ALBERT

Description: Since 2007 a large decline in Arctic sea ice has been observed. The large-scale atmospheric circulation response to this decline is investigated in ERA-Interim reanalyses and HadGEM3 climate model experiments. In winter, post-2007 observed circulation anomalies over the Arctic, North Atlantic and Eurasia are small compared to interannual variability. In summer, the post-2007 observed circulation is dominated by an anticyclonic anomaly over Greenland which has a large signal-to-noise ratio. Climate model experiments driven by observed SST and sea ice anomalies are able to capture the summertime pattern of observed circulation anomalies, although the magnitude is a third of that observed. The experiments suggest warm SSTs and reduced sea ice in the Labrador Sea lead to warm temperature anomalies in lower troposphere which weaken the westerlies over North America through thermal wind balance. The experiments also capture cyclonic anomalies over north-western Europe, which are consistent with downstream Rossby wave propagation.

Description: Stratospheric Sounding Units (SSU) on the NOAA polar orbiting satellites measured infrared radiances in the 15 micron CO 2 band between late 1978 and mid-2006. From these radiances a time series of layer mean stratospheric temperatures has been derived by several groups. Discrepancies in these temperature analyses have been highlighted recently and efforts are now underway to resolve the differences between them. This paper is the Met Office response summarising the issues to be resolved in creating a climate data record from the different SSUs, including corrections for radiometric, spectroscopic and tidal differences. Calibration issues identified include the SSU space view anomaly and radiometric anomalies in the NOAA-9 observations. The spectroscopic correction required for changing pressures in the pressure modulator cells is also outlined. The most important correction for the time series is for the solar diurnal and semi-diurnal tides as the satellite overpass local times change. Comparisons with other stratospheric temperature trend analyses are made and the reasons for the differences discussed. The time series presented here show sustained drops in stratospheric temperatures at all levels after the El Chichon and Pinatubo eruptions but only small trends to lower temperatures between eruptions.

Description: A high-order global shallow water model on Yin-Yang grid has been developed by using the multi-moment constrained finite volume (MCV) method. Different from the traditional finite volume method, more degrees of freedom (DOFs) which are the values at the solution points within each mesh element are defined and updated in time. The time evolution equations for these point values are derived from a set constraint conditions in terms of the so-called multi-moment quantities, such as the point value (PV), the volume-integrated average (VIA) and derivative (DV). Different moments use different forms of equations which are all consistent with the shallow water equations, among which the VIA moment is computed from a finite volume formulation of flux form that guarantees the rigorous numerical conservation. A fourth-order formulation is devised with the third-order reconstruction built over each element using the DOFs locally available. A simple and orthogonal overset grid, the Yin-Yang grid, is used to represent the spherical geometry with quasi-uniform grid spacing. The resulting global shallow water model is attractive in algorithmic simplicity and computational efficiency. The model has been validated by widely used benchmark tests. The numerical results of the present model are competitive to most existing advanced models.

Description: Linear instability of warm core eddies of constant potential vorticity (PV) is studied in a two layer, finite depth, shallow water ocean. The basic state flow in the constant PV eddy that obeys the gradient balance cannot be described by explicit expressions and can only be solved numerically. The various cases of gradient balance are classified by constructing a canonical formulation that relates any PV value to a value of the angular velocity that has to prevail near the center of the constant PV eddy. The growth-rates of perturbations imposed on the basic state are calculated for a variety of values of the (constant) PV and the depth of the surrounding ocean. The growth-rates i.e. the eigenvalues are calculated numerically by employing a shooting to fitting point method that guarantees that the corresponding eigenfunctions are regular at all singular points. The maximal growth-rates are contoured as functions of PV and ocean depth for azimuthal wavenumber 2 and 3 and the maximum of these growth-rates is of the order of 1 day which is similar to that of a solidly rotating eddy. However, the range angular velocity and ocean depth where the constant PV eddy is unstable is greatly reduced compared to that of a solidly rotating eddy. The instabilities found here are classified in terms of wave-wave interactions by comparing our results in each PV value with the known instabilities of the solidly rotating eddy with the same angular velocity. In the constant PV eddy the Baroclinic instability is filtered out and the range of angular velocity where the Hybrid instability exists is significantly reduced. All instabilities decay monotonically with the increase in ocean depth.

Description: We review what is presently known about the climate system response to stratospheric ozone depletion and its projected recovery, focusing on the responses of the atmosphere, ocean and cryosphere. Compared to well-mixed greenhouse gases (GHGs), the radiative forcing of climate due to observed stratospheric ozone loss is very small: in spite of this, recent trends in stratospheric ozone have caused profound changes in the Southern Hemisphere (SH) climate system, primarily by altering the tropospheric midlatitude jet, which is commonly described as a change in the Southern Annular Mode. Ozone depletion in the late twentieth century was the primary driver of the observed poleward shift of the jet during summer, which has been linked to changes in tropospheric and surface temperatures, clouds and cloud radiative effects, and precipitation at both middle and low latitudes. It is emphasized, however, that not all aspects of the SH climate response to stratospheric ozone forcing can be understood in terms of changes in the midlatitude jet. The response of the Southern Ocean and sea ice to ozone depletion is currently a matter of debate. For the former, the debate is centered on the role of ocean eddies in possibly opposing wind-driven changes in the mean circulation. For the latter, the issue is reconciling the observed expansion of Antarctic sea ice extent during the satellite era with robust modeling evidence that the ice should melt as a result of stratospheric ozone depletion (and increases in GHGs). Despite lingering uncertainties, it has become clear that ozone depletion has been instrumental in driving SH climate change in recent decades. Similarly, ozone recovery will figure prominently in future climate change, with its impacts expected to largely cancel the impacts of increasing GHGs during the next half-century.

Description: The demand for substantial increases in the spatial resolution of global weather- and climate- prediction models makes it necessary to use numerically efficient and highly scalable algorithms to solve the equations of large scale atmospheric fluid dynamics. For stability and efficiency reasons several of the operational forecasting centres, in particular the Met Office and the ECMWF in the UK, use semi-implicit semi-Lagrangian time stepping in the dynamical core of the model. The additional burden with this approach is that a three dimensional elliptic partial differential equation (PDE) for the pressure correction has to be solved at every model time step and this often constitutes a significant proportion of the time spent in the dynamical core. In global models this PDE must be solved in a thin spherical shell. To run within tight operational time scales the solver has to be parallelised and there seems to be a (perceived) misconception that elliptic solvers do not scale to large processor counts and hence implicit time stepping can not be used in very high resolution global models. After reviewing several methods for solving the elliptic PDE for the pressure correction and their application in atmospheric models we demonstrate the performance and very good scalability of Krylov subspace solvers and multigrid algorithms for a representative model equation with more than 10 10 unknowns on 65536 cores on HECToR, the UK’s national supercomputer. For this we tested and optimised solvers from two existing numerical libraries ( DUNE and hypre) and implemented both a Conjugate Gradient solver and a geometric multigrid algorithm based on a tensor-product approach which exploits the strong vertical anisotropy of the discretised equation. We study both weak and strong scalability and compare the absolute solution times for all methods; in contrast to one-level methods the multigrid solver is robust with respect to parameter variations.

Description: This paper is concerned with a dust-raising cold pool over the Sahara desert that occurred on August 3-5, 2006. Both the quantity of the uplifted dust and its spatio-temporal evolution are examined using satellite observations and a numerical simulation. The dust emission during this event was initiated by a mesoscale cold pool emanating from mesoscale convective systems (MCSs) that developed over northern Niger and Mali on August 3. This event is one of several exceptional northward surges of the West African Monsoon (WAM) during the 2006 wet season. We examine the propagation of the cold pool and associated dust lofting using high temporal resolution false-color dust product images from the Meteosat Second Generation Spinning Enhanced Visible and Infrared Imager (MSG-SEVIRI). Observations from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) are used to characterize the vertical structure of the dust cloud as it spreads over the Sahara and across the Atlantic coast. The European Centre for Medium-Range Weather Forecasting African Monsoon Multidisciplinary Analysis (ECMWF-AMMA) special reanalysis was used to describe the synoptic conditions that accompanied this event. Furthermore, a numerical simulation using the mesoscale model MesoNH was performed to estimate the emissions and the westward transport of dust during this event. MODIS AOD satellite imagery has then been used to track the dust plume across the Atlantic Ocean to Barbados, where comparisons are made with the local dust record there. The dusty cold pool covered southern Algeria and a large part of northern Mali and Western Niger attaining a total area close to 2 x 10 6 km 2 . It extended over 2-3 km in altitude and had an aerosol optical depth on the order of 1.5 and an estimated total dust load of about 1.5 Tg on average. Following daytime heating, the dusty cold pool and associated northward surge of moisture favored the development of new convection and additional precipitation over the Sahara. The northward extension of the dusty cold pool was accompanied by a collapse of the Saharan heat low, a characteristic feature of monsoon surges. A model-estimated quantity of 0.4 Tg of the dust produced during this event was subjected to westward transport toward the Atlantic Ocean after being mixed upward in the thickening boundary layer by the daytime heating over the Sahara to altitudes as high as 5-6 km. The arrival of the dust plume in Barbados in the Caribbean Sea 9 days after its departure from the west coast of Africa was characterized by a peak in dust concentration of 48.5 µg. m -3 .

Description: It is customary in atmospheric britishmodelling to approximate the equi-geopotential surfaces of apparent gravity as spheres, and to use spherical polar coordinates to represent the global atmosphere. However Earth's mean surface is more accurately approximated by a spheroid of revolution than by a sphere, and therefore the geopotential surfaces are better represented as spheroidal surfaces than spherical ones. Several authors have considered how to develop a spheroidal coordinate system. The keystone for this is a sufficiently-accurate, yet simple and flexible, mathematical approximation of the geopotential for a spheroidal Earth. Geopotential approximation is a compromise between the extremes of being either too simple, with deficient representation of the essentials of the underlying physics, or too complicated, leading to overly-complicated coordinate systems. A new spheroidal geopotential approximation is proposed herein. It is relatively simple, and analytically tractable, yet properly represents the underlying physics. Using this new approximation, a new, relatively simple, quasi-orthogonal spheroidal coordinate system is developed. It is then straightforward to obtain the customary spherical geopotential approximation, with its associated use of spherical polar coordinates, as an asymptotic limit of this new formulation. This confirms a previous finding, obtained using a different quasi-orthogonal coordinate system. The spheroidal geopotential approximation, and quasi-orthogonal coordinate system, proposed herein are however much simpler, yet no less accurate. They thereby lead to a much simpler, more direct, yet equally rigorous, justification for the spherical geopotential approximation.

Description: We propose a strategy to couple a stochastic lattice-gas model of a cloud system to a rather general class of convective parameterization schemes. As proposed in similar models recently presented in the literature, a cloud system in a grid-box of general circulation model (GCM) is modelled as a sub-grid lattice of N elements which can be in one out of S states, each corresponding to a different convective regime. The time evolution of each element of the lattice is represented as a Markov process characterized by transition rates dependent on large-scale fields and/or local interactions. In order to make applications to GCMs computationally feasible, we propose a reduction method leading to a system of S -1 stochastic differential equations with multiplicative noise. The accuracy of the reduction method is tested in a minimal version of the model. The coupling to a convective scheme is performed in such a way that in the limit of space ant time scale separation the modified stochastic parameterization converges to the original deterministic version of the host scheme. Experiments with a real GCM are then performed coupling the minimal version of the stochastic model to the Betts-Miller scheme in an aqua-planet version of the Planet Simulator. In this configuration the stochastic extension of the parameterization keeps the climatology of its deterministic limit but strongly impacts the statistics of the extremes of daily convective precipitation.

Description: We examine the physical constraints that must be satisfied to allow for a steady-state tropical cyclone in an isolated environment, paying particular attention to the need to replenish absolute angular momentum exactly at the rate that it is consumed and to the vanishing of the spin-up function above the frictional boundary layer. We conclude that it is unlikely that these conditions will be met simultaneously and question whether globally steady-state tropical cyclone solutions have merit. The implications for previous studies are discussed.

Description: This review assesses storm studies over the North Atlantic and Northwestern Europe regarding the occurrence of potential long-term trends. Based on a systematic review of available articles, trends are classified according to different geographical regions, data sets, and time periods. Articles that used measurement and proxy data, reanalyses, regional and global climate model data on past and future trends are evaluated for changes in storm climate. The most important result is that trends in storm activity depend critically on the time period analysed. An increase in storm numbers is evident for the reanalyses period for the most recent decades, whereas most long-term studies show merely decadal variability for the last 100-150 years. Storm trends derived from reanalyses data and climate model data for the past are mostly limited to the last four to six decades. The majority of these studies find increasing storm activity north of about 55-60 ° N over the North Atlantic with a negative tendency southward. This increase from about the 1970s until the mid-1990s is also mirrored by long-term proxies and the North Atlantic Oscillation and constitutes a part of their decadal variability. Studies based on proxy and measurement data or model studies over the North Atlantic for the past which cover more than 100 years show large decadal variations and either no trend or a decrease in storm numbers. Future scenarios until about the year 2100 indicate mostly an increase in winter storm intensity over the North Atlantic and Western Europe. However, future trends in total storm numbers are quite heterogeneous and depend on the model generation used.

Description: In the weak-wind nocturnal boundary layer, the wind direction near the surface may vary substantially with height. The directional shear is caused by transient modes and shallow, terrain-induced motions. This study examines the large directional shear for nocturnal data collected in the lowest 10 m from three field programs. The directional shear in two broad valleys with high sidewalls is dominated by transient motions within the valley cold pool. In contrast, the directional shear within a shallow narrow valley is dominated by terrain-induced downvalley drainage flows, in spite of the fact that magnitude and the height of the valley sidewalls are much smaller compared to the other two valleys. The relative contribution of the directional shear to the total vector shear is examined as a function of wind speed and stability.

Description: We demonstrate that summer precipitation biases in the South Asian monsoon domain are sensitive to increasing the convective parametrisation's entrainment and detrainment rates in the Met Office Unified Model. We explore this sensitivity to improve our understanding of the biases and inform efforts to improve convective parametrisation. We perform novel targeted experiments in which we increase the entrainment and detrainment rates in regions of especially large precipitation bias. We use these experiments to determine whether the sensitivity at a given location is a consequence of the local change to convection or is a remote response to the change elsewhere. We find that a local change leads to different mean-state responses in comparable regions. When the entrainment and detrainment rates are increased globally, feedbacks between regions usually strengthen the local responses. We choose two regions of tropical ascent that show different mean-state responses, the western equatorial Indian Ocean and western north Pacific, and analyse them as case studies to determine the mechanisms leading to the different responses. Our results indicate that several aspects of a region's mean-state, including moisture content, sea surface temperature and circulation, play a role in local feedbacks that determine the response to increased entrainment and detrainment.

Description: In general particle filters need large numbers of model runs in order to avoid filter degeneracy in high-dimensional systems. The recently proposed, fully nonlinear equivalent-weights particle filter overcomes this requirement by replacing the standard model transition density with two different proposal transition densities. The first proposal density is used to relax all particles towards the high-probability regions of state space as defined by the observations. The crucial second proposal density is then used to ensure that the majority of particles have equivalent weights at observation time. Here the performance of the scheme in a high, 65,500 dimensional, simplified ocean model is explored. The success of the equivalent-weights particle filter in matching the true model state is shown using the mean of just 32 particles in twin experiments. It is of particular significance that this remains true even as the number and spatial variability of the observations are changed. The results from rank histograms are less easy to interpret and can be considerably influenced by the parameter values used. This article also explores the sensitivity of the performance of the scheme to the chosen parameter values and the effect of using different model error parameters in the truth compared to the ensemble model runs.

Description: Northern Hemisphere mean monthly temperature anomalies for the 1890–2010 period are examined for seasonality. The statistical method of biplotting visually synthesises the major features of the time series for the twelve calendar months into a single plot. The common upward trend in all months and the winter-summer temperature contrast capture more than eighty percent of the total data variance. A temperature seasonalisation is established: winter (January-March), summer (May-October), and transitional months (November, December, and April). Two uncorrelated factors underlie this seasonality. The first is the North Atlantic thermohaline circulation, which is indicated by the Atlantic Multidecadal Oscillation, and which statistically determines the summer temperature anomalies. The second is the cold ocean-warm land pattern, which is indicated by the hemispheric land-ocean temperature contrast, and which statistically determines the winter anomalies. The interannual effect of the El Niño-Southern Oscillation, volcanism as well as the Arctic Oscillation/North Atlantic Oscillation mode competes with the background trend to produce a few extreme and outlier years.

Description: A partition of the geostrophic vorticity into shear and curvature components is employed to consider the influence of differential vorticity advection on the development of upper level jet-front systems in northwesterly flow in an idealized and an observed case. The analysis reveals that negative geostrophic shear vorticity advection by the thermal wind, inextricably coincident with regions of geostrophic cold air advection in cyclonic shear, forces subsidence that is distributed in narrow, quasi-linear, frontal scale bands aligned along the warm edge of the upper baroclinic zone. In each case examined, this component of the quasi-geostrophic (QG) subsidence makes the largest contribution to upper frontogenetical tilting. Additionally, since QG omega forced by geostrophic vorticity advection by the thermal wind is of the shearwise variety, the analysis shows that the traditional emphasis on the role of laterally displaced transverse circulations is an incomplete description of the upper frontogenetic tilting that arises in such environments. In fact, the results suggest that Mudrick's (1974) emphasis on negative vorticity advection increasing with height combined with Shapiro's (1981) insight regarding the lateral displacement of frontogenetic transverse circulations offers the most comprehensive way to conceptualize the forcings that promote rapid upper level jet-front development in regions of geostrophic cold air advection in cyclonic shear.

Description: In this paper, a nonlinear multi-scale interaction (NMI) model is used to propose an eddy-blocking matching (EBM) mechanism to account for how synoptic eddies reinforce or suppress a blocking flow. It is shown that the spatial structure of the eddy vorticity forcing (EVF) arising from upstream synoptic eddies determines whether an incipient block can grow into a meandering blocking flow through its interaction with the transient synoptic eddies from the west. Under certain conditions, the EVF exhibits a low-frequency oscillation on timescales of 2–3 weeks. During the EVF phase with a negative-over- positive dipole structure, a blocking event can be resonantly excited through the transport of eddy energy into the incipient block by the EVF. As the EVF changes into an opposite phase, the blocking decays. The NMI model produces life cycles of blocking events that resemble observations. Moreover, it is shown that the eddy north–south straining is a response of the eddies to a dipole- or Ω-type block. In our model, as in observations, two synoptic anticyclones (cyclones) can attract and merge with one another as the blocking intensifies, but only when the feedback of the blocking on the eddies is included. Thus, we attribute the eddy straining and associated vortex interaction to the feedback of the intensified blocking on synoptic eddies. The results illustrate the concomitant nature of the eddy deformation, whose role as a PV source for the blocking flow becomes important only during the mature stage of a block. Our EBM mechanism suggests that an incipient block flow is amplified (or suppressed) under certain conditions by the EVF coming from the upstream of the blocking region. This also suggests that weather and climate models need to be run with a grid size below 100 km in order to simulate the matching EVF and thus atmospheric blocking.

Description: Low-wind spectra are investigated through the analysis of sonic anemometer observations gathered in three experimental campaigns: the Urban Turbulent Project, the Northern hemisphere climate Processed land-surface Experiment and the Graz experiment. From the comparison of the horizontal Eulerian experimental spectra computed in this work with those available in the literature it is shown that in low-wind condition the spectra present an evident peak in the lower frequency range, that it is not accounted for in the classical formulations. This is a consequence of the oscillatory behaviour of the horizontal wind and the peak frequency depends on the meandering time scale of the horizontal wind components. In contrast with previous studies the meandering behaviour is observed in both stable and unstable conditions. A new formula for the low-wind velocity spectrum is proposed and tested.

Description: Eddy kinetic-energy (EKE) redistribution within mid-latitude cyclones is investigated using a two-layer quasi-geostrophic model. The flow is composed of synoptic localized cyclones at both layers interacting with a baroclinic zonal basic flow. The effects of the planetary-vorticity gradient β and the relative-vorticity gradient of the westerly basic flow are to reduce the upstream ageostrophic geopotential fluxes at the lower layer and to intensify the downstream fluxes at the upper layer. Furthermore, they act to reduce the downward ageostrophic geopotential fluxes and to intensify the upward ageostrophic fluxes. When cyclones are embedded in a cyclonic shear, EKE rapidly accumulates on the southwestern flank of the lower-layer cyclone before being cyclonically redistributed by the rearward cyclonically-oriented ageostrophic fluxes and nonlinear advection. On the contrary, when cyclones are embedded in an anticyclonic shear, the lower-layer ageostrophic fluxes are mainly westward oriented and the pressure work perfectly combines with nonlinear advection to maintain the EKE increase at the same place upstream or downstream of the lower-layer cyclone, depending on the value of β . Finally, a simulation showing the crossing of a westerly jet by cyclones condenses all the above effects. At the early stages, when cyclones lie on the anticyclonic side of the jet, a strong EKE increase occurs downstream of the lower-layer cyclone. Just after the jet crossing, the EKE is rapidly rearward and cyclonically redistributed by the ageostrophic fluxes and nonlinear advection leading to the formation of a lower-layer jet to the south of the cyclone.

Description: A set of relaxation experiments using the ECMWF atmospheric model is used to analyse the severe European winter of 1962/63. We argue that the severe winter weather was associated with a wave train that originated in the tropical Pacific sector (where weak La Nina conditions were present) and was redirected towards Europe, a process we suggest was influenced by the combined effect of the strong easterly phase of the Quasi-Biennial Oscillation (QBO ) and unusually strong easterly winds in the upper equatorial troposphere that winter. A weak tendency towards negative North Atlantic Oscillation (NAO) conditions in December, associated with extratropical sea surface temperature and sea-ice anomalies, might have acted as a favourable preconditioning. The redirection of the wave train towards Europe culminated in the stratospheric sudden warming at the end of January 1963. We argue that in February, the sudden warming event helped maintain the negative NAO regime, allowing the severe weather to persist for a further month. A possible influence from the Madden-Julian Oscillation, as well as a role for internal atmospheric variability, is noted.

Description: We present results from a new synthesis (GECCO2) that covers the years 1948-2011 employing a similar configuration of the Massachusetts Institute of Technology general circulation model as the previous 50-yr (1952-2001) GECCO synthesis. In GECCO2, the resolution was increased, it now includes the Arctic Ocean and a dynamic/thermodynamic sea ice model. The synthesis uses the adjoint method to bring the model into consistency with available hydrographic and satellite data as well as prior estimates of surface fluxes. In comparison to GECCO, GECCO2 provides a better agreement with the assimilated data, however, the estimated flux adjustments remain similar to GECCO. Global heat content changes are in agreement with recent observational estimates and the estimate of the global heatflux is close to a radiative forcing estimate. Both show a clear effect of the radiative forcing from volcanic eruptions and a weak relation to ENSO events. In contrast to GECCO, the importance of the Denmark Strait overflow for the variability of the Atlantic Meridional Overturning Circulation (AMOC) is replaced in GECCO2 by water mass transformation in the subpolar gyre, which is shown to be part of the thermohaline circulation if the overturning is defined as function of density . Heat and freshwater transport estimates in the Atlantic are more consistent with previous estimates than the unconstrained run. Decomposing heat and freshwater transports into overturning and gyre components by averaging on density coordinates demonstrates that in these coordinates the contribution from the gyre circulation largely disappears for heat transport and is reduced for the freshwater transport.

Description: Observations of turbulent fluxes of momentum, heat and moisture from low-level aircraft data are presented. Fluxes are calculated using the eddy covariance technique from flight legs typically ~40 m above the sea surface. Over 400 runs of 2 minutes (~12 km) from twenty-six flights are evaluated. Flight legs are mainly from around the British Isles although a small number are from around Iceland and Norway. Sea surface temperature observations from two on-board sensors (the ARIES interferometer and a Heimann radiometer) and a satellite-based analysis (OSTIA) are used to determine an improved SST estimate. Most of the observations are from moderate to strong wind speed conditions: the latter being a regime short of validation data for the bulk flux algorithms that are necessary for numerical weather prediction and climate models. Observations from both statically stable and unstable atmospheric boundary-layer conditions are presented. There is a particular focus on several flights made as part of the DIAMET (Diabatic influence on mesoscale structures in extratropical storms) project. Observed neutral exchange coefficients are in the same range as previous studies, although higher for the momentum coefficient and are broadly consistent with the COARE 3.0 bulk flux algorithm, as well as the surface exchange schemes used in the ECMWF and Met Office models. Examining the results as a function of aircraft heading shows higher fluxes and exchange coefficients in the across-wind direction, compared to along-wind (although this comparison is limited by the relatively small number of along-wind legs). A multi-resolution spectral decomposition technique demonstrates a lengthening of spatial scales in along-wind variances in along-wind legs, implying the boundary-layer eddies are elongated in the along-wind direction. The along-wind runs may not be able to adequately capture the full range of turbulent exchange that is occurring because elongation places the largest eddies outside of the run length.

Description: Among the dynamical cores of the NWP communities many different discretization methods can be distinguished to solve numerically the equations governing the motions in the atmosphere. One of them, the Z-grid approach, is based on solving the equations formulated in terms of divergence and vorticity on an Arakawa A-grid, a grid where all the variables are defined at the same gridpoints. To permit an efficient semi-implicit (SI) treatment, Z-grid schemes were proposed in literature which first perform the SI time discretization on the momentum equations formulated in terms of velocity components to construct from this a discretized divergence equation. This publication shows that a careful formulation of such SI Z-grid schemes is required to conserve appropriate dispersion relations for the inertia-gravity, inertia-Lamb and Rossby waves. It is proven analytically for a two time-level (2TL) SI Z-grid scheme of the 1D shallow water equations that the spatial discretization must respect temporal symmetry, meaning that the spatial discretization must be identical in the implicit and explicit part of the scheme. If not, the discretized waves are damped or amplified and their phase and group velocity may be seriously distorted. These findings are discussed in detail and both 1D and 2D numerical tests are carried out to demonstrate that a symmetric formulation is an important modelling constraint in order to obtain an appropriate geostrophic adjustment.

Description: Serial correlation of extreme mid-latitude cyclones observed at the storm track exits is explained by deviations from a Poisson process. To model these deviations, we apply Fractional Poisson Processes (FPP) to extreme mid-latitude cyclones, which are defined by the 850-hPa relative vorticity of the ERA interim reanalysis during boreal winter (DJF) and summer (JJA) seasons. Extremes are defined by a 99%-quantile threshold in the grid point time series. In general, FPPs are based on long term memory and lead to non-exponential return time distributions. The return times are described by a Weibull distribution to approximate the Mittag-Leffler function in the FPPs. The Weibull shape parameter yields a dispersion parameter, which agrees with results found for mid-latitude cyclones. The memory of the FPP, which is determined by detrended fluctuation analysis, provides an independent estimate for the shape parameter. Thus, the analysis exhibits a concise framework of the deviation from Poisson statistics (by a dispersion parameter), non-exponential return times, and memory (correlation) on the basis of a single parameter. The results have potential implication for the predictability of extreme cyclones.

Description: Originally the only surface data assimilated in the Met Office global forecasting system were pressure and marine winds but now most temperatures, humidities and land winds are also used. Adjustments for differences between station and model height are essential for pressure and temperature; new height adjustments for humidity and wind were introduced. These changes brought the global and regional forecasting systems much closer in their use of surface data and forecast performance for surface variables. Winds from islands and headlands not resolved in the forecast model and tropical land winds are excluded. Extra reports (notably Metars) have been introduced into the system. The assimilation of land station temperature and humidity reports gave a clear improvement to short range forecasts of "screen" temperature and humidity and small improvements to pressure forecasts. The assimilation of winds over land areas had little impact – wind speed biases, especially at night, are part of the problem. The surface pressure assimilation improves pressure and upper atmosphere forecasts but has little effect on other surface variables. The observation innovations reveal aspects of observation and model errors and other factors such as the proximity to the coast and the importance of the diurnal cycle.

Description: The present study seeks to understand how the initial vortex intensity and outer winds influence tropical cyclone (TC) size, which is defined as the azimuthally-averaged radius of the 10-m 17 m s −1 wind from the TC centre (R17), using a full baroclinic model in a quiescent f -plane environment. The initial vortex intensity is found to influence the size growth rate in the developing phase of the vortex life cycle. However, when the vortex comes to the mature phase and/or decaying phase of the vortex life cycle, the initial vortex intensity (ranges between 20 and 40 m s −1 in this study) does not strongly affect TC size. On the other hand, vortex intensification or re-intensification resulting from inner-core dynamics is apparently favourable for size growth in most instances. In addition, the lower-tropospheric outer winds of a vortex (i.e. winds beyond R17; e.g. the environmental flow around the TC) are found to be an important factor governing size change. The outer winds closer to R17 are more effective and can influence the vortex size at an earlier stage, especially if the winds are strong. The impact of the initial lower-tropospheric outer winds on the TC size evolution appears to be more prominent than that of the initial vortex intensity. The size change is much more sensitive to the outer-core, rather than inner-core, dynamics. The higher the angular momentum (AM) beyond R17, the more the AM can be brought towards the centre and hence favours size growth, and vice versa.

Description: While many investigations of the atmospheric zonal mean circulation have been published using various vertical coordinates this paper concentrates on other meridional coordinates than latitude. Potential vorticity q and potential temperature θ are selected with height as vertical coordinate. Both q and θ are conserved in adiabatic frictionless flow. Although this helps in the interpretation of the results, both choices are problematic because of the contortions in the contours. The related problems are solved by integrating over suitable zonal tubes. The isertelic mean circulation exhibits global cells in the upper troposphere and separate shallow hemispheric cells near the ground. Mean q-fluxes are derived from the mass circulation. The mean circulation in ( θ ,z) coordinates is similar to that in ( φ , θ ) coordinates and has hemispheric direct cells. Mean θ -fluxes follow from that. Both circulations are forced in the sense that they can be derived and understood if the zonal mean heating is known. This is demonstrated by explaining, for example, the equatorward surface flow in ( θ ,z)-coordinates and the shallow boundary layer ciculation in the isertelic system.

Description: Forecast verification is important across scientific disciplines as it provides a framework for evaluating the performance of a forecasting system. In the atmospheric sciences, probabilistic skill scores are often used for verification as they provide a way of unambiguously ranking the performance of different probabilistic forecasts. In order to be useful, a skill score must be proper — it must encourage honesty in the forecaster, and reward forecasts which are reliable and which have good resolution. A new score, the Error-spread Score ( ES ), is proposed which is particularly suitable for evaluation of ensemble forecasts. It is formulated with respect to the moments of the forecast. The ES is confirmed to be a proper score, and is therefore sensitive to both resolution and reliability. The ES is tested on forecasts made using the Lorenz ’96 system, and found to be useful for summarising the skill of the forecasts. The European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble prediction system (EPS) is evaluated using the ES . Its performance is compared to a perfect statistical probabilistic forecast — the ECMWF high resolution deterministic forecast dressed with the observed error distribution. This generates a forecast that is perfectly reliable if considered over all time, but which does not vary from day to day with the predictability of the atmospheric flow. The ES distinguishes between the dynamically reliable EPS forecasts and the statically reliable dressed deterministic forecasts. Other skill scores are tested and found to be comparatively insensitive to this desirable forecast quality. The ES is used to evaluate seasonal range ensemble forecasts made with the ECMWF System 4. The ensemble forecasts are found to be skilful when compared with climatological or persistence forecasts, though this skill is dependent on region and time of year.

Description: The orographic gravity wave drag produced in flow over an axisymmetric mountain when both vertical wind shear and non-hydrostatic effects are important was calculated using a semi-analytical two-layer linear model, including unidirectional or directional constant wind shear in a layer near the surface, above which the wind is constant. The drag behaviour is determined by partial wave reflection at the shear discontinuity, wave absorption at critical levels (both of which exist in hydrostatic flow), and total wave reflection at levels where the waves become evanescent (an intrinsically non-hydrostatic effect), which produces resonant trapped lee wave modes. As a result of constructive or destructive wave interference, the drag oscillates with the thickness of the constant-shear layer and the Richardson number within it ( Ri ), generally decreasing at low Ri and when the flow is strongly non-hydrostatic. Critical level absorption, which increases with the angle spanned by the wind velocity in the constant-shear layer, shields the surface from reflected waves, keeping the drag closer to its hydrostatic limit. While, for the parameter range considered here, the drag seldom exceeds this limit, a substantial drag fraction may be produced by trapped lee waves, particularly when the flow is strongly non-hydrostatic, the lower layer is thick and Ri is relatively high. In directionally sheared flows with Ri  =  O (1), the drag may be misaligned with the surface wind in a direction opposite to the shear, a behaviour which is totally due to non-trapped waves. The trapped lee wave drag, whose reaction force on the atmosphere is felt at low levels, may therefore have a distinctly different direction from the drag associated with vertically propagating waves, which acts on the atmosphere at higher levels.

Description: The difference between the derivations of the Coriolis effect on a rotating turntable and on the rotating earth is discussed. In the latter case a real force, the component of the earth's gravitational attraction, non-parallel to the local vertical, plays a central role by balancing the centrifugal force. That a real force is involved leaves open, not only the question on the inertial nature of the “inertial oscillations” , but also the way we tend to physically conceptualize the terrestrial Coriolis effect.

Description: Nocturnal low-level jets (LLJs) are maxima in the wind profile, which often form above the stable nocturnal boundary layer. Over the Sahara, the world's largest source of mineral dust, this phenomenon is of particular importance to the emission and transport of desert aerosol. We present the first ever detailed large-eddy simulations of dust-generating LLJs. Using sensitivity studies with the UK Met Office large-eddy model (LEM), two key controls of the nocturnal LLJ are investigated: surface roughness and the Coriolis force. Functional relationships derived from the LEM results help to identify optimal latitude-roughness configurations for a maximum LLJ enhancement. Ideal conditions are found in regions between 20 and 27°N with roughness lengths 〉 0.0001 m providing long oscillation periods and large jet amplitudes. Typical LLJ enhancements reach up to 3.5 m s −1 for geostrophic winds of 10 m s −1 . The findings are largely consistent with results from a theoretical LLJ model applied for comparison. The results demonstrate the importance of latitude and roughness in creating regional patterns of LLJ influence. Combining the functional relationships with high-resolution roughness data over northern Africa gives good agreement with the location of morning dust uplift in satellite observations. It is shown that shear-induced mixing plays an important role for the LLJ evolution and surface gustiness. With decreasing latitude the LLJ oscillation period is longer and, thus, shear-induced mixing is weaker, allowing a more stable nocturnal stratification to develop. This causes a later and more abrupt LLJ breakdown in the morning with stronger gusts, which can compensate for the slower LLJ evolution that leads to a weaker jet maximum. The findings presented here can serve as first step towards a parameterisation to improve the representation of the effects of nocturnal LLJs on dust emission in coarser-resolution models.

Description: This paper describes a local similarity theory for developed turbulence in the stably stratified boundary layer that is based on the Brunt-Väisälä frequency and the dissipation rate of turbulent kinetic energy instead the turbulent fluxes used in the traditional Monin-Obukhov similarity theory. Based on dimensional analysis (Pi theorem), it is shown that any properly scaled statistics of the small-scale turbulence are universal functions of a stability parameter defined as the ratio of a reference height z and the Dougherty-Ozmidov length scale, which in the limit of z -less stratification is linearly proportional to the Obukhov length scale. Measurements of atmospheric turbulence made at five levels on a 20-m tower over the Arctic pack ice during the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) are used to examine the behaviour of different similarity functions in the stable boundary layer. In the framework of this approach the non-dimensional turbulent viscosity is equal to the gradient Richardson number whereas the non-dimensional turbulent thermal diffusivity is equal to the flux Richardson number. These results are a consequence of the approximate local balance between production of turbulence by shear in the mean flow and viscous dissipation. The turbulence framework based on the Brunt-Väisälä frequency and the dissipation rate of turbulent kinetic energy may have practical advantages for estimating turbulence when the fluxes are not directly available.

Description: Atmospheric reanalyses covering the European region are mainly available as part of comparably coarse global reanalysis. The aim of this paper is to present the development and evaluation of a next generation regional reanalysis for the European CORDEX EUR-11 domain with a horizontal grid spacing of approximately 6 km. In this context, a reanalysis is understood as assimilation of heterogeneous observations with a physical model such as a numerical weather prediction (NWP) model. The reanalysis system presented here is based on the NWP model COSMO by the German Meteorological Service DWD (Deutscher Wetterdienst) using a continuous nudging scheme. In order to assess the added value of data assimilation, a dynamical downscaling experiment has been conducted, i.e., an identical model set-up but without data assimilation. Both systems have been evaluated for a one-year test period, employing standard measures such as analysis increments, biasses, or log-odds ratios, as well as tests for distributional characteristics. An important aspect is the evaluation from different perspectives and with independent measurements such as satellite infrared brightness temperatures using forward operators, integrated water vapour from GPS stations, and ceilometer cloud cover. It can be shown that the reanalysis better resolves local extreme events, which is basically an effect of the higher spatio-temporal resolution, as it is known from dynamical downscaling approaches. An important criterion for regional reanalyses, though, is the coherence with independent observations of high temporal and spatial resolution, which has significantly improved compared to dynamical downscaling. The system is intended to become operational within a year, continuously reprocessing and evaluating longer time periods. The reanalysis data is planned to become available to the research community within a year.

Description: A multiple linear regression analysis of nine different reanalysis datasets has been performed to test the robustness of variability associated with volcanic eruptions, the El Niño Southern Oscillation (ENSO), the quasi-biennial oscillation (QBO) and with a specific focus on the 11-year solar cycle. The analysis covers both the stratosphere and troposphere and extends over the period 1979–2009. The characteristic signals of all four sources of variability are remarkably consistent between the datasets and confirm the responses seen in previous analyses. In general, the solar signatures reported are primarily due to the assimilation of observations, rather than the underlying forecast model used in the reanalysis system. Analysis of the 11-year solar response in the lower stratosphere confirms the existence of the equatorial temperature maximum, although there is less consistency in the upper stratosphere, probably reflecting the reduced level of assimilated data there. The solar modulation of the polar jet oscillation is also evident, but only significant during February. In the troposphere, vertically banded anomalies in zonal mean zonal winds are seen in all the reanalyses, with easterly anomalies at 30 ∘ N and 30 ∘ S suggesting a weaker and possibly broader Hadley circulation under solar maximum conditions. This structure is present in the annual signal and is particularly evident in NH wintertime. As well as the ’top-down’ solar contribution to NAM variability, we show the potential contribution from the surface conditions allowing for a ’bottom-up’ pathway. Finally, the reanalyses are compared with both observed global-mean temperatures from the Stratospheric Sounding Unit (SSU) and from the latest general circulation models from CMIP-5. The SSU samples the stratosphere over three different altitudes, and the 11-year solar cycle fingerprint is identified in these observations using detection and attribution techniques.

Description: The paper examines the flow generated by time-periodic variations in surface temperature along an infinite slope in an initially unperturbed, stably stratified atmosphere at rest. Uniform boundary conditions at the surface are conducive to an along-slope parallel flow, governed by a periodically reversing local imbalance between along-slope advection and slope-normal fluxes of momentum and heat. It is shown that solutions include both a transient part and a periodic regime and that three different flow regimes may occur. The properties of the solutions in each regime are examined and discussed, outlining novelties with respect to previously known results.

Description: The structure of leeside warming during foehn events is investigated as a function of cross-barrier flow regime linearity. Two contrasting cases of westerly flow over the Antarctic Peninsula (AP) are considered – one highly non-linear, the other relatively linear. Westerly flow impinging on the AP provides one of the best natural laboratories in the world for the study of foehn, owing to its maritime setting and the Larsen C Ice Shelf (LCIS) providing an expansive, homogenous and smooth surface on its east side. Numerical simulations with the Met Office Unified Model (at 1.5 km grid size) and aircraft observations are utilised. In case A relatively weak southwesterly cross-Peninsula flow and an elevated upwind inversion dictate a highly non-linear foehn event. The consequent strongly-accelerated downslope flow leads to high amplitude warming and ice shelf melt in the immediate lee of the AP. However, this foehn warming diminishes rapidly downwind, due to upward ascent of the foehn flow via a hydraulic jump. In case C strong northwesterly winds dictate a relatively linear flow regime. There is no laterally extensive hydraulic jump and strong foehn winds are able to flow at low levels across the entire ice shelf, mechanically mixing the near-surface flow, preventing the development of a strong surface inversion and delivering large fluxes of sensible heat to the ice shelf. Consequently in case C ice melt rates are considerably greater over the LCIS as a whole than in case A. Our results imply that whilst non-linear foehn events cause intense warming in the immediate lee of mountains, linear foehn events will commonly cause more extensive leeside warming and, over an ice surface, higher melt rates. This has major implications for the AP, where recent east coast warming has led to the collapse of two ice shelves immediately north of the LCIS.

Description: A record-breaking high temperature of 39.8 °C in June in Japan was observed at 1420 Japan Standard Time (JST) on 24 June 2011 60 km northwest of central Tokyo. In this EHT event, surface air temperatures above 37.0 °C were recorded in and around Kumagaya, an area just north of the convergence line between westerly winds from the Chubu Mountains and southwesterly wind from the Pacific Ocean. To determine the mechanism of this EHT event, we applied various analyses using the Weather Research and Forecasting (WRF) model and observational data. According to the heat budget analysis obtained from the WRF model, during the morning most of the sensible heat supply to the mixed layer came from the net heat input, due to surface sensible heat transported by subgrid-scale turbulent diffusion. However, most of the net heat input came from advective heat transport after noon, when the westerly wind penetrated the EHT area. This westerly wind, according to backward trajectory, Lagrangian energy budget, and Eulerian forward tracer analyses, arose from a combination of two kinds of foehn flow (“hybrid” type foehn wind). Specifically, the westerly wind became foehn wind that was caused by dry-adiabatic heating and wet-diabatic heating with water vapour condensation. This “hybrid” foehn wind was an important factor in causing the present EHT event.

Description: Current understanding of the behaviour of sea breezes in the offshore environment is limited but rapidly requires improvement due, not least, to the expansion of the offshore wind energy industry. Here we report on contrasting characteristics of three sea breeze types on five coastlines around the southern North Sea from an eleven year model-simulated climatology. We present and test an identification method which distinguishes sea breeze types which can, in principle, be adapted for other coastlines around the world. The coherence of the composite results for each type demonstrates that the method is very effective in resolving and distinguishing characteristics and features. Some features, such as jets and calm zones, are shown to influence offshore wind farm development areas, including the sites of the proposed wind farms up to 200 km offshore. A large variability in sea breeze frequency between neighbouring coastlines up to a factor of 3 is revealed. Additionally, there is a strong association between sea breeze type on one coastline and that which may form coincidentally on another nearby. This association can be as high as 86% between, for example, North Norfolk and East Norfolk coasts. We show, through associations between sea breeze events on coastlines with contrasting orientations, that each coastline can be important for influencing the wind climate of another. Furthermore, we highlight that each sea breeze type needs separate consideration in wind power resource assessment and that future larger turbines will be more sensitive to sea breeze impacts.

Description: This note discusses a rigorous mathematical formulation for the evolution of the Eliassen balanced vortex. It is first shown that a stable balanced vortex of finite extent can be embedded in an ambient fluid at rest, and that such a vortex exists for prescribed angular momentum and potential temperature on fluid parcels. This uses a method developed by Shutts, Booth and Norbury. This is a different way of viewing the problem from the normal methods, which analyse the stability of a prescribed vortex. The stability of the vortex depends on the presence of background rotation and on the azimuthal velocity at the boundary of the vortex being less than that in the surrounding ambient fluid. It is then shown that the evolution of this vortex under axisymmetric forcing can be written as a conservation law for a potential pseudo-density in the transformed coordinates introduced by Schubert and Hack. The stability of the vortex to non-axisymmetric perturbations is also discussed.

Description: Active satellite sensors, such as Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and CloudSat, provide cloud properties that are not available from passive sensors, such as Moderate Resolution Imaging Spectroradiometer (MODIS). While active sensors provide vertical profiles of clouds, their spatial coverage is limited to their narrow, nadir ground-track. As a result, estimation of radiation by combining active sensors and broadband instrument has limitations due to their different spatial coverages. This study uses a scene construction algorithm (SCA) and MODIS data to extend two-dimensional (2D) nadir cloud profiles into the cross-track direction, and examines how the resulting constructed three-dimensional (3D) cloud fields improve simulation of solar radiative transfer. Clouds and Earth's Radiant Energy System (CERES) radiances are used as references to assess the improvements. While use of constructed 3D cloud fields only slightly impact mean-bias errors for instantaneous 20 km CERES footprint-averaged top-of-atmosphere (TOA) radiances, reductions in random errors are about 40%. The largest improvements in TOA radiance simulation are for clouds with small-scale horizontal inhomogeneity such as Stratocumulus and Cumulus. In contrast, uniform clouds such as Nimbostratus, and Deep convective clouds (Dc) show little response to the SCA. The impact of using the SCA on instantaneous surface irradiances is significant for Stratocumulus and Cumulus, but weak for Nimbostratus and Dc. Conversely, SCA significantly influences atmospheric absorption and heating rates for Nimbostratus and Dc. Differences in TOA radiances simulated by one-dimensional (1D) and 3D transfer models are smaller than differences due to use of only the 2D nadir cross-sections and the 3D constructed fields. This is because of smoothing of 3D radiative effects when averaged up to CERES footprints. For surface irradiance and atmospheric absorption, however, differences simulated by 1D and 3D transfer models are more comparable to differences that stem from use of 2D and 3D cloud information.

Description: In the present study, the cases of atmospheric circulation evolution favouring the occurrence of Desert Aerosol Episodes (DAEs) over the broader Mediterranean region are investigated using an objective and dynamic algorithm, with daily satellite data for the period 2000–2013. After identifying strong and extreme DAEs, at 1°x1° geographical cell level, 255 Dust Aerosol Episode Days (DAEDs) and 148 cases of consecutive DAEDs namely Desert Aerosol Episode Cases (DAECs) are defined. For each DAEC, the lower tropospheric circulation evolution one and two days before, during the initiation and after the cease of the DAEC is considered. S-mode Factor Analysis and K-Means Cluster Analysis are applied on NCEP/NCAR Mean Sea Level Pressure and 700 hPa geopotential height fields, classifying the 148 cases of atmospheric circulation evolution into 6 homogeneous and discrete clusters. The mean intra-annual variation of the DAECs reveals a primary maximum in May (18.9%), while their mean annual number is equal to 11.4 DAECs. On a seasonal basis, the highest percentage of the DAECs is found in spring (51.4%). The 58.8% of DAECs last one day while their maximum duration is 7 days. Annually, the mean monthly number of DAEs varies from 35.8 (September) to 58.0 (April). The western parts of the Mediterranean are affected by DAEs when cyclonic conditions prevail in the western Mediterranean and northwestern Africa. On the contrary, the central and eastern parts of the study region are affected by dust storms when a low pressure system in the central Mediterranean or central Europe and an anticyclone in the eastern Mediterranean prevail. As to the mean regional intensity (aerosol optical depth at 550 nm) the strong DAEs vary from 0.67 to 0.77, while the extreme ones from 1.14 to 2.06. Generally, strong DAEs are more frequent than extreme ones (in five out of six clusters).

Description: Analyses of Lagrangian model simulations and atmospheric sonde data reveal a key component of the Madden Julian Oscillation (MJO): circumnavigating equatorial Kelvin waves with dynamics that transform between dry and moist, which initiate and dissipate MJO convection. The same compositing procedure is applied to simulated and observed MJOs, which uses a coordinate system that moves with the precipitation center, and treats developing, mature, and dissipating stages separately. MJO structure and evolution are similar in the simulation and the observations. To the west of the developing convection, there is a broad region of low-level (upper-level) perturbation westerlies (easterlies), which is accompanied by a deep negative temperature perturbation. As this feature enters the MJO-formation region, convection intensifies on its eastern edge, and the zonal wind perturbations decrease in zonal extent and propagation speed. This process is shown to be a dynamical consequence of a largely dry, fi rst baroclinic Kelvin wave entering a region where large-scale upward motion is mostly balanced by convective heating. As the MJO matures, a Kelvin wave of opposite sign emerges from its eastern edge, and makes the opposite transition (from moist to dry). The resulting wave, which includes low-level (upper-level) easterlies (westerlies) and a deep positive temperature perturbation, rapidly propagates around the world and dissipates MJO convection. The Kelvin wave that initiates MJO convection is shown to originate from the inactive phase of a previous MJO, so that the complete MJO cycle is characterized by the two kinds of Kelvin waves emerging from active and suppressed phases of MJO convection, circumnavigating the tropics, and triggering the opposite phase.

Description: A feature-based approach is introduced to assess the quality of the representation of warm conveyor belts (WCBs) in high-resolution forecasts of the European Centre for Medium-range Weather Forecasts (ECMWF). WCBs are moist ascending airstreams in extratropical cyclones, which transport boundary layer air within 1-2 days poleward to the upper troposphere. The WCB outflow is typically characterised by low potential vorticity (PV), amplifying upper-level ridges and in turn modifying the jet stream and downstream Rossby wave evolution. Therefore the correct representation of WCBs can be essential for medium-range weather prediction. A three-component verification measure PAL is introduced, measuring errors in the WCB outflow's PV-anomaly ( P -component), the amplitude of the WCB ( A -component), and the location of the outflow ( L -component). The PAL approach is applied to North Atlantic WCBs in ECMWF forecasts during three winters between 2002/2003 and 2010/2011. For the latest winter, the PAL results are also compared to the anomaly correlation coefficient (ACC) of upper-level PV. It is shown that (i) the representation of WCBs improves for forecasts with a shorter lead time, (ii) PAL values are on average better for more recent forecasts, (iii) recent model versions show no systematic over- or underestimation of WCB intensity, (iv) single individual medium-range forecasts are can be associated with large errors in particular in the amplitude of WCBs, and (v) the comparison of PAL and the ACC indicates that indeed several poor forecasts in terms of ACC are associated with significant errors in the representation of WCBs. Limitations of this study are also discussed.

Description: The energy-Casimir method is employed to examine the stability of both non-parallel and parallel basic flows in the semi-geostrophic (SG) model in isentropic coordinates (ICs) based on the wave-activity invariants. It is found that the stability criteria for non-parallel basic flow under small-amplitude disturbances are more complicated than those for parallel basic flow and spatial structure of basic flow at horizontal boundary has a great impact on the stability of both parallel and non-parallel basic flow. Results obtained in this work extend the stability results of Eliassen which are valid for parallel basic flow with non-normal geostrophic flow as lateral boundary conditions. It is shown that although the there is a strong similarity between the SG model in ICs and shallow water SG (SWSG) model the finite-amplitude stability theorems for the latter model cannot be extended to the SG model in ICs and the reasons for the failure are discussed.

Description: The increasing size of wind turbines, their height and the area swept by their blades have revised the needs for understanding the vertical structure of wind gusts. Information is needed for the whole profile. In this study, we analysed turbulence measurements from a 100 m high meteorological mast at the Danish National Test Station for Large Wind Turbines at Hø vsø re in Denmark. The site represents flat, homogeneous grassland with an average gust factor of 1.4 at 10 m and 1.2 at 100 m level. In a typical surface layer gust parameterization, the gust factor is composed of two components, the peak factor and turbulence intensity, from which the turbulence intensity was found to rule over the peak factor in determining the effects of stability and height above surface on the gust factor. The peak factor only explained 15 % or less of the vertical decrease of the gust factor, but determined the effect of a gust duration on the gust factor. The statistical method to estimate the peak factor did not reproduce the observed vertical decrease in near neutral and stable conditions and near constant one in unstable conditions. Despite of this inconsistensy, the theoretical method provides estimates for a peak factor when comparing gust durations of 1 s and 3 s with averaging period lengths of 10 min and 1 h. A new technique to study the timing of maxima at different levels relative to the maximum gust at some level was developed. Results showed that a 10 m level maximum gust was typically preceded by maxima at higher levels, and vice versa, a 100 m gust was usually followed by a maximum at lower levels.

Description: The impact of observations on analysis uncertainty and forecast performance was investigated for austral spring 2010 over the southern polar area for four different systems (NRL, GMAO, ECMWF and Météo-France), at the time of the Concordiasi field experiment. The largest multi-model variance in 500 hPa height analyses is found in the southern sub-Antarctic oceanic region, where there are rapidly evolving weather systems, rapid forecast error growth, and fewer upper air wind observation data to constrain the analyses. The total impact of all observations on the model forecast was computed using the 24 hour-forecast sensitivity to observation diagnostic. Observation types that contribute most to the reduction of the forecast error are shown to be AMSU, IASI, AIRS, GPS-RO, radiosonde, surface and atmospheric motion vector observations. For sounding data, radiosondes and dropsondes, one can note a large impact on the analysis and forecasts of temperature at low levels and a large impact of wind at high levels. Observing system experiments using the Concordiasi dropsondes show a large impact of the observations over the Antarctic plateau extending to lower latitudes with the forecast range, with the largest impact around 50 to 70° South. These experiments indicate there is a potential benefit from using radiance data better over land and sea-ice and from innovative atmospheric motion vectors obtained from a combination of various satellites to fill the current data gaps and improve NWP analyses in this region.

Description: In this study two commonly used automated methods to detect atmospheric fronts in the lower troposphere are compared in various synoptic situations. The first method is a thermal approach, relying on the gradient of equivalent potential temperature (TH), while the second method is based on temporal changes in the 10 m wind (WND). For a comprehensive objective comparison of the outputs of these methods of frontal identification, both schemes are firstly applied to an idealised strong baroclinic wave simulation in the absence of topography. Then, two case studies (one in the Northern Hemisphere (NH) and one in the Southern Hemisphere (SH)) were conducted to contrast fronts detected by the methods. Finally, we obtain global winter and summer frontal occurrence climatologies (derived from ERA-Interim for 1979–2012) and compare the structure of these. TH is able to identify cold and warm fronts in strong baroclinic cases that are in good agreement with manual analyses. WND is particularly suited for the detection of strongly elongated, meridionally oriented moving fronts, however has very limited ability to identify zonally oriented warm fronts. We note that the areas of the main TH frontal activity are shifted equatorwards compared to the WND patterns and are located upstream of regions of main WND front activity. The number of WND fronts in the NH shows more interseasonal variations compared to TH, where it decreases by more than 50% from winter to summer. In the SH there is a weaker seasonal variation of the number of observed WND fronts, however TH front activity reduces from summer (DJF) to winter (JJA). The main motivation is to give an overview of their performance, such that researchers can choose the appropriate method for their particular interest.

Description: The AROME-NWC nowcasting system has been developed in order to cover the nowcasting 0–6 hours range. It is based on the existing AROME mesoscale model from which the lateral boundary conditions and the first guess file are taken. The difference between those two systems is basically the observation window length and a very short cutoff time. Studies have been carried out to show that in practical it is not necessary to compute a set of background error statistics matrix as a function of the forecast lead time of the first guess file. The spin-up of the system has been proven to be small and the 15 minutes cutoff to be long enough to keep a good forecast quality. This nowcasting system has been validated against the operational mesoscale model AROME, it has been shown that it performs better for nowcasting ranges as regards temperature, humidity and wind due to its use of more recent observations. The precipitation forecasts are less satisfactory with some improvement during the day and some degradation at night.

Description: We test the behavior of a unified continuous/discontinuous Galerkin (CG/DG) shallow water model in spherical geometry with curved elements on three different grids of ubiquitous use in atmospheric modeling: (A) the cubed-sphere, (B) the reduced latitude-longitude, and (C) the icosahedral grid. Both conforming and non-conforming grids are adopted including static and dynamically adaptive grids for a total of twelve mesh configurations. The behavior of CG and DG on the different grids are compared for a non-linear mid-latitude perturbed jet and for a linear case that admits an analytic solution. Because the inviscid solution on certain grids shows a high sensitivity to the resolution, the viscous counterpart of the governing equations are also solved and the results compared. The logically unstructured element-based CG/DG model described in this paper is flexible with respect to arbitrary grids. However, we were unable to define a best grid configuration that could possibly minimize the error regardless of the characteristic geometry of the flow. This is especially true if the governing equations are not regularized by the addition of a sufficiently large, fully artificial, diffusion mechanism, as will be shown. The main novelty of this study lies in the unified implementation of two element-based Galerkin methods that share the same numerical machinery and that do not rely on any specific grid configuration to solve the shallow water equation on the sphere.

Description: Because of its non-conformity to Monin-Obukhov Similarity Theory (MOST), the effects of thermal stratification on scaling laws describing the stream-wise turbulent intensity σ u normalized by the turbulent friction velocity ( u * ) continues to draw research attention. A spectral budget method has been developed to assess the variability of σ u / u * under unstable atmospheric stratification. At least three different length scales- the distance from the ground ( z ), the height of the atmospheric boundary layer ( δ ), and the Obukhov length ( L ) are all found to be controlling parameters in the variation of σ u / u * . Analytical models have been developed and supported by experiments for two limiting conditions: z / δ  〈 0.02, −  z / L  〈 0.5 and 0.02 ≪  z / δ  〈 0.1, −  z / L  〉 0.5. Under the first constraint, the turbulent kinetic energy spectrum is predicted to follow three regimes: k 0 , k − 1 and k − 5/3 divided in the last two-regimes by a break-point at kz  = 1, where k denotes wavenumber. The σ u / u * is shown to follow the much discussed logarithmic scaling reconciled to Townsend's attached eddy hypothesis , where the coefficients B 1 and A 1 are modified by MOST for mildly unstable stratification. Under the second constraint, the turbulent energy spectrum tends to become quasi inertial, displaying a k 0 and a k − 5/3 with a breakpoint predicted to occur 0.3 〈  kz  〈 1. The work here brings together well established but seemingly unrelated theories of turbulence such as Kolmogorov's hypothesis, Townsend's attached eddy hypothesis, MOST, and Heisenberg's eddy viscosity under a common framework.

Description: Météo-France has implemented a short-range ensemble prediction system known as PEARP. This system is a global ensemble performing forecasts up to 4.5 days. It uses the operational global numerical weather prediction model ARPEGE and benefits from its variable horizontal resolution, so that it is comparable to some limited area mesoscale systems over France. Perturbations to the initial conditions are computed by combining an Ensemble Data Assimilation system with singular vectors. Model uncertainties are represented through a “multi-physics” approach with 10 different physical parameterization sets. The paper describes the set-up of the system and provides an assessment of the approaches used to represent initial conditions and model uncertainties. The positive impact of the variable horizontal resolution of PEARP is also illustrated. As a global Ensemble Forecasts System (EFS), PEARP is also used to forecast cyclones track. It is shown that it has correctly predicted the landfall of hurricane Sandy. The performance of PEARP as it is run operationnally with these features in 2014 is objectively assessed and compared to that of four operational global Ensemble Forecasts Systems using classical probabilistic scores. This comparison is based on the TIGGE data. This is one of the first evaluation of EFS for short-range forecast. The reliability and the global skill of the five EFS are evaluated over a three-months period with scores computed against observations. PEARP shows comparable or better skill than the other EFS.

Description: This paper describes results from a new four-year (2009–2012) radar-based precipitation climatology for the southeastern United States (SE US). The climatology shows that a size-based classification between mesoscale precipitation features (MPF) and isolated precipitation reveals distinct seasonal and diurnal variability of precipitation. On average, from 70%-90% of precipitation is associated with MPF, generally less in the summertime and in southern coastal regions. MPF precipitation has a relatively small seasonal cycle except in Florida and the warm offshore waters of the Gulf Stream. In contrast, isolated precipitation has a dramatic seasonal cycle that outlines the SE US coastline whereas the MPF precipitation does not, consistent with a thermodynamic mechanism for onshore isolated storms in coastal regions. In summer, the isolated precipitation preferentially forms offshore at night, and dramatically “flips” inland by early afternoon. In contrast, MPF precipitation has no clear diurnal variations except in the southern coastal region in the summer, likely associated with sea breeze convection organized on the mesoscale. These results suggest that the MPF versus isolated precipitation system framework provides a useful basis for future studies of large scale and local controls on precipitation and resulting implications for long-range predictability of precipitation.

Description: This study assesses the impact of urban land use on the climatological distribution of thunderstorm initiation occurrences in the humid subtropical region of the Southeast United States, which includes the Atlanta, Georgia metropolitan area. Initially, an automated technique is developed to extract the locations of isolated convective initiation (ICI) events from 17 years (1997–2013) of composite reflectivity radar data for the study area. Nearly 26,000 ICI points were detected during 85 warm-season months, providing the foundation for first long-term, systematic assessment of the influence of urban land use on thunderstorm development. Results reveal that ICI events occur more often over the urban area compared to its surrounding rural counterparts, confirming that anthropogenic-induced changes in land cover in moist tropical environments lead to more initiation events, resulting thunderstorms, and affiliated hazards over the developed area. The ICI risk for Atlanta is greatest during the late afternoon and early evening in July and August in synoptically benign conditions. Greater ICI counts downwind of Atlanta suggest that prevailing wind direction also influences the location of these events. Moreover, ICI occurrences over the city were significantly higher on weekdays compared to weekend days—a result that was not apparent in a rural control region located west of the city. This suggests that the weekly commuting cycle and associated aerosol levels of Atlanta may amplify ICI rates. The investigation provides a methodological framework for future studies that examine the effect of land use, land cover, and terrain discontinuities on the spatiotemporal character of ICI events.

Description: To understand why tropical islands are rainier than nearby ocean areas, we explore how a highly idealized island, which differs from the surrounding ocean only in heat capacity, might respond to the diurnal cycle and influence the tropical climate, especially the spatial distribution of rainfall and the thermal structure of the troposphere. We perform simulations of three-dimensional radiative-convective equilibrium with the System for Atmospheric Modeling (SAM) cloud-system-resolving model, with interactive surface temperature, where a highly idealized, low heat capacity circular island is embedded in a slab-ocean domain. The calculated precipitation rate over the island can be more than double the domain average value, with island rainfall occurring primarily in an intense, regular thunderstorm system that forms in the afternoon to early evening each day. Island size affects the magnitude of simulated island rainfall enhancement, the intensity of the convection, and the timing of the rainfall maximum relative to solar noon. A combination of dynamic and thermodynamic mechanisms leads to a monotonic enhancement of domain-averaged tropospheric temperature with increasing fraction of island surface, which may contribute to localization of ascent over the Maritime Continent and its relationship to the Walker Circulation.

Description: This study explores simulations using the numerical Weather Research and Forecasting (WRF) model with respect to the representation of the nocturnal low-level jet (LLJ) over the Sahel. Three sets of experiments are designed to investigate the sensitivity with respect to (1) the boundary-layer and surface-layer schemes including local and non-local closures, (2) the horizontal grid spacing and the number of vertical levels within the lowest kilometre and (3) the role of initial and boundary data. In total 27 simulations are performed on one host domain and two nested domains for a representative LLJ case study on 9 November 2006. The ability of the individual simulations to represent the life-cycle of the nocturnal LLJ is validated against observations carried out in the framework of the African Monsoon Multidisciplinary Analysis special observation periods: Surface wind observations from Agoufou, Bamba and Banizoumbou, atmospheric wind profiles derived from Atmospheric Radiation Measurement Mobile Facility wind radar measurements at Niamey and profiles from radiosondes launched at Niamey. All runs reproduce the general characteristics of the observed LLJs satisfactorily. In contrast to earlier studies, results are more sensitive to the choice of the initial and boundary data (here GFS and ECMWF), than to the used boundary-layer and surface schemes or to model grid resolution. The sensitivity to the model grid resolution is surprisingly minor. Considerable differences between the individual stations suggest that local surface conditions such as roughness length, albedo or soil moisture may play an important role for the observed mismatch between model simulations and observations.

Description: A large fraction of the rain received by continental India is produced by cyclonic vortices with outer radii of about 1,000 km that are contained within the larger scale South Asian monsoon flow. The more intense occurrences of these vortices are called monsoon depressions; these consist of bottom-heavy columns of relative vorticity that propagate to the northwest in time-mean low-level eastward flow. Previous studies have argued that this apparent upstream propagation is caused by dynamical lifting west of the vortex center, with the resulting ascent producing vortex stretching that shifts the vortex to the west. Here analysis of over 100 Indian monsoon depressions is used to show that low-level vortex stretching has a spatial structure inconsistent with the observed propagation and is balanced by other terms in the low-level vorticity budget. Instead, monsoon depressions are shown to consist of potential vorticity maxima that have peak amplitude in the middle troposphere and that propagate westward by nonlinear, horizontal adiabatic advection (i.e. beta drift). The precipitating ascent in monsoon depressions makes a more minor contribution to the total storm motion and primarily acts to maintain the upright structure of the vortex. These results suggest a new view of Indian monsoon depressions as potential vorticity columns that propagate primarily by adiabatic dynamics.

Description: Many studies evaluating model boundary-layer schemes focus either on near-surface parameters or on short-term observational campaigns. This reflects the observational datasets that are widely available for use in model evaluation. In this paper we show how surface and long-term Doppler lidar observations, combined in a way to match model representation of the boundary layer as closely as possible, can be used to evaluate the skill of boundary-layer forecasts. We use a 2-year observational dataset from a rural site in the UK to evaluate a climatology of boundary layer type forecast by the UK Met Office Unified Model. In addition, we demonstrate the use of a binary skill score (Symmetric Extremal Dependence Index) to investigate the dependence of forecast skill on season, horizontal resolution and forecast leadtime. A clear diurnal and seasonal cycle can be seen in the climatology of both the model and observations, with the main discrepancies being the model overpredicting cumulus capped and decoupled stratocumulus capped boundary-layers and underpredicting well mixed boundary-layers. Using the SEDI skill score the model is most skillful at predicting the surface stability. The skill of the model in predicting cumulus capped and stratocumulus capped stable boundary layer forecasts is low but greater than a 24 hr persistence forecast. In contrast, the prediction of decoupled boundary-layers and boundary-layers with multiple cloud layers is lower than persistence. This process based evaluation approach has the potential to be applied to other boundary-layer parameterisation schemes with similar decision structures.

Description: Birmingham is the second most populous city in the UK and observations indicate it has a pronounced urban heat island (UHI), i.e. higher ambient temperatures in the city centre compared to surrounding suburban and rural areas, particularly at night. The effects of UHIs are often amplified during anticyclonic summer weather conditions, which can cause or exacerbate heatwaves. Enhanced temperatures in highly populated regions can mean that significant numbers of people are at risk from heat related illness during hot weather. Climate change projections often do not include the effects of the UHI, which can mean that assessments of heat related health effects using climate change projections underestimate the actual magnitude of future health impacts. We present numerical simulations of the UHI in Birmingham and the West Midlands Metropolitan region during the heatwave of August 2003 using a high resolution, regional meteorological model (WRF) with an urban canopy scheme. We evaluated the model using local air temperature observations, and found good model performance in capturing the temporal and spatial signature of the UHI. We performed a sensitivity test, replacing urban land categories with rural ones, and found the difference in temperature between the 2 model runs throughout the heatwave period (2 nd to 11 th August 2003) was around 3°C on average, and reached a maximum of 7°C. Finally, we present a novel generic methodology to enable the examination of the extent of horizontal advection of warm air downwind of the conurbation area. We found that during the heatwave, temperatures downwind of Birmingham were up to 2.5°C warmer than those upwind. This methodology has the potential for improvements to or parameterizations for diagnostic models which do not explicitly include dynamics and where local conditions are driven largely by land surface type.

Description: Accounting the height of a release in the source term estimation is important since a ground level approximation of the release leads to errors in capturing the actual extent of a plume. A least-squares inversion technique, free from initial guess, is adapted here for the reconstruction of an elevated point release in a discretized space. Primarily, this involves estimation of the effective height of the release above the ground along with its location and strength from a limited set of noisy concentration measurements. The methodology is evaluated here with the nine runs from the Idaho diffusion experiment (1974) corresponding to low wind, stable conditions. Both real and model generated synthetic data are used to test the method. With synthetic data, the methodology can exactly reproduce the input source terms. With real data, the average release height is estimated as 3.1 m, which is very close to the effective release height (3 m) reported in the Idaho data. The release location is retrieved with an average error of 30 m, whereas the minimum distance between source and detectors is 100 m; strength is retrieved within a factor of two in all the runs. The deviations in the source parameters from their prescribed values are explained in the context of model representativeness, wind variability and available monitoring network. The sensitivity of the source term estimation is evaluated against several parameters: (i) receptors height either neglected or duly taken as 0.76 m, (ii) measurement noises and (iii) number of receptors utilized in the inversion. In addition, the limitations and meteorological issues related to the inversion of an elevated release are highlighted.

Description: The first global climatology of monsoon low pressure systems is presented here, based on the ERA-Interim reanalysis. Low pressure systems are classified into three intensity categories, and particular focus is given to systems in the category corresponding to a traditional definition of monsoon depressions. Vortex tracks are identified using an automated algorithm applied to the distributions of 850 hPa relative vorticity, sea level pressure, and surface wind speed for 1979 through 2012. Roughly two to three times as many monsoon low pressure systems form in the northern hemisphere as in the southern hemisphere during local summer. The frequency of genesis typically peaks in local summer, but low pressure systems form throughout the year in every monsoon region. Interannual variability is weak, with standard deviations of summer counts typically being below ten percent of the long-term summer mean. Regional composites reveal that monsoon depressions in India, the western Pacific, and northern Australia share a common structure consisting of a warm-over-cold core and a top-heavy column of potential vorticity that extends from the surface to the upper troposphere. A separate class of monsoon low pressure systems develops over dry regions of West Africa and western Australia, with a shallow composite structure having a warm core in the lower troposphere and cyclonic potential vorticity confined to a thin near-surface layer. Low pressure systems in nearly all monsoon regions are estimated to account for a large fraction, from about 40% to more than 80%, of summer precipitation on the poleward edge of the climatological mean precipitation maxima.

Description: This paper discusses the possible response of the large-scale atmospheric structure to a warmer climate. Using integrations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5) in conjunction with physical arguments, we try to identify what changes are likely to be robust and what the underlying mechanisms might be. We focus on the large-scale zonally-averaged circulation, in particular on height of the tropopause, the strength and position of the surface westerlies and the strength and extent of the Hadley Cell. We present analytic arguments and numerical calculations that suggest that under global warming the height of the tropopause will increase in both the transient response and final equilibrium state, and an increase is clearly found in all the comprehensive models in CMIP5. Upper stratospheric cooling is also found in the comprehensive models, and this too can be explained by a radiative argument. Regarding the circulation, most models show a slight expansion and weakening of the Hadley Cell, depending on season and hemisphere. The expansion is small and largely confined to winter but with some expansion in Southern Hemisphere summer. The weakening occurs principally in Northern Hemisphere but the intermodel scatter is large. There is also a general polewards shift in surface westerlies, but the changes are small and again are little larger than the inter-model variability in the change. This shift is positively correlated with the Hadley Cell expansion to a degree that depends somewhat on the metric chosen for the latter. There is a robust strengthening in the Southern Hemisphere surface winds across seasons. In the Northern Hemisphere there is a slight strengthening in the westerlies in most models in winter but a consistent weakening of the westerlies in summer. We present various physical arguments concerning these circulation changes but none that are both demonstrably correct and that account for the model results. We conclude that the above-mentioned large-scale thermodynamic/radiative changes in the large-scale atmospheric structure are generally robust, in the sense of being both well understood and consistently reproduced by comprehensive models. In that sense the dynamical changes are less robust given the current state of knowledge and simulation, although one cannot conclude that they are, in principle, unknowable or less predictable.

Description: Demonstrating the effect that climate change is having on regional weather is a subject which occupies climate scientists, government policy makers and the media. After an extreme weather event occurs, the question is often posed, “Was the event caused by anthropogenic climate change?” Recently, a new branch of climate science (known as attribution) has sought to quantify how much the risk of extreme events occurring has increased or decreased due to climate change. One method of attribution uses very large ensembles of climate models computed via volunteer distributed computing. A recent advancement is the ability to run both a global climate model and a higher resolution regional climate model on a volunteer's home computer. Such a setup allows the simulation of weather on a scale that is of most use to studies of the attribution of extreme events. This paper introduces a global climate model that has been developed to simulate the climatology of all major land regions with reasonable accuracy. This then provides the boundary conditions to a regional climate model (which uses the same formulation but at higher resolution) to ensure that it can produce realistic climate and weather over any region of choice. The development process is documented and a comparison to previous coupled climate models and atmosphere only climate models is made. The system (known as weather@home) by which the global model is coupled to a regional climate model and run on volunteer's home computers is then detailed. Finally, a validation of the whole system is performed, with a particular emphasis on how accurately the distributions of daily mean temperature and daily mean precipitation are modelled in a particular application over Europe. This builds confidence in the applicability of the weather@home system for event attribution studies.

Description: We developed an empirical relative wind direction (RWD) model function to represent azimuthal variations of oceanic microwave radiances of vertical and horizontal polarizations. The RWD model function was based on radiance measurements from the Advanced Microwave Scanning Radiometer and Special Sensor Microwave Imager Sounder (SSMIS). Ocean surface wind vector data from SeaWinds on board the Advanced Earth Observing Satellite – II and European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System were utilized for the RWD model function development. The RWD model function was introduced to a microwave ocean emissivity model; a FAST microwave Emissivity Model (FASTEM) in a radiative transfer model for satellite radiance assimilation. Performances of the RWD model function were much more realistic than present azimuthal model functions in FASTEM for low wind speed and high frequency channels. Assimilation experiments using the RWD model function were performed in the ECMWF system. The experiment demonstrated reductions of first guess departure biases arising from modelling of the azimuthal variations in areas of high wind speed and low variability of wind direction. For example, bias reductions in ascending and descending SSMIS 19 GHz vertical polarized radiance in Somali jet at the Arabian Sea were approximately 0.6 K and 0.7 K. The bias reductions were found for all assimilated microwave imager channels in a wide wind speed range. Moreover, analysis increments of specific humidity in the lower troposphere were reduced (e.g. 0.2 g kg −1 reduction at 1000 hPa in the Somali jet). We found improvements of relative humidity and temperature in short-range forecasts in the lower troposphere. The experiment results clearly showed the importance of modelling the azimuthal variation of emissivity for assimilation of microwave imager observations. The new RWD model function, combined with the other components of FASTEM, will be available as FASTEM-6.

Description: Synoptic-scale precursors of typical East Asia/ Pacific (EAP) teleconnection pattern responsible for persistent extreme precipitation events (PEPEs) in the Yangtze River Valley (YRV) are investigated based on a composite analysis. The results reveal that about one week prior to PEPEs, a blocking high develops near the Sea of Okhotsk owing to an eastward energy dispersion and further strengthens markedly due to a poleward energy dispersion from low latitudes. Subsequently, a meridional tripole structure of typical EAP pattern becomes well established by this blocking and a westward-migrated strong negative anomaly at mid-latitudes/positive anomaly at lower latitudes. In the lower troposphere, a westward-progressive anomalous anticyclone-cyclone pair can be identified up to about a week prior to PEPEs, contributing to greatly enhanced moisture transport towards the YRV with a magnitude anomaly over 3 standard deviations above normal. A mid-latitude anomalous cyclone associated with the EAP pattern evolution and the eastward-extended South Asia High combine to provide favorable upper-level divergence. Correspondingly, strong ascent of low-level warm/moist air along a quasi-stationary front leads to PEPEs in the YRV. A contrastive analysis between evolution of typical wet and dry EAP regimes indicates that EAP-induced PEPEs are more likely to occur in the YRV with the ridge of the western Pacific subtropical high typically staying around northeastern quadrant of the South China Sea. This contrastive analysis also highlights the importance of the upstream pre-existing ridge to early strengthening of the Okhotsk blocking.

Description: The connection between the large-scale tropical circulation of the atmosphere, convective mixing, and climate sensitivity is explored in a wide range of climates through a perturbed-parameter ensemble of a comprehensive Earth-System Model. Four parameters related to the representation of atmospheric moist convection are found to dominate the response of the model. Their values govern the strength of the tropical circulation, the surface temperature, atmospheric humidity, and the strength of the tropical overturning circulation, largely through their influence on the atmospheric stability. The same convective parameters, albeit in different combinations, also have a strong influence on the equilibrium climate sensitivity of the model, which ranges from a little over 3 ∘ C to more than 10 ∘ C. The importance of the most poorly represented processes in determining important aspects of the behavior of the model argues for the need to move beyond statistical approaches to estimating climate sensitivity and focusing on the development of a better understanding and representation of convective mixing, particularly in the tropics.

Description: We propose a method to account for model error due to unresolved scales in the context of the Ensemble Transform Kalman Filter (ETKF). The approach extends to this class of algorithms the deterministic model error formulation recently explored in variational schemes and extended Kalman filter. The model error statistic required in the analysis update is estimated using historical reanalysis increments and a suitable model error evolution law. Two different versions of the method are described; a time-constant model error treatment where the same model error statistical description is time-invariant, and a time-varying treatment where the assumed model error statistics is randomly sampled at each analysis step. We compare both methods with the standard method of dealing with model error through inflation and localization, and illustrate our results with numerical simulations on a low order nonlinear system exhibiting chaotic dynamics. The results show that the filter skill is significantly improved through the proposed model error treatments, and that both methods require far less parameter tuning than the standard approach. Furthermore, the proposed approach is simple to implement within a pre-existing ensemble based scheme. The general implications for the use of the proposed approach in the framework of square-root filters such as the ETKF are also discussed.

Description: Global numerical weather prediction skill over polar areas is assessed, mostly based on the ECMWF system but also Met Office, JMA, Env. Canada and NCEP analysis data. Polar forecast verification against analyses shows a similar trend of forecast improvement over the past 12 years compared to improvements at lower latitudes. These improvements are presumably due to increased model resolution and model sophistication, improved data assimilation methods, and increased observational data coverage and better data quality. By comparing ECMWF’s real-time forecast skill changes against those from reforecasts initialized from reanalyses, it is possible to quantify how much of the improvement is from system improvements and how much is attributable to weather variability. Ensemble skill also improved over time and, again, consistently across latitudes. The quality of analyses serving for forecast verification and initialization has been further investigated. An intercomparison of TIGGE (THORPEX Interactive Grand Global Ensemble) analyses and forecasts revealed substantial differences for surface parameters but also at lower levels in the troposphere where most of the physical processes relevant to weather in the short-to-medium range take place over the poles. The differences between the TIGGE analyses was generally much larger than differences between members of the ECMWF 4D-Var ensemble of analyses generated internally at ECMWF. This suggests neither the multi-analysis approach nor the ensemble data assimilation may properly represent polar analysis uncertainty. This is particularly visible at the surface and lower levels in the atmosphere. Forecast spread and error match much better north of 65 N along the entire forecast range while in areas of significant synoptic activity spread also appears too low.

Description: Stationarity is a fundamental assumption in the statistical investigation of turbulence and the development of similarity functions that are widely used in surface-layer parameterization schemes. Nonstationary time series should therefore be removed from the analysis before assessing turbulence statistics used in similarity functions. Many approaches have been developed over the years to determine nonstationarity of means and (co-)variances but there has been no systematic investigation of the differences and similarities between the approaches. In this paper, we contrast several frequently used approaches, including two statistical tests to determine trends, the determination of a nonstationarity ratio, and the determination of differences in turbulence statistics calculated for different averaging times. We apply these approaches to near-surface time series of wind and temperature collected during the Terrain-induced Rotor Experiment (T-REX). Our results show that the degree of nonstationarity varies considerably with the used approach. Investigation of the time series that are declared stationary by all the above approaches simultaneously reveals that in many cases such time series still show a behavior indicative of intermittent turbulence, both for stable and unstable conditions. When these tests are combined with an additional condition for the intermittency level, a rigorous approach for the detection of nonstationarity is developed.

Description: The sensitivity of an operational numerical weather prediction model to parametrized microphysical properties of ice hydrometeors is examined. The effects of varying ice particle size distribution, fallspeed, mass and depositional capacitance are considered in kilometer-scale simulations of mid-latitude cloud systems and twenty-year global climate integrations. It is shown that the observed sensitivity can be obtained from steady state arguments whereby the vertical moisture flux is balanced by the hydrometeor sedimentation flux and supersaturation production is in equilibrium with depositional growth. The high resolution simulations are compared to insitu measurements from the Constrain field campaign (Prestwick, UK (2010)).

Description: This paper analyses ubiquitous flow structures that affect the dynamics of stable atmospheric boundary layers. These structures introduce non-stationarity and intermittency to turbulent mixing, thus invalidating the usual scaling laws and numerical model parametrizations, but their characteristics and generating mechanisms are still generally unknown. Detecting these unknown events from time series requires techniques that do not assume particular geometries or amplitudes of the flow structures. We use a recently developed such method with some modifications to study the nighttime structures over a three-month period during the FLOSSII experiment. The structures cover about 26% of the dataset, and can be categorized using clustering into only three classes with similar characteristics. The largest class, including about 50% of the events, contains smooth structures, often with wave-like shapes, that occur in stronger winds and weak stability. The second class includes sharper structures with large kurtosis. It is characterized by weaker winds and stronger stability. The smallest class, including about 20% of the events, contains predominantly sharp step-like structures, or microfronts. They occur in weakest winds with strong stability. Sharper, and particularly shallower, structures are related to transient low-level wind maxima that create inflection points and may affect generation of turbulence. Furthermore, large wind directional shear, which is another source of transient inflection points, is generated even by deep coherent structures when the background wind is weaker than the structure intensity. These results show that the complexity of structures can be reduced for the purpose of further analysis using a proper classification. Mapping common characteristics of such events leads to their better understanding, which, if combined with similar analyses of other boundary layer data, could lead to improving their effects in numerical models.

Description: A strong ocean–atmosphere coupling exists in the eastern equatorial region of the Tropical Atlantic at intraseasonal timescales, with a significant contribution in the functioning and partitioning of the water cycle in spring over the ocean, as well as later in the season over West Africa. Uncertainties in simulating the air-sea interaction in the Gulf of Guinea and its impact on the water cycle are studied using modeling experiments during spring-summer 2006 with the Weather Research and Forecasting model (WRF). Tested parameters include physical package of cumulus ( Cu ), planetary boundary layer ( PBL ), microphysics ( MP ) and radiative ( RAD ) schemes. The simulations are compared with satellite-based observations, ship-based radiosonde data and the state of art of atmospheric model reanalyses. Results show that Cumulus, Microphysics and Radiatives parameterizations exert a large influence in the simulated seasonal distribution of regional convective rainfall. Non-local PBL schemes are determinant to simulate the correct surface wind pattern and water vapor distribution in order to get realistic precipitation from intra-seasonal to diurnal scales, especially over the ocean where the nocturnal rainfall representation is improved.

Description: The influence of the valley geometry on thermally driven flows is studied by means of high resolution simulations. An idealised valley-plain topography and a spatially constant but time dependent surface sensible heat flux is used to generate upslope, upvalley and plain-to-mountain winds. A systematic variation of valley depth, width and length induces differences in the cross- and along-valley flow field and thermal structure of the boundary layer. The deeper the valley the stronger are the upvalley winds and the more favoured is the formation of vertically stacked circulation cells and an elevated valley inversion layer. Upvalley winds become weaker for wide valleys. The development of plain-to-mountain circulations considerably increases vertical exchange processes between the boundary layer and the free atmosphere compared to vertical transport processes over a plain. The analysis of mass flux budgets and forward trajectories indicates that mass is transported three to four times more effectively from the surface to the free atmosphere over valleys than over flat terrain. Vertical transport processes are strongest for deep and narrow valleys.

Description: Air-sea exchanges play an important role during intense weather events over the Mediterranean Sea, especially in supplying heat and moisture for heavy precipitation events that often affect the area. Observations collected during the first HyMeX Special Observation Period (SOP1) over the Western Mediterranean area in autumn 2012 provide an unprecedented dataset for assessing the capabilities of numerical weather prediction systems to represent the air-sea interface and marine boundary layer during the heavy precipitation season. A HyMeX-dedicated version of AROME covering the whole western Mediterranean basin, named AROME-WMED , was evaluated through comparisons against moored buoys, drifting buoys and ship measurements deployed during the HyMeX campaign. A generally good agreement is found for near-surface meteorological parameters, whereas significant discrepancies are observed during strong air-sea exchange periods. The two main reasons are (i) SST is kept constant during the model runs; and (ii) sensible heat flux is overestimated in strong wind regimes by the AROME turbulent flux parameterisation. Air-sea exchanges during SOP1 were characterised thanks to AROME-WMED short-range (1-24 h) forecasts. It shows some areas of strong air-sea fluxes in the Gulf of Lion, and the Balearic, Ligurian and Tyrrhenian Seas. The Gulf of Lion is the area showing the highest variability of air-sea fluxes, due to dominant strong regional winds (Mistral/Tramontane). Whereas some heavy precipitation events occur without significant air-sea fluxes, all strong air-sea exchange events include, or occur only one or two days before, heavy precipitation events. A detailed analysis of an Intense Observation Period (IOP) dedicated to a heavy precipitation event (IOP13, from 12 to 15 October) illustrates how both dynamic (wind) and thermodynamic (temperature and humidity gradient effect) contributions influence air-sea flux evolution.

Description: European and North American winter weather is dominated by year to year variations in the North Atlantic Oscillation (NAO) which controls the direction and speed of the prevailing winds. An ability to forecast the time-averaged NAO months to years ahead would be of great societal benefit, but current operational seasonal forecasts show little skill. However, there are several elements of the climate system that potentially influence the NAO and may therefore provide predictability for the NAO. We review these potential sources of skill, present emerging evidence that the NAO may be usefully predictable (with correlations exceeding 0.6) on seasonal timescales, and discuss prospects for improving skill and extending predictions to multi-year timescales.

Description: Data assimilation experiments of radiances from the microwave sounder SAPHIR on-board the Megha-Tropiques satellite (MT) are conducted with the Numerical Weather Prediction global model operational at Météo-France (ARPEGE) for the June-to-August 2012 period. The combination of MT low-inclined orbit with SAPHIR’ six channels was found to increase the number of assimilated humidity-sensitive microwave observations in the Tropics by a factor of 3.8, compared to a control experiment where three AMSU-B/MHS microwave sounders are already assimilated. Impacts of SAPHIR on both analyses through the 4D-Var system and forecasts are found to be positive with respect to radiosonde data and ECMWF analyses up to 12 hours and neutral to positive up to 72 hours. In particular, the improvement of innovation statistics for other humidity-sensitive satellite observations when SAPHIR is assimilated indicates a positive synergy of MT with the present observing system. In addition to providing new humidity observations in the Tropics, SAPHIR data can also be used to define new diagnostics for numerical weather prediction models. In particular, (i) MT’s orbit being non-heliosynchronous, all local times are observed by the same sensor for a given area which makes diurnal cycle diagnostics straightforward to set up, (ii) SAPHIR providing more information on the vertical (6 channels) than similar sensors, it can be easily used to study water vapour vertical structures in NWP models. The first diagnostic applied to the ARPEGE model reveals a good consistency between observed and modelled diurnal cycles in analyses over Western Africa, but also reveals a 2–4 hour time lag between mean observed and equivalent model brightness temperatures . The second diagnostic highlights a well-represented vertical correlation of simulated brightness temperatures from ARPEGE with respect to SAPHIR over Western Africa, but a too low vertical correlation over drier regions such as the Southern Pacific Ocean during the June-to-August 2012 period.

Description: Tropical precipitation is caused by many processes that occur over a wide-range of temporal and spatial scales. Such processes vary from local, diurnal convection driven by a destabilisation of the boundary layer to planetary-scale systems that result in rainfall over many days. It is therefore important to assess whether general circulation models (GCMs) can represent these processes given that such models are routinely used to project future rainfall in the low-latitudes. In this study, we evaluate the rainfall and circulation characteristics of ten GCMs from the Coupled Model Intercomparison Project Phase 5 (CMIP5) over northern Australia. This work shows that the diurnal cycle of the low-level (925 hPa) flow around the heat low is represented well by the models but the timing of precipitation is not (triggered too early). There is also evidence that mid-level synoptic systems that are responsible for initiating rain in the observations are also present in all of the models. Nevertheless, the biases in the modelled seasonal mean precipitation seem to be linked to the strength of both the meridional flow into northern Australia and the vertical mass flux. Furthermore, there is also evidence that the representation of convection in these models is likely to be contributing to both the precipitation and circulation errors over northern Australia.

Description: We introduce an innovative wavelet-based approach to dynamically adjust the local grid resolution to maintain a uniform specified error tolerance. Extending the work of [Dubos and Kevlahan(2013)], a wavelet multi-scale approximation is used to make dynamically adaptive the TRiSK model [Ringler ~ al.(2010)Ringler, Thuburn, Klemp and Skamarock] for the rotating shallow water equations on the sphere. This paper focuses on the challenges encountered when extending the adaptive wavelet method to the sphere and ensuring an efficient parallel implementation using mpi. The wavelet method is implemented in fortran95 with an emphasis on computational efficiency and scales well up to O (10 2 ) processors for load-unbalanced scenarios and up to at least O (10 3 ) processors for load-balanced scenarios. The method is verified using standard smooth test cases [Williamson ~ al.(1992)Williamson, Drake, Hack, Jakob and Swarztrauber] and a nonlinear test case proposed by [Galewsky ~ al.(2004)Galewsky, Scott and Polvani]. The dynamical grid adaption provides compression ratios of up to 50 times in a challenging homogenous turbulence test case. The adaptive code is about three times slower per active grid point than the equivalent non-adaptive TRiSK code and about four times slower per active grid point than an equivalent spectral code. This computationally efficient adaptive dynamical core could serve as the foundation on which to build a complete climate or weather model.

Description: This paper shows how one can formulate the representation problem starting from Bayes theorem. The purpose of this paper is to raise awareness to the formal solutions, so that approximations can be placed in a proper context. The representation errors appear in the likelihood, and the different possibilities for the representation of reality in model and observations are discussed, including nonlinear representation probability density functions. Specifically, the assumptions needed in the usual procedure to add a representation error covariance to the error covariance of the observations are discussed, and it is shown that when several sub-grid observations are present their mean still has a representation error, so so-called ’superobbing’ does not resolve the issue. Connection is made to the off-line or on-line retrieval problem, providing a new simple proof of the equivalence of assimilating linear retrievals and original observations. Furthermore, it is shown how nonlinear retrievals can be assimilated without loss of information. Finally we discuss how errors in the observation operator model can be treated consistently in the Bayesian framework, connecting to previous work in this area.

Description: Simulation models are widely employed to make probability forecasts of future conditions on seasonal to annual lead times. Added value in such forecasts is reflected in the information they add either to purely empirical statistical models, or to simpler simulation models. An evaluation of seasonal probability forecasts from the DEMETER and the ENSEMBLES multi-model ensemble experiments is presented. Two particular regions are considered (Nino3.4 in the Pacific and Main Development Region in the Atlantic); these regions were chosen before any spatial distribution of skill were examined. The ENSEMBLES models are found to have skill against the climatological distribution on seasonal time scales. For models in ENSEMBLES which have a clearly defined predecessor model in DEMETER, the improvement from DEMETER to ENSEMBLES is discussed. Due to the long lead times of the forecasts and the evolution of observation technology, the forecast-outcome archive for seasonal forecast evaluation is small; arguably evaluation data for seasonal forecasting will always be precious. Issues of information contamination from in-sample evaluation are discussed, impacts (both positive and negative) of variations in cross-validation protocol are demonstrated. Other difficulties due to the small forecast-outcome archive are identified. The claim that the multi-model ensemble provides a “better" probability forecast than the best single model is examined and challenged. Significant forecast information beyond the climatological distribution is also demonstrated in a persistence probability forecast. The ENSEMBLES probability forecasts add significantly more information to empirical probability forecasts on seasonal time scales than on decadal scales. Current operational forecasts might be enhanced by melding information both from simulation models and from empirical models. Simulation models based on physical principles are sometimes expected, in principle, to outperform empirical models; direct comparison of their forecast skill provides information on progress toward that goal.

Description: This paper describes the UK Met Office Global Seasonal forecast system version 5 (GloSea5). GloSea5 upgrades include an increase in horizontal resolution in the atmosphere (N216–0.7 ∘) and the ocean (0.25 ∘ ), and implementation of a 3D VAR assimilation system for ocean and sea-ice conditions. GloSea5 shows improved year-to-year predictions of the major modes of variability. In the tropics, predictions of the El Niño-Southern Oscillation are improved with reduced errors in the west Pacific. In the extratropics, GloSea5 shows unprecedented levels of forecast skill and reliability for both the North Atlantic Oscillation and the Arctic Oscillation. We also find useful levels of skill for the Western North Pacific Subtropical High which largely determines summer precipitation over East Asia.

Description: Linear post-processing approaches are proposed and fundamental mechanisms by which the probabilistic skill of an ensemble forecast can be improved are analyzed. The ensemble mean of the corrected forecast is a linear function of the ensemble mean(s) of the predictor(s). Likewise, the ensemble spread of the corrected forecast depends linearly on the one of the uncorrected forecast. The regression coefficients are obtained by maximizing the likelihood function for the error distribution. Comparing different calibration approaches on simple systems that exhibit chaotic features (the Kuramoto-Sivashinsky equation, the spatially-extended Lorenz system), four correction mechanisms are identified: the ensemble-mean scaling and nudging using the predictor(s), and, the ensemble-spread scaling and nudging. Ensemble-spread corrections turn out to yield improvement only when “reliability” constraints are imposed on the corrected forecast. First of all climatological reliability is enforced and is satisfied when the total variability of the forecast is equal to the variability of the observations. Second, ensemble reliability or calibration of the ensembles is enforced such that the squared error of the ensemble mean coincides with the ensemble variance. In terms of continuous ranked probability skill score, spread calibration provides much more gain in skill than the traditional ensemble-mean calibration and extends for lead times far beyond the error doubling time. The skill performance is better or as good as the benchmark calibration method which derives from statistical assumptions, Nonhomogeneous Gaussian Regression. In addition to the member-by-member nature of the approach, benefits can be pinpointed as compared to the benchmark method. In particular, although the post-processing methods are performed for each lead time, location and variable independently, they preserve the rank correlations and thus take dependencies across space, time, and different variables into account. In addition, higher-order ensemble moments like kurtosis and skewness correspond to the ones of the uncorrected forecasts.

Description: Warm conveyor belts are the main ascending airmass within an extratropical cyclone. They often exhibit strong condensation and precipitation, associated with ascent on large-scales or embedded convection. Most of the air outflows into the upper troposphere as part of a ridge. Such ridges are an integral part of Rossby waves propagating along the tropopause and are identified with a negative potential vorticity (PV) anomaly and associated anti-cyclonic circulation. It has been argued that diabatic modification of PV in WCB’s has an important influence on the extent of the ridge, propagation of Rossby waves and weather impacts downstream. Following the coherent ensemble of trajectories defining a WCB, PV is expected to increase with time while below the level of maximum latent heating and then decrease as trajectories ascend above the heating maximum. In models it is found that the net change is approximately zero so that the average PV of the WCB outflow is almost equal to the PV of its inflow. Here, the conditions necessary for this evolution are explored analytically using constraints arising from the conservation of circulation. It is argued that the net PV change is insensitive to the details of diabatic processes and the PV maximum midway along a WCB depends primarily on the net diabatic transport of mass from the inflow to outflow layer. The main effect of diabatic processes within a WCB is to raise the isentropic level of the outflow, rather than modification of PV.

Description: The Prandtl model couples, probably in the most succinct way, basic boundary-layer dynamics and thermodynamics for pure anabatic and katabatic flows over inclined surfaces by assuming 1D steady-state balance between buoyancy and turbulent friction. While the classical Prandtl model is linear, having an a priori assigned vertically constant eddy diffusivity and heat conductivity, K , in this analytic work we partly relax both of these restrictions. The first restriction is loosened by using a weakly non-linear approach where a small parameter, ε , controls feeding of the flow-induced potential temperature gradient back to the environmental potential temperature gradient since the former, below the katabatic jet, can be 20 to 50 times stronger than the latter, background or free-flow gradient. An appropriate range of values for ε , controlling the weak nonlinearity for pure katabatic flow, is provided. In this way, the near-surface potential temperature gradient becomes stronger and the corresponding katabatic jet somewhat weaker (at a slightly lower height) than that in the classical Prandtl solution. The second restriction is partly relaxed by using a prescribed, gradually varying K with distance from the underlying surface, all within the usual validity of the zero-order WKB approximation to solve the coupled differential equations. The new model is compared to the glacier wind data from PASTEX-94, Austria. Further discussion includes gradient Richardson number consideration and an application to simple anabatic flows. The model may be applied for estimation and interpretation of the wind affecting glacier mass balance and air-pollution.

Description: Two ensemble data assimilation methods are used to assimilate Doppler radar observations into a convection-allowing model. The analyses and subsequent forecasts from the two systems are compared. The Local Ensemble Transform Kalman Filter (LETKF) simultaneously assimilates all observations that can impact the model state at a given location. It is compared to the Ensemble Square Root Filter (EnSRF), which assimilates observations sequentially and has commonly been used for convective-scale Doppler radar data assimilation. While the filters should behave the same for ideal systems, a comparison between the serial and simultaneous filters has not previously been explored at the convective-scale where significant non-linear effects are present. Observing System Simulation Experiments (OSSEs) are first used to compare the assimilation systems for the analysis and forecast of a supercell thunderstorm. Both the EnSRF and LETKF produce reasonable analyses from the Doppler velocity and reflectivity observations of the true supercell. Small improvements in analysis errors and system noise from the LETKF simultaneous update do not significantly impact the subsequent forecasts. This result is consistent across a range of localization length scales and is independent of the manner in which localization is applied. Tests comparing the EnSRF and LETKF for a real-data case also have small differences. The magnitudes of these differences are similar to those that arise from the sampling variability associated with a finite ensemble. Overall, the results suggest the EnSRF and LETKF approaches are equally capable methods for radar data assimilation at convective-scales.

Description: We develop a dynamical-statistical downscaling approach by coupling the PRECIS regional modeling system and a statistical method – SCADS to construct very high resolution climate projections for studying climate change impacts at local scales. The coupled approach performs very well in hindcasting the mean temperature of present-day climate, while the performance for precipitation is relatively poor due to its high spatial variability and nonlinear nature but its spatial patterns are well captured. We then apply the coupled approach for projecting the future climate over the province of Ontario, Canada at a fine resolution of 10 km. The results show that there would be a significant warming trend throughout this century for the entire province while less precipitation is projected in most of the selected weather stations. The projections also inform apparent spatial variability in the amount of precipitation but no noticeable changes are found in its spatial patterns.

Description: Large-Eddy Simulation (LES) is used to examine the complex interactions between cloud properties and boundary layer structure in Arctic low-level mixed-phase clouds using idealised conditions based on the Indirect and Semi-Direct Aerosol Campaign (ISDAC, April 2008). The persistence of steady mixed-phase conditions depends mostly on a balance between ice vertical redistribution and ice growth by vapour deposition in such a way that ice crystals cannot accumulate within the cloud layer to consume the available liquid water. An external source of water vapour is necessary to balance the net sink of total water in the cloud layer. Two main local sources of moisture are present: the initial moist surface layer and the free troposphere. In the studied case, the surface layer is found to be the dominant source of vapour to the cloud, the temperature inversion preventing significant entrainment from above. In most of the cases, the simulated boundary layer becomes rapidly well-mixed despite the stabilising effect of ice sublimation and latent cooling close to the surface. The minor effect of near-surface latent cooling on stability is connected to the initially moist surface layer limiting ice sublimation. Water vapour supply in the sub-cloud layer resulting from entrainment of moisture from aloft, reduces ice sublimation above the surface layer and contributes to the maintenance of some degree of boundary layer decoupling. In contrast, moisture surface fluxes reduce sublimation in the surface layer and accelerate cloud-surface coupling. Overall, the persistence of cloud-surface decoupling remains mostly driven by large-scale heat and moisture advection.

Description: A bulk air-sea flux algorithm couples the ocean and the lower atmosphere through flux boundary conditions and can be used in various analyses and in numerical models. The algorithm described here has two features not present in any other existing bulk flux algorithm. First, it has a new air-sea drag relation. Here, for wind speeds above about 9 m s −1 , the friction velocity u * , which is related to the square root of the surface stress, is linearly related to U N10 , the neutral-stability, 10-metre wind speed. When extrapolated to hurricane-strength winds, this drag relation has better properties than relations formulated in terms of a drag coefficient or a roughness length. The second unique feature of this flux algorithm is that it recognizes two routes by which heat and moisture cross the air-sea interface: one is the interfacial route that is controlled by molecular processes right at the air-sea interface; the second is the spray -mediated route that is governed by microphysical processes at the surface of sea spray droplets. Through microphysical theory and our analysis of 4000 sets of eddy-covariance measurements of the scalar fluxes, we separate the measured fluxes into the interfacial and spray contributions and thereby produce the only spray flux algorithm tested and validated against oceanic data. Because all components of our flux algorithm are physics based and validated with data for winds up to 25 m s −1 , one application is extrapolating this algorithm to hurricane-strength winds, where sea spray plays a dominant role in scalar transfer.

Description: Near-surface and lower-tropospheric warming of the Arctic over the past 35 years is examined for several datasets. The new estimate for the near surface from Cowtan and Way (2014) agrees reasonably well with the ERA-Interim reanalysis for this region. Both provide global averages with a little more warming over recent years than indicated by the widely used HadCRUT4 dataset, which has sparse coverage of the high Arctic. ERA-Interim is more sensitive than the Cowtan and Way estimate to the state of the underlying Arctic Ocean. Observational coverage of the Arctic varies considerably over the period. Surface air-temperature data of identified types are generally fitted well by ERA-Interim, especially data from ice stations, which appear of excellent quality. ERA-Interim nevertheless has a warm wintertime bias over sea-ice. Mean fits vary in magnitude as coverage varies, but their overall changes are much smaller than analysed temperature changes. This is also largely the case for fits to data for the free troposphere. Much of the information on trends and low frequency variability provided by ERA-Interim comes from its background forecast, which carries forward information assimilated from a rich variety of earlier observations, rather than from its analysis of surface air-temperature observations. ERA-Interim agrees quite well with the new JRA-55 reanalysis, and with the MERRA reanalysis until recent years when MERRA exhibits weaker surface warming. Temperatures vary coherently between the surface and middle troposphere, with largest amplitude at the surface except in summer, when air temperatures are constrained by sea-ice and open-sea temperatures that differ little from 0 °C. Much of the recent near-surface warming of the Arctic is associated with reduced cold-season sea-ice cover, with low temperatures over ice replaced by much higher ones over open sea. This occurs primarily in a relatively well-observed region around the northernmost islands of Europe and western Asia.

Description: Calculation of momentum flux using Monin Obukhov similarity-theory over forested areas is well known to underestimate the flux. Several suggestions of corrections to the standard flux-profile expression have been proposed in order to increase the magnitude of turbulent flux. The aim of this paper is to find a simple, analytical representation for the characteristics of the flow within the canopy layer and the surface layer, including the roughness sublayer. A new form of the roughness sublayer correction is proposed, based on the desire to connect the shape of the roughness sublayer correction to forest characteristics. The new flux profile relation can be used to find the flux or the wind profile whenever simple and fast estimations are needed, as for meso-scale modelling, scalar transport models, or sound propagation models.

Description: Satellite infra-red sounders are invaluable tools for making observations of the structure of the atmosphere. They provide much of the observational data used to initialise atmospheric models, especially in regions that do not have extensive surface-based observing systems, such as oceans. However, in the presence of cloud, information is lacking, as the cloud layer is opaque to infra-red radiation. This means that where information is most desired (such as a developing storm), it is often in shortest supply. In order to explore the mathematics of assimilating data from cloudy radiances, a study has been performed using an idealised single column atmospheric model. The model simulates cloud development in an atmosphere with vertical motion, allowing the characteristics of a 2D-Var data assimilation system using a single simulated infra-red satellite observation taken multiple times to be studied. The strongly non-linear nature of cloud formation poses a challenge for variational methods. The adjoint method produces an accurate gradient for the cost function, and minimisation is achieved using preconditioned conjugate gradients. The conditioning is poor, and varies strongly with the atmospheric variables, and the cost function has multiple minima, but acceptable results are achieved. The assimilation system is provided with a prior forecast simulated by adding random correlated Gaussian error to the truth. Assimilating observations comparable to those available from current geostationary satellites allows vertical motion to be retrieved with error of less than a centimetre per second in most conditions. Moreover, evaluating the second derivative of the cost function at the minimum provides an estimate of the uncertainty in the retrieval. This allows atmospheric states that do not provide sufficient information for retrieval of vertical motion to be detected (such as a cloudless atmosphere or a non-moving opaque cloud layer in the upper troposphere). Retrieval is most accurate with upwards motion.

Description: On 29th October 2013 a severe squall line developed to the west of the Balearic Islands (Spain) ahead of an advancing cold front and then crossed the archipelago. We first provide an observational characterization of the event based on surface reports, remote sensing products, radiosoundings and synoptic information. We also achieve, by means of numerical experiments, new insights into the kinematic and thermodynamic factors that governed the genesis and evolution of the linear convective system. Radar and satellite images confirm the fast movement and linear shape of the system, with an indication of possible transition into a bow echo structure during its later stages. The synoptic setting at mid-upper tropospheric levels was dominated by a cold trough extended over western Europe associated with a jet stream located downstream. Convection evolved under the right-entrance region of the jet and initiated under the crucial influence of a surface low developed over the Mediterranean Sea ahead of the cold front. The low not only cooperated with the upper-level dynamical forcing to erode a capping inversion initially present over the Balearics and to moisten the atmospheric column above, but also shaped and enhanced a convergence line along which the first convective cells grew and self-aggregated. This scenario is confirmed by numerical simulations of the case, which also emphasize the important role of the regional topography in the production of the aforementioned maritime convergence through the mesoscale modulation of the low-level flow. Additional simulations show that (i) the destabilization of the low-level air mass necessary for triggering and feeding an organized convective system on 29th October 2013 was attributable to the prior evaporation from the Mediterranean and (ii) sea surface temperatures appear to be critical for a successful fine-grid numerical forecast of the mode, degree of severity, timing and track of the convective precipitation system.

Description: A new method to estimate the vertical part of the background error covariance matrix for an ocean variational data assimilation system is presented and tested in the Mediterranean operational daily analysis system. The operational, seasonally varying error covariances are compared with high frequency estimates from a Bayesian Hierarchical Model (BHM) that estimates distributions for the vertical error covariances from two data stage inputs: model anomalies and differences between model background and observations, i.e., so-called misfits. It is found that the posterior mean BHM-error covariance estimates that vary on 5-day timescales reduce the misfits root mean square of the analysis vertical profiles of temperature and salinity by 10-20% versus analyses arising from covariances that vary on seasonal timescales or those from the BHM given only model anomalies as data stage inputs.

Description: The Spherical Geopotential Approximation (SGA) used in most meteorological global models assumes a spherical shape for the Earth and its geopotential field, together with a horizontally-uniform gravity field (for physical consistency). This approximation has largely been used from the origins and still is used in operational meteorological forecast systems. However, the magnitude of the errors linked to this approximation is debated, especially for long integrations. Consistent mathematical models of the atmosphere in Ellipsoidal geopotential approximation (EGA) have recently been developed, thus allowing a clean assessment of the errors linked to using SGA. Two types of errors may be anticipated, those arising due to geometrical distortions, and those arising due to the spatial differences in the magnitude of the gravity field. The shallow-water system, which is the minimal framework where both types of errors are likely to manifest themselves, is used in this study. The link between ‘pure forecast errors’ examined here and ‘NWP errors’ that would arise in a concrete NWP application (with a Data Assimilation system) is discussed, in relation with the experimental protocol chosen here. Medium-range forecast errors for idealised fully deterministic flows are first considered in a phenomenological way (phase errors, distortions, etc.), then some widely documented real cases are examined.

Description: This paper describes the implementation of an incremental first guess at appropriate time 3DVAR data assimilation scheme, NEMOVAR, in the Met Office's operational 1/4 degree global ocean model. NEMOVAR assimilates observations of SST (sea surface temperature), SSH (sea surface height), in situ temperature and salinity profiles and sea ice concentration. The Met Office is the first centre to implement NEMOVAR at 1/4 degree and the required developments are discussed, with particular focus on the specification of the background error covariances. Background error correlations in NEMOVAR are modelled using a diffusion operator. The horizontal background error correlations for temperature, salinity and sea ice concentration are parameterised using the Rossby radius which produces relatively short correlation length-scales at mid to high latitudes, while a flow dependent mixed layer depth parameterisation is used to define the vertical length-scales for the 3D variables. Results from a one year reanalysis with NEMOVAR are presented and compared to the preceding operational data assimilation scheme at the Met Office. NEMOVAR is shown to provide significant improvements to SST, SSH and sea ice concentration fields with the largest improvements seen in regions of high variability such as eddy shedding and frontal regions and the marginal ice zone. This improvement is associated with shorter correlation length-scales in the extratropics and an improved fit to observations in NEMOVAR. Some degradation to subsurface temperature and salinity fields where data are sparse is identified and this will be the focus of future improvements to the system.

Description: Earth's extra-tropical troposphere is equilibrated by turbulent eddy fluxes of potential temperature and momentum. The equilibrated state has the remarkable characteristic that isentropic slopes leaving the surface in the sub-tropics reach the tropopause near the poles. It has been speculated that turbulent eddy fluxes maintain this state for a wide range of radiative forcing and planetary parameters. In a previous study the authors showed that this state needs to be associated with an eddy diffusivity of Ertel potential vorticity that is largest at the surface and decays through the troposphere to approximately zero at the tropopause. This result is confirmed in this study using atmospheric reanalysis and idealized numerical simulations. However, it is also shown that the vertical profile of the eddy diffusivity can change, resulting in different isentropic slopes and climates. This is illustrated with a series of idealized numerical simulations with varying planetary scales and rotation rates.

Description: Ertel's potential vorticity (PV) is used as a diagnostic tool to give a direct comparison between the treatment of PV in the dynamics and the integration of PV as a passive tracer, yielding a systematic evaluation of a model's consistency between the dynamical core's integration of the equations of motion and its tracer transport algorithm. Several quantitative and qualitative metrics are considered to measure the consistency including error norms and grid-independent probability density functions. Comparisons between the four dynamical cores of the National Center for Atmospheric Research's (NCAR) Community Atmosphere Model version 5.1 (CAM) are presented. We investigate the consistency of these dynamical cores in an idealized setting; the presence of a breaking baroclinic wave. For linear flow, before the wave breaks, the consistency for each model is good. As the flow becomes nonlinear the consistency between dynamic PV and tracer PV breaks down, especially at small scales. Large values of dynamic PV are observed that do not appear in the tracer PV. The results indicate that the spectral-element (CAM-SE) dynamical core is the most consistent of the dynamical cores in CAM, however, the consistency between dynamic PV and tracer PV is related to and sensitive to the diffusive properties of the dynamical cores.

Description: Predictions of the snow water equivalent (SWE) over Siberia in spring (March-April-May) and its relationship with the accompanied summer (June-July-August) precipitation over China are investigated using the reforecast (1983–2010) from the National Centers for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2). The CFSv2 for one month lead (LM0) well predicts the leading pattern of Eurasian SWE in spring, characterized by homogeneous variations over most of Eurasia centered in Siberia. Overall, the CFSv2 for LM0 exhibits a reasonable level of prediction skill for variations of Siberian spring SWE, but the initial SWE in the CFSv2 shows a dominant effect on the re-forecasted Siberian SWE climatology. Using the initial condition in April, the CFSv2 can predict the SWE over the Siberia in May and the corresponding summer rainfall pattern over China. Positive SWE in May in the CFSv2 is associated with negative summer rainfall anomalies in South and North China and part of Northeast China, and with positive anomalies between the Yangtze River and the Huaihe River and over Inner Mongolia and parts of the north of the Tibetan Plateau and southeastern Tibetan Plateau. The result is consistent with observed features.

Description: The formation of lower-tropospheric wind speed maxima is analyzed during the mature stage of two distinct windstorms using ERA-interim reanalysis datasets and performing an eddy kinetic-energy (EKE) budget. Both storms developed according to the Shapiro-Keyser conceptual model and crossed the large-scale low-frequency jet from its warm-air towards its cold-air side. The formation of strong wind regions are shown to depend on the position of the storms relative to the large-scale jet axis, which confirm theoretical results of a companion study. As long as the storms are travelling south of the low-frequency jet or close to the jet axis, the most intense EKE maxima as well as the total kinetic-energy maxima are located in the warm sector of the surface cyclones on their southeastern side. As soon as the surface cyclones move to the north of the low-frequency jet, EKE is cyclonically redistributed in the lower troposphere, first to the north-northwest of the cyclones center, and then to the southwest along the bent-back warm fronts. At this later stage, EKE, which is generated by baroclinic conversion in the mid-troposphere, is downward redistributed by the vertical ageostrophic geopotential fluxes before being southwestward redistributed in the lower troposphere by the ageostrophic geopotential fluxes. This EKE redistribution led to the formation of a low-level westerly jet to the south of each cyclone center behind the cold front. These common features among both storms happened in spite of differences in their shape and environment.

Description: Idealized simulations of deep moist convection are performed with the Meso-NH model at kilometric scales to assess the impact of horizontal grid spacing (4 km, 2 km, 1 km and 500 m) and turbulence scheme (one-dimensional (T1D) versus three-dimensional (T3D) turbulence). The simulations generate one cell which splits into two convective systems: a leftward-moving multicellular system and a rightward-moving supercell. Objective criteria based on statistical properties and bulk quantities are examined on both systems to characterize the convection. They show that the accumulated and corresponding rainfall surface area increase with increasing resolution as does the area covered by the updrafts, while the 90th quantile of the intensity of updraft cores decreases. The 4-km horizontal grid spacing is set apart as it clearly under resolves the convective motions, but the difference between 2-km and 1-km horizontal resolutions is larger than between 1-km and 500-m, suggesting a beginning of convergence at 500-m. Also 1-km appears better to represent the deep convective structures correctly for a practical weather forecast of organized convective systems . T3D induces more mixing and enhances the microphysical processes compared to T1D, producing larger amounts of cloud cover and precipitation. Also, the magnitude of the pressure anomaly on the southeastern flank of the supercell is stronger, accentuating the path curvature. The difference between T1D and T3D becomes perceptible at 2-km, pointing out the necessity to deal with horizontal turbulent fluxes at kilometric resolutions. Although a stronger numerical diffusion added to T1D allows the necessary damping to be introduced at the spectral energy tail, it removes a part of the physical mixing and still misses some variance at larger scales. The ratio between resolved and total turbulent kinetic energy (TKE) decreases with increasing resolution for both T1D and T3D, which is unexpected. The main explanation is the insufficient turbulent mixing inside convective clouds, more pronounced at coarser resolution, which is also confirmed by the vertical velocity spectra. At 500-m horizontal resolution, the subgrid TKE is mainly due to dynamical processes, with maxima located at the upper level of the convective systems in areas of stronger potential temperatures associated with downdrafts. But thermal production is mostly negative, underlying the lack of entrainment at the cloud edges.

Description: We construct a two-variable model which describes the interaction between local baroclinicity and eddy heat flux in order to understand aspects of the variance in storm tracks. It is a heuristic model for diabatically forced baroclinic instability close to baroclinic neutrality. The two-variable model has the structure of a nonlinear oscillator. It exhibits some realistic properties of observed storm track variability, most notably the intermittent nature of eddy activity. This suggests that apparent threshold behaviour can be more accurately and succinctly described by a simple nonlinearity. An analogy is drawn with triggering of convective events.