developing eyewall region are shown to contribute to an enhanced overturning circulation and an intensifying storm. These eddy processes are distinctly agradient effects that are not features of the classical spin up mechanism. It remains to be understood how the rotating convective updraughts combine to produce the diagnosed structures of the eddy terms, themselves, and how vortex Rossby waves and other eddies contribute to the alignment of the vortex during intensification.