ALBERT

Description: This paper proposes a method to identify blocking onset and decay by means of two stability indicators: enstrophy advection and its integral. The key to this technique is the use of local Lyapunov exponents for the barotropic vorticity equation which can be approximated by the integral of enstrophy (IRE) over a fixed, finite, region. The IRE can then be viewed as a measure of stability. However, by differentiating the IRE with respect to time, two measures of stability can be derived to assess blocking onset and decay: (i) the integral of enstrophy advection (DIRE), for which a time series is used to assess stability; and (ii) enstrophy advection, for which contours are plotted in conjunction with 500 hPa heights to locate blocking. One year of Northern Hemisphere blocking events from July 2011-July 2012 are studied to demonstrate that the integral of enstrophy advection is a useful diagnostic. In particular, time series of IRE and DIRE for four of the blocking cases are presented, while contour plots of enstrophy advection for one case are presented. In all cases studied, the diagnostics were seen to detect the instability in an incipient blocking event and in its decay.

Description: The Weather Research and Forecasting (WRF) model was used to simulate the evolution of Tropical Storm Ivan (2004) in the southeast (SE) US using both the Yonsei University (YSU) and Mellor-Yamada-Janjić (MYJ) boundary layer parameterizations. In contrast with tropical cyclone (TC) simulations over the ocean, the effect of surface layer becomes secondary for a dissipating hurricane along its terrestrial track. Although these two schemes can reproduce Ivan reasonably well, our results suggest that the mixing properties for damped mechanical turbulent conditions (weakly stable) are strongly underestimated by both parameterizations. This underestimation impacts the thermodynamic properties of the storm, leading to significant differences in the storm areal extent and the simulated precipitation fields. Suggestions for further improvements are provided. An evaluation of the impact of using or not using a convective parameterization, specifically the Kain-Fritsch (KF) scheme, at 3 km grid spacing shows marginal impact on storm coverage, intensity and precipitation, except for the presence of widespread light rainfall in the Piedmont east of the mountains when the KF is employed. Analysis of the thermal structure of the simulated storm indicates that, in the inner-storm region, the KF is either not activated or primarily produces ( parameterized ) shallow convection. As a result, the net heating tendency associated with adiabatic and diabatic processes is almost unaltered inside the storm, together with a nearly equivalent surface momentum sink, leading to similar storm areal extent and intensity. Light rainfall to the east of the mountains can be due to the trigger mechanism of KF, which depends on boundary layer convergence, forcing parameterized deep convection near the coast, where surface roughness changes enhance convergence.

Description: Heavy precipitation events (HPEs) affect the south-eastern area of France frequently during the months of September to November. Very high amounts of rain can fall during these events, with the ensuing flash-floods causing widespread damage. The cases of the 6th of September 2010 and the 1st to the 4th of November 2011 represent the different large-scale conditions in which these episodes can occur. These HPEs are forecast with differing levels of skill by the Méso-NH model at a 2.5 km resolution. The case of the 6th of September is used to test different methods of addressing cloud physics parameterisation uncertainties. Three ensembles are constructed, where the warm process microphysical time tendencies are perturbed by different methods. Results are compared by examining the spatio-temporal distribution of the precipitation field as well as looking at ensemble statistics. The ensemble methodology which induces the most dispersion in the rainfall field is deemed the most suitable. This method is then used to examine the sensitivity of four cases from November 2011 to errors in the microphysical and turbulent parameterisations. It appears that according to the model skill for the HPE, the sensitivity to microphysical perturbations varies. Events where the model skill is high (low) show low (moderate) sensitivity. These cases show a stronger sensitivity to perturbations performed upon the turbulent tendencies, while perturbing the microphysical and turbulent tendencies together produces even further dispersion. The results show the importance and the usefulness of ensembles with perturbed physical parameterisations in the forecasting of HPEs.

Description: This paper analyses the annual mean vertical and latitudinal structure of the Brewer-Dobson circulation in the CMIP5 models. The strength of the tropical mass upwelling is found to increase at all altitudes throughout the stratosphere due to climate change. However, the width of the tropical upwelling region narrows below about 20hPa, and widens above 20hPa, suggesting different physical mechanisms may play a role in this change above and below 20hPa. In the lower stratosphere, an equatorward shift in the stationary wave critical line allows waves to propagate further into the tropics. However, in the upper stratosphere, where the behaviour is dominated by what happens during the winter, an increase in the extratropical zonal mean westerly jet leads to a reduced equatorward refraction of planetary waves. The seasonal cycle of the change in the Brewer-Dobson circulation is also considered, and differences are found in the latitudinal structure of the increased extratropical downwelling between the Northern and Southern Hemispheres in winter.

Description: Sub-seasonal forecasts have been routinely produced at ECMWF since 2002 with re-forecasts produced "on the fly" to calibrate the real-time sub-seasonal forecasts. In this study, the skill of the re-forecasts from April 2002 to March 2012 and covering a common set of years (1995 to 2001) has been evaluated. Results indicate that the skill of the ECMWF re-forecasts to predict the Madden Julian Oscillation has improved significantly since 2002, with an average gain of about 1 day of prediction skill per year. The amplitude of the MJO has also become more realistic, although the model still tends to produce MJOs which are weaker than in the ECMWF re-analysis. As a consequence, the ability of the ECMWF model to simulate realistic MJO teleconnections over the northern and southern Extratropics has improved dramatically over the 10-year period. Forecast skill scores have also improved in the Extratropics. For instance, weekly mean forecasts of the North Atlantic Oscillation Index are more skillful in recent years than ten years ago. A large part of this improvement seems to be linked to the improvements in the representation of the Madden Julian Oscillation. Skill to predict 2-metre temperature anomalies over the northern Extratropics has also improved almost continuously since 2002. Changes in the horizontal and vertical resolutions of the atmospheric model had only a small impact on the skill scores, suggesting that most of the improvements in the ECMWF sub-seasonal forecasts were due to changes in model physics which were primarily designed to improve the model climate and medium-range forecasts. The impact of changes in the data assimilation system and in the observing data has not been considered in this study, since all the re-forecasts used for this study were initialized from the same re-analysis over a common set of years.

Description: Dual-polarisation radar measurements provide valuable information about the shapes and orientations of atmospheric ice particles. For quantitative interpretation of these data in the Rayleigh regime, common practice is to approximate the true ice crystal shape with that of a spheroid. Calculations using the discrete dipole approximation for a wide range of crystal aspect ratios demonstrate that approximating hexagonal plates as spheroids leads to significant errors in the predicted differential reflectivity, by as much as 1.5dB. An empirical modification of the shape factors in Gans's spheroid theory was made using the numerical data. The resulting simple expressions, like Gans's theory, can be applied to crystals in any desired orientation, illuminated by an arbitrarily polarised wave, but are much more accurate for hexagonal particles. Calculations of the scattering from more complex branched and dendritic crystals indicate that these may be accurately modelled using the new expression, but with a reduced permittivity dependent on the volume of ice relative to an enclosing hexagonal prism.

Description: The parameterisation of diabatic processes in numerical models is critical for the accuracy of weather forecasts and for climate projections. A novel approach to the evaluation of these processes in models is introduced in this contribution. The approach combines a suite of on-line tracer diagnostics with off-line trajectory calculations. Each tracer tracks accumulative changes in potential temperature associated with a particular parameterised diabatic process in the model. A comparison of tracers therefore allows the identification of the most active diabatic processes and their downstream impacts. The tracers are combined with trajectories computed using model-resolved winds, allowing the various diabatic contributions to be tracked back to their time and location of occurrence. We have used this approach to investigate diabatic processes within a simulated extratropical cyclone. We focus on the warm conveyor belt, in which the dominant diabatic contributions come from large-scale latent heating and parameterised convection. By contrasting two simulations, one with standard convection parameterisation settings and another with reduced parameterised convection, the effects of parameterised convection on the structure of the cyclone have been determined. Under reduced parameterised convection conditions, the large-scale latent heating is forced to release convective instability that would otherwise have been released by the convection parameterisation. Although the spatial distribution of precipitation depends on the details of the split between parameterised convection and large-scale latent heating, the total precipitation amount associated with the cyclone remains largely unchanged. For reduced parameterised convection, a more rapid and stronger latent heating episode takes place as air ascends within the warm conveyor belt.

Description: Motivated by observations of the mean state of tropical precipitable water (PW), a moist, first baroclinic mode, shallow water system on an equatorial β -plane with a background saturation profile that depends on latitude and longitude is studied. In the presence of a latitudinal moisture gradient, linear analysis of the non-rotating problem reveals large-scale, symmetric, eastward and westward propagating unstable modes. The introduction of a zonal moisture gradient breaks the east–west symmetry of the unstable modes. The effects of rotation are then included by numerically solving the resulting eigenvalue problem on an equatorial β -plane. With a purely meridional moisture gradient, the system supports large-scale, low-frequency, eastward and westward moving neutral modes. Some of the similarities, and some of the discrepancies of these modes with intraseasonal tropical waves are pointed out. Finally, a zonal moisture gradient in the presence of rotation renders some of the aforementioned neutral modes unstable. In particular, as per observations of large-scale, low-frequency tropical variability, it is seen that regions where the background saturation profile increases (decreases) to the east favour eastward (westward) moving moist modes.

Description: In this study changes in the Northern Hemisphere winter storm tracks during the 20th century are investigated based on the individual 56 ensemble-members of the 20th Century Reanalysis dataset. It is found that the 20th century trends in storm track activities exhibit large discrepancies between the upper and lower troposphere. In the upper troposphere, a substantial intensification is identified at the poleward and downstream regions of the North Pacific and North Atlantic storm track activities, indicating a large northeastward expansion of storm tracks in the late 20th century. However, in the lower troposphere the synoptic eddy activities, especially in terms of the eddy kinetic energy (EKE) and meridional eddy heat flux, tend to be significantly weakened over the high-latitudes of central-western North Pacific and the upstream regions of the North Atlantic storm tracks. Further inspections find that such strengthening (weakening) of storm tracks in the upper (lower) troposphere are mainly attributed to the increase (decrease) of the baroclinic instability, which is predominantly determined by the meridional temperature gradient changes. Moreover, from a local energetic perspective, the baroclinic generation and barotropic damping of the synoptic eddies are found to be substantially enhanced at the upstream and downstream regions of the two storm tracks in the upper troposphere, respectively, while in the lower troposphere the baroclinic energy conversion to eddies are generally decreased.

Description: We derive a family of ideal (nondissipative) 3D sound-proof fluid models that includes both the Lipps-Hemler anelastic approximation (AA) and the Durran pseudo-incompressible approximation (PIA). This family of models arises in the Euler-Poincaré framework involving a constrained Hamilton's principle expressed in the Eulerian fluid description. The derivation in this framework establishes the following properties of each member of the entire family: the Kelvin-Noether circulation theorem, conservation of potential vorticity on fluid parcels, a Lie-Poisson Hamiltonian formulation possessing conserved Casimirs, a conserved domain integrated energy and an associated variational principle satisfied by the equilibrium solutions. Having set the stage with the derivations of 3D models using the constrained Hamilton's principle, we then derive the corresponding 2D vertical slice models for these sound-proof theories.

Description: The surface wind response to SST and SST meridional gradient is investigated in the Gulf of Guinea by using daily observations and reanalyses in the 2000–2009 decade, with a focus on boreal spring and summer months (May to August), where quasi-biweekly fluctuations in the position of the northern front of the equatorial cold tongue induce quasi-biweekly equatorial sea surface temperature (SST) anomalies. Following a large-scale wind acceleration (deceleration), an equatorial SST cold (warm) anomaly is created within a few days. In order to explain the local atmospheric response to this SST anomaly, the two following mechanisms are invoked: first, a colder (warmer) ocean decreases (increases) the vertical stability in the marine atmospheric boundary layer, which favors a weaker (stronger) surface wind; and second, a negative (positive) anomaly of SST meridional gradient induces a positive (negative) anomaly of sea level pressure meridional gradient, which decelerate (accelerate) the surface wind. The first mechanism has an immediate effect in the equatorial belt between 1°S-1°N (and to a lesser extent between 3°S and 1°S), while the second takes one or two days to adjust and damps anomalous southeasterlies up to 800 hPa in the low troposphere between 7°S and 1°N, through reversed anomalies of meridional SST and pressure gradient. This negative feedback leads to weaker (stronger) winds in the southeastern Tropical Atlantic, which forces the opposite phase of the oscillation within about a week. Around the equator, where the amplitude of the oscillation is found maximal, both mechanisms combine to maximize the wind response to the front fluctuations. Between the equator and the coast, a low-level secondary atmospheric circulation takes control of the surface wind acceleration or deceleration around 3°N, which reduces the influence of the SST front fluctuations.

Description: The purpose of the operational-oriented system COALITION ( C ontext and Scale Oriented Thunderstorm Satellite Predictors Development) is to automatically detect severe thunderstorms early in their development and consequently help weather forecasters to increase lead time when issuing severe weather warnings. This new object-oriented system integrates data provided by different sources. Data from Meteosat Second Generation Rapid Scan Service, weather radar and numerical weather prediction, as well as climatology are utilized by the system. One of its primary purposes is to use all the best operationally available information about convective processes and to integrate it into a heuristic model. Furthermore the orographic forcing, which is often neglected in heuristic nowcasting models, is taken into account and included in the system as an additional convective triggering mechanism. This is particularly important for areas characterized by complex orography like the Alpine region. The COALITION algorithm merges evolving thunderstorm properties with selected predictors. The forecasted evolution of the storm is the result of the interaction between convective signatures and surrounding storm environment. Eight different "object-environment" interactions are analysed in eight modules, providing ensemble nowcasts of thunderstorm attributes (satellite- and radar-based) for the following 60 minutes. All ensemble nowcasts are then combined through a weighting and thresholding scheme and the results are summarized into a single graphical map in order to facilitate user interpretation. The COALITION nowcast system has an update frequency of 5 minutes. The output highlights the cells having a high probability of severe thunderstorm development within the next 30 minutes. Verification statistics confirm that COALITION is able to nowcast the intensity of developing convective cells with sufficient skill up to a lead time of about 20 minutes.

Description: In late February 2010 the extraordinary windstorm Xynthia crossed over Southwestern and Central Europe and caused severe damage, affecting particularly the Spanish and French Atlantic coasts. The storm was embedded in uncommon large-scale atmospheric and boundary conditions prior to and during its development, namely enhanced sea surface temperatures (SST) within the low-level entrainment zone of air masses, an unusual southerly position of the polar jet stream, and a remarkable split jet structure in the upper troposphere. To analyse the processes that led to the rapid intensification of this exceptional storm originating close to the subtropics (30°N), the sensitivity of the cyclone intensification to latent heat release is determined using the regional climate model COSMO-CLM forced with ERA-Interim data. A control simulation with observed SST shows that moist and warm air masses originating from the subtropical North Atlantic were involved in the cyclogenesis process and led to the formation of a vertical tower with high values of potential vorticity (PV). Sensitivity studies with reduced SST or increased laminar boundary roughness for heat led to reduced surface latent heat fluxes. This induced both a weaker and partly retarded development of the cyclone and a weakening of the PV-tower together with reduced diabatic heating rates, particularly at lower and mid levels. We infer that diabatic processes played a crucial role during the phase of rapid deepening of Xynthia and thus to its intensity over the Southeastern North Atlantic. We suggest that windstorms like Xynthia may occur more frequently under future climate conditions due to the warming SSTs and potentially enhanced latent heat release, thus increasing the windstorm risk for Southwestern Europe.

Description: We present a series of idealized numerical model experiments to investigate aspects of deep convection in tropical depressions, including the effects of a boundary layer wind structure on storm structure, especially on vertical vorticity production and updraught splitting, and the combined effects of horizontal and vertical shear on vertical vorticity production, with and without background rotation. In warm-cored disturbances such as tropical depressions, the vertical shear and horizontal vorticity change sign at some level near the top of the boundary layer so that, unlike in the typical middle-latitude ‘supercell’ storm, the tilting of horizontal vorticity by a convective updraught leads not only to dipole patterns of vertical vorticity, but also to a reversal in sign of the updraught rotation with height. This finding has implications for understanding the merger of convectively-induced vorticity anomalies during vortex evolution. Ambient cyclonic horizontal shear and/or cyclonic vertical vorticity favour amplification of the cyclonically-rotating gyre of the dipole. Consistent with an earlier study, storm splitting occurs in environments with pure horizontal shear as well as pure vertical shear, but the morphology of splitting is different. In both situations, splitting is found to require a relatively unstable sounding and relatively strong wind shear.

Description: Observations have been obtained within an intense (precipitation rates 〉 50 mm hour -1 ) narrow cold-frontal rainband (NCFR) embedded within a broader region of stratiform precipitation. In-situ data were obtained from an aircraft which flew near a steerable dual-polarisation Doppler radar. The observations were obtained to characterise the microphysical properties of cold frontal clouds, with an emphasis on ice and precipitation formation and development. Primary ice nucleation near cloud top (−55°C) appeared to be enhanced by convective features. However, ice multiplication led to the largest ice particle number concentrations being observed at relatively high temperatures (〉 −10°C). The multiplication process (most likely rime-splintering) occurs when stratiform precipitation interacts with supercooled water generated in the NCFR. Graupel was notably absent in the data obtained. Ice multiplication processes are known to have a strong impact in glaciating isolated convective clouds, but have rarely been studied within larger organised convective systems such as NCFRs. Secondary ice particles will impact on precipitation formation and cloud dynamics due to their relatively small size and high number density. Further modelling studies are required to quantify the effects of rime splintering on precipitation and dynamics in frontal rainbands. Available parameterizations used to diagnose the particle size distributions do not account for the influence of ice multiplication. This deficiency in parameterizations is likely to be important in some cases for modelling the evolution of cloud systems and the precipitation formation. Ice multiplication has significant impact on artefact removal from in-situ particle imaging probes.

Description: This paper proposes a selective ensemble mean technique for tropical cyclone (TC) track forecast based on the errors of ensemble prediction system (EPS) members at short lead times (SLTs, 12 h in this study). The means (SEAV) and weighted means (SEWE) of selected EPS members are applied to EPS products from the European Centre for Medium-Range Weather Forecasts (ECMWF), Japan Meteorological Agency (JMA), National Centers for Environmental Prediction (NCEP), and China Meteorological Administration for 35 TCs in the Western North Pacific in 2010 and 2011. Verification results show that SEAV behaves better than SEWE, with a skill of 5% to 30% over relevant ensemble means of EPS within 72 h. The SEAV method is the most effective for the JMA EPS, with a skill of 10% even at 96 h. SEAV predictions are compared with the high-resolution deterministic model predictions of ECMWF and several official forecasts, with special consideration given to the time delay associated with numerical model products in operation. The SEAV for the ECMWF EPS can overcome the high-resolution ECMWF deterministic model at 24 h. Case analyses and sensitivity tests on the error thresholds of member selection and SLT lead times are also presented in this paper.

Description: The predictive quality of an ensemble model of cirrus ice crystals to model passive and active measurements of ice cloud, from the ultraviolet (UV) to the microwave, is tested. The ensemble model predicts ice mass ∝ D 2 (m-D), where D is the maximum dimension of the ice crystal, and m is the mass. This predicted m-D relationship is applied to a moment estimation parametrization of the particle size distribution (PSD), to estimate the PSD shape, given ice water content (IWC) and in-cloud temperature. The same microphysics is applied across the electromagnetic spectrum to model UV, infrared, microwave and radar observations. The short-wave measurements consist of airborne UV backscatter lidar estimates of the volume extinction coefficient, total solar optical depth, and space-based multi-directional spherical albedo measurements, at 0.865 µm, between the scattering angles 85 o and 125 o . The airborne long-wave measurements consist of high-resolution interferometer upwelling brightness temperatures, obtained between the wavelengths of about 3.45 µm and 4.1 µm, and 8.0 µm to 12.0 µm. The low frequency measurements consist of ground-based Chilbolton 35 GHz radar reflectivity measurements and space-based upwelling 190 GHz brightness temperature measurements. The predictive quality of the ensemble model is demonstrated to be generally within the experimental uncertainty of the lidar backscatter estimates of the volume extinction coefficient and total solar optical depth. The ensemble model prediction of the high-resolution brightness temperature measurements is generally within ±2 K and ±1K, at solar and infrared wavelengths, respectively. The 35 GHz radar reflectivity and 190 GHz brightness temperatures are generally simulated to within ±2 dBZ e , and ±2 K, respectively. The directional spherical albedo observations suggest that the scattering phase function of the most randomized ensemble model gives the best fit to the measurements (generally within ±3%). This paper demonstrates that the ensemble model, assuming the same microphysics , is physically consistent across the electromagnetic spectrum.

Description: This paper investigates two schemes that perturb sea-surface temperatures (SSTs) and soil moisture content (SMC) in the Met Office Global and Regional Ensemble Prediction System (MOGREPS), to address a known deficiency of a lack of ensemble spread near the surface. Results from a two-month long trial during the northern hemisphere summer show positive benefits from these schemes. These include a decrease in the spread deficit of surface temperature and improved probabilistic verification scores. SST perturbations exhibit a stronger impact than SMC perturbations, but when combined the increased spread from the two schemes is cumulative. A regional ensemble system driven by the global ensemble members largely reflects the same changes seen in the global ensemble but cycling fields, like SMC, between successive regional forecasts does show some benefit.

Description: Two statistical downscaling models were developed for downscaling monthly GCM outputs to precipitation at a site in north-western Victoria, Australia. The first downscaling model was calibrated and validated with the NCEP/NCAR reanalysis outputs over the periods of 1950–1989 and 1990–2010 respectively. The projections of precipitation into future were produced by introducing the outputs of HadCM3, ECHAM5, GFDL2.0 and GFDL2.1, pertaining to A2 and B1 greenhouse gas emission scenarios to this downscaling model. In this model, the input data used in the development and future projections are not homogeneous, as they originate from two different sources. As a solution to this issue, the second downscaling model was developed and precipitation projections into future were produced with a homogeneous set of inputs. To produce a homogeneous set of inputs to this model, regression relationships were formulated between the NCEP/NCAR reanalysis outputs and the 20 th century climate experiment outputs corresponding to the variables used in the first downscaling model obtained from the ensemble consisted of HadCM3, ECHAM5 and GFDL2.0. The outputs of these relationships pertaining to the periods of 1950–1989 and 1990–1999, were used for the calibration and validation of this downscaling model respectively. Using the outputs of HadCM3, ECHAM5 and GFDL2.0 pertaining to A2 and B1 emission scenarios on these relationships, inputs for the second downscaling model pertaining to the period of 2000–2099 were generated. The first downscaling model with NCEP/NCAR reanalysis outputs, showed a high Nash-Sutcliffe Efficiency (NSE) of 0.75 over the period 1950–1999. When this downscaling model was run with the 20 th century climate experiment outputs of HadCM3, ECHAM5, GFDL2.0 and GFDL2.1, it exhibited limited performances over the period 1950–1999, which was indicated by relatively low NSEs of −0.62, -2.54, -0.40 and −0.48 respectively. The second downscaling model displayed a NSE of 0.35 over the period 1950–1999.

Description: The U.S. Navy's relocatable (RELO) ensemble prediction system is fully described and is examined in the Gulf of Mexico for 2010. After briefly describing the Ensemble Transfer (ET) method for the initial perturbation generation, we introduce a new time-deformation technique to generate the surface forcing perturbations from the atmospheric model fields. The extended forecast time (EFT) is introduced to quantify the advantages of the ensemble mean forecasts over a single deterministic forecast. The ensemble spread and its growth are investigated together with their relations with the ensemble forecast accuracy, reliability and skill. Similar to many other operational ensemble forecast systems at Numerical Weather Prediction (NWP) centers, the initial analysis error is underestimated by the technique used in the data-assimilation (DA) system. Growth of the ocean ensemble spread is also found to lag the growth of the ensemble mean error, a tendency attributed to insufficiently accounting for model-related uncertainties. As an initial step, we randomly perturb the two most important parameters in the ocean model mixing parameterizations, namely the Smagorinsky horizontal and Mellor-Yamada vertical mixing schemes. We examine three different parameter perturbation schemes based on both uniform and Gaussian distributions. It is found that all three schemes improve the ensemble spread to a certain extent, particularly the scheme with Gaussian distribution of perturbations imposed on both the horizontal and vertical mixing parameters. The findings in this paper indicate that the RELO ensemble forecast demonstrates superior accuracy and skill relative to a single deterministic forecast for all the variables and over all the domains considered in this paper. The ensemble spread provides valuable estimate of forecast uncertainty. However, the RELO uncertainty forecast capability could be further improved by accounting for more model-related uncertainties, for example, by the development of an error parameterization that imposes stochastic forcing at each model grid point.

Description: The gamma family of probability densities has recently been used to model raindrop size. The traditional approach of using method of moments to estimate the gamma distribution parameters, however, is known to be biased and can have substantial errors. A recently-developed approach combining moment information and a weighted least squares analysis generally produces substantially better results. Other procedures superior to the method of moments approach include maximum likelihood. In particular, maximum likelihood estimates have been shown to outperform method of moments estimators both in the case in which the full range of drop sizes are observed as well as the case in which small drop sizes fail to be observed because of the inability of disdrometers to record observations below a threshold. The foregoing comments on maximum likelihood concern the situation in which drop sizes are measured on a continuous scale. In this work we consider drop sizes from gamma distributions which are classified into broad size bins, as would be the case with data obtained from many disdrometers; this requires some modification of the maximum likelihood procedure. We do also allow for the possibility of drop sizes below a threshold or above another threshold not being observed. Maximum likelihood performance in this case is investigated through simulation of volume sampling from gamma distributions with known parameters. We compare the performance of the maximum likelihood estimates with those of method of moments (only a truncated-data version is viable) and the recently developed weighted least squares procedure, and also apply the three estimation procedures to some experimental data. Since the experimental data are surface data we indicate how drop fall velocity may be incorporated to obtain parameters for the volume distributions from the surface data.

Description: Despite the availability of several atmospheric reanalyses (e.g. ERA-Interim) there exists both considerable uncertainty in surface forcing fields for ice/ocean modelling and sensitivity to the choice of product used. Here we introduce a relatively high-resolution alternative forcing data set for ice-ocean models derived from the Canadian Meteorological Centre's (CMC) Global Deterministic Prediction System (GDPS). A set of daily 30 hr reforecasts is produced using the GDPS 33 km resolution model providing hourly atmospheric forcing fields for the period 2002–2011. The CMC GDPS Reforecasts (CGRF) are compared to ERA-Interim and several observational datasets to evaluate their suitability for forcing ocean models. In particular, the CGRF surface temperature, humidity and winds show equivalent biases to those found in ERA-interim. Moreover, the higher resolution of the CGRFs permit a more detailed representation of atmospheric structures and topographic steering resulting in finer-scale coastal features and wind stress curl. While the CGRF dataset is not a reanalysis and thus is expected to be less well constrained by available observations, its higher resolution and small bias make it an attractive alternative for forcing ice/ocean models.

Description: We examine the El Niño/Southern Oscillation (ENSO) influence on the quasi-biennial oscillation (QBO) modulation of the cold-point tropopause (CPT) temperatures. An analysis of approximately 5 decades (in most cases 1950s to near-present) of radiosonde data from eleven near-equatorial stations, distributed along the Equator, shows that the ENSO influence on the QBO is quite zonally symmetric. At all stations analyzed, the QBO has larger amplitude and longer period during La Niña conditions than during El Niño over this total period. We also show that, as a consequence of the ENSO influences on QBO periods and amplitudes, the differences between the warmer CPT temperatures during QBO westerly shear conditions and colder temperatures during QBO easterly shear conditions, are larger during La Niña than during El Niño for all stations considered for the entire period considered here. This strengthens earlier findings that the greatest dehydration of air entering the stratosphere from the troposphere occurs during the winter under La Niña and easterly QBO conditions. Also, stratosphere /troposphere wind and temperature profiles are derived to establish the degree of QBO downward penetration to influence zonal winds and temperatures in the upper troposphere.

Description: The variability of turbulent momentum flux in neutral and unstable atmospheric boundary layers is characterised by analysing surface-layer measurements and data from large-eddy simulations (LES). The method involves multiresolution (MR) decomposition of vertical wind and advected variables into eddy fluctuations on different scales. It provides a measure of the amount of flux variability that stems from same-scale correlations and from combinations of different scale eddy fluctuations. Combining two analysis methods enabled MR component cospectra to be introduced in order to study the contribution of downward and upward flux on different scales. These component cospectra were used to investigate at which scales most of the upward and downward momentum flux occurs. By using MR spectra, cospectra, and flow visualisation this investigation provides insights into turbulence structure and fluxes in neutral and unstable stratification. It is shown that most of the flux variability in the lower part of the boundary layer can be characterised as a combination of larger scale streamwise elongated horizontal wind streaks and smaller scale vertical wind fluctuations. These streaks are found to account for a large part of downward momentum flux at relatively large, energy-containing scales. Most of the upward momentum flux is found to occur at smaller scales. This can be interpreted as showing that upward momentum flux in these conditions is caused by the generation of smaller scale secondary motions when larger scale turbulence elements break down and dissipate. Differences in the height dependence of turbulence structure and momentum flux for neutral and unstably stratified conditions are also investigated and related to the existence of wind streaks and horizontal rolls in these different conditions.

Description: Idealized simulations on an f -plane of tropical cyclone (TC) landfall under a quiescent environment in the Southern Hemisphere are performed to investigate the effects of land-sea surface contrast on precipitation. In the control simulation with realistic roughness and moisture over land which is to the south of the TC, the simulated vortex moves toward land due to a land-induced steering flow. The abrupt decrease (increase) of tangential wind at the surface leads to convergence (divergence) on the onshore (offshore) flow side. Enhanced convergence at the top of the planetary boundary layer is found both in the onshore and offshore side, which is caused by the advection from the surface and the enhanced offshore radial wind. The boundary layer top convergence pattern is consistent with the rainfall distribution. Vertical wind shear develops during the landfall process associated with the low- and upper-level asymmetric flows across the model domain. The wavenumber-1-like low-level asymmetric flow is introduced by an asymmetric geopotential height field that is generated by the large area of frictionally-induced convergence at the onshore side and divergence at the offshore side. The upper-level asymmetric flow is attributed to asymmetric convection and associated diabatic heating after landfall. Most rainfall is found in the down-shear right quadrant, which is consistent with previous studies that focused on the effect of environmental shear. Although the existence of feedback from the vertical wind shear to rainfall remains an open question, the relation between maximum rainfall and vertical wind shear is robust especially when the shear magnitude is large after landfall.

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: The impact of dust on a six-day pulsation of the West African heat low (WAHL) in summertime (14–20 July 2006) is investigated, with convective rainfall and dust bursts being observed over the Sahel at the beginning and end of the episode. Three Meso-NH simulations were designed which differed in their dust representation. All the simulations capture the variation in the WAHL intensity well, including the simulation without any dust effects. This shows the primary role of large-scale forcing on the WAHL pulsation. In spite of additional daytime heating and night-time cooling effects over the Sahara, the simulation with dust climatology resembles the simulation without any dust effects. In contrast, the simulation using a prognostic dust scheme enhances alternating northward advection of warm and dry air and southward advection of cold and wet air associated with the propagation of an African easterly wave, leading to a strengthening of the WAHL variabilities. This study highlights two effects of dust on the WAHL over the Sahara: a so-called direct effect associated with dust radiative heating, which increases the WAHL thickness, and a so-called indirect effect that intensifies both the African easterly jet and a related African easterly wave. Copyright © 2011 Royal Meteorological Society

Description: The Geostationary Earth Radiation Budget Intercomparison of Longwave and Shortwave radiation (GERBILS) was an observational field experiment over North Africa during June 2007. The campaign involved 10 flights by the FAAM BAe-146 research aircraft over southwestern parts of the Sahara Desert and coastal stretches of the Atlantic Ocean. Objectives of the GERBILS campaign included characterisation of mineral dust geographic distribution and physical and optical properties, assessment of the impact upon radiation, validation of satellite remote sensing retrievals, and validation of numerical weather prediction model forecasts of aerosol optical depths (AODs) and size distributions. We provide the motivation behind GERBILS and the experimental design and report the progress made in each of the objectives. We show that mineral dust in the region is relatively non-absorbing (mean single scattering albedo at 550 nm of 0.97) owing to the relatively small fraction of iron oxides present (1–3%), and that detailed spectral radiances are most accurately modelled using irregularly shaped particles. Satellite retrievals over bright desert surfaces are challenging owing to the lack of spectral contrast between the dust and the underlying surface. However, new techniques have been developed which are shown to be in relatively good agreement with AERONET estimates of AOD and with each other. This encouraging result enables relatively robust validation of numerical models which treat the production, transport, and deposition of mineral dust. The dust models themselves are able to represent large-scale synoptically driven dust events to a reasonable degree, but some deficiencies remain both in the Sahara and over the Sahelian region, where cold pool outflow from convective cells associated with the intertropical convergence zone can lead to significant dust production. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

Description: We studied the use of IASI data to improve the forecasts of extreme weather events in the Arctic region. For this purpose, the HARMONIE/Norway regional model was used. A set of 366 IASI channels was initially chosen from the ECMWF archived database. Active channels showing the best fit with the analysis system were selected by applying a multi-step monitoring technique. The IASI data were assimilated together with most of the available conventional and operational satellite observations using a three-dimensional variational data assimilation system. Four experiments with cyclic assimilations and subsequent 48-hour forecasts were performed during the IPY-THORPEX campaign period to evaluate the impact of the IASI data and the campaign observations on the hydrostatic HARMONIE/Norway analyses and forecasts. The assessment of the degrees of freedom for signals on the analysis showed that incorporating the IASI data in the assimilation system improved the contribution of the other observations. The utilization of an energy norm-based approach proved the sensitivity of the forecasts to the IASI channels in cases dominated by dynamic instabilities leading to quickly developing weather systems like, for example, polar lows. Comparison of the HARMONIE/Norway forecasts against independent observations and the ECMWF analyses showed a clear positive impact of the IASI data on geopotential fields in mid-troposphere and in the troposphere in general, respectively. We found small but significant positive impact on the temperature and humidity in the lower troposphere. A case-study showed positive impact of IASI radiances on the analysis and forecasts of a polar low. The impact on the forecasts lasted up to 24 hours when extra in situ campaign data were excluded from the analysis, and up to 36 hours when the campaign data were assimilated together with the IASI radiances. Copyright © 2011 Royal Meteorological Society

Description: Extreme mesoscale weather in the Arctic region consists mainly of cases with shallow fronts that often form in the vicinity of the ice-edge and intense storms called polar lows. This article describes high-resolution numerical simulations of a severe weather event that occurred on 1 March 2008 over the Barents Sea. The event was recorded during the IPY–THORPEX field experiments carried out during February and March 2008. The numerical simulations indicated the formation of a low-pressure system over the Barents Sea on 29 February 2008 due to baroclinic instability. On 1 March, the surface low moved onto the sea-ice around Spitsbergen and decayed later on. The conditions that prevailed before the dissipation of the surface low were favourable for the formation of a polar low. Two experiments were performed to test the possibilities of triggering a polar low through certain modifications to the surface conditions. In the first experiment, the sea-ice around Spitsbergen was removed. No polar low developed in this case, since the static stability was too high. In the second experiment, an attempt to reduce the static stability was made by raising the sea-surface temperature by 5 K. The surface low persisted over the Barents Sea area due to the increased surface heating and led to a strong outbreak of Arctic air over the Norwegian Sea on 2 March. The Arctic-air outbreak formed a sharp baroclinic zone which was absent in the control simulation. A secondary mesoscale low was triggered near the baroclinic zone over the Norwegian Sea, which grew into an intense polar low with surface winds reaching hurricane force. Formation of the polar low was due to baroclinic instability, whereas convective instability was important during the growth of the low. Copyright © 2011 Royal Meteorological Society

Description: Ground-based radar observations at three distinct geographical locations in West Africa along a common latitudinal band (Niamey, Niger (continental), Kawsara, Senegal (coastal), and Praia, Republic of Cape Verde (maritime)) are analyzed to determine convective system characteristics in each domain during a 29-day period in 2006. Ancillary datasets provided by the African Monsoon Multidisciplinary Analyses (AMMA) and NASA-AMMA (NAMMA) field campaigns are also used to place the radar observations in context. Results show that the total precipitation is dominated by propagating mesoscale convective systems. Convective characteristics vary according to environmental properties, such as vertical shear, CAPE, and the degree of synoptic forcing. Data are bifurcated based on the presence or absence of African easterly waves. In general, African easterly waves appear to enhance mesoscale convective system strength characteristics (e.g. total precipitation and vertical reflectivity profiles) at the inland and maritime sites. The wave regime also resulted in an increased population of the largest observed mesoscale convective systems observed near the coast, which led to an increase in stratiform precipitation. Despite this increase, differentiation of convective strength characteristics was less obvious between wave and no-wave regimes at the coast. Owing to the propagating nature of these advecting mesoscale convective systems, interaction with the regional thermodynamic and dynamic environment appears to result in more variability than enhancements due to the wave regime, independent of location. Copyright © 2011 Royal Meteorological Society

Description: A wavelet formulation on the sphere is being considered for modelling heterogeneous background-error correlations for the Météo-France global numerical weather prediction (NWP) model. This approach is compared with the operational spectral formulation, which is horizontally homogeneous to a large extent. Diagnostic studies have been conducted to examine geographical variations of three-dimensional correlations over the whole globe. Results indicate that the contrast between relatively broad horizontal correlations in the Tropics and sharp ones in midlatitudes is well represented by the wavelet formulation. Heterogeneities in vertical correlations are also better captured in the wavelet approach than in the spectral one, with visible changes as functions of e.g. latitude and land/sea contrasts. In addition, wavelet-based correlation estimates are shown to be partly sensitive to the choice of the calibration period. The impact of the wavelet formulation on the forecast quality has been investigated during a three-week calibration period, and also during the following three weeks. While the impact of the wavelet formulation is globally positive during the two periods, it tends to be more spectacular during the calibration time interval, as expected. These results indicate that an on-line calibration should be considered in the future, in order to exploit fully the ability of wavelets to extract correlation heterogeneities from ensemble data. Copyright © 2011 Royal Meteorological Society

Description: FY-3A, launched in May 2008, is the first in a series of seven polar-orbiting meteorological satellites due to be launched by China's Meteorological Administration in the period leading up to 2020. The FY-3A payload includes four instruments of particular interest for numerical weather prediction (NWP): microwave temperature and humidity sounders, a microwave imager, and an infrared sounder. The main features of these instruments are described. Data from the calibration–validation phase of the FY-3A mission were introduced into the ECMWF Integrated Forecasting System in order to assess the data quality and the influence of the data on analyses and forecasts. An analysis of first-guess departures has shown the data to be of good quality overall. Several issues with instrument performance and ground segment processing have been identified. The most serious of these are: uncertainties in the temperature sounder passbands on-orbit, orbital biases in the infrared instrument affecting the highest peaking channels, and scan biases in the microwave humidity sounder. Variational bias correction partially corrects for these errors, but more work remains to be done to correct the problems before the full benefit of the data is realised. In observing system experiments, the FY-3A instruments, both individually and as a package, show considerable skill when added to observation depleted control experiments. When added to a full observing system, the impacts are neutral to slightly positive, as expected. These initial results are encouraging and build confidence that the following series of FY-3 instruments will be widely used in NWP data assimilation systems. Copyright © 2011 Royal Meteorological Society

Description: Snow particle size distributions (particle size 〉400 µm) in the western Arctic measured with in situ aircraft instrumentation during the Surface Heat Budget of the Arctic/First ISCCP Regional Experiment - Arctic Clouds Experiment and Mixed-Phase Arctic Cloud Experiment are analysed. Three cases of shallow, precipitating mixed-phase boundary-layer clouds and two cases of deep, precipitating frontal clouds are examined. Overall, the shallow cases had much lower values of particle concentration and ice water content than the deep cases, indicating large differences in ice initiation and growth between these regimes. Within a given case for both the shallow and deep frontal systems, and for the dataset as a whole, crystal concentration had little correlation with temperature (height), despite an active aggregation process that was indicated by large aggregates (〉5 mm) observed in four out of the five cases. Exponential size distributions are fitted to the observations, allowing a direct comparison with the snow particle size distributions that are represented with exponential functions in many bulk microphysics schemes used in weather and climate models. Values of the fitted intercept parameter N 0 are generally 2–10 times smaller for the shallow compared to the deep frontal cases as a result of differences in crystal concentration between these regimes. Values of N 0 ∼ 10 7 m −4 specified for snow in many bulk microphysics schemes are broadly consistent with fitted N 0 for the deep cases but larger than values for the shallow cases. The deep frontal cases also exhibit a relationship between N 0 and temperature consistent with previous observations of midlatitude frontal systems. However, there are no consistent differences in N 0 between the shallow and deep cases when partitioned by ice water content. Fitted values of slope parameter λ for the shallow and deep cases are generally consistent with previous studies of lower-latitude cloud systems. Copyright © 2011 Royal Meteorological Society

Description: A bulk flux algorithm predicts the turbulent surface fluxes of momentum and sensible and latent heat from mean measured or modelled meteorological variables. The bulk flux algorithm resulting from data collected over winter sea ice during SHEBA, the experiment to study the Surface Heat Budget of the Arctic Ocean, failed, however, in its first trial to predict the turbulent momentum flux over sea ice in the Antarctic. This result suggests that the main parameter for predicting the momentum flux, the aerodynamics roughness length z 0 , does not respond just to the friction velocity, as in the SHEBA algorithm, but is closely related to the physical roughness of snow-covered sea ice and may need to be site-specific. I investigate this idea with simultaneous measurements of z 0 and the physical roughness of the surface, ξ , at Ice Station Weddell. The metric ξ derives from surveys of surface elevation and is related to but always less than the standard deviation in surface elevation. On combining the z 0 – ξ pairs from Ice Station Weddell with similar data obtained over Arctic sea ice, I show that the Arctic and Antarctic z 0 – ξ data lie along a continuum such that measuring ξ could provide a means for estimating a site-specific z 0 for any global sea ice surface. Backscatter data from satellite-borne synthetic aperture radar might provide a remotely sensed estimate of ξ . Copyright © 2011 Royal Meteorological Society

Description: During 3–4 March 2008, the Norwegian IPY-THORPEX field campaign successfully carried out three flight missions that observed the full life cycle of a polar low over the Norwegian Sea. Here the three-dimensional structure of the polar low has been investigated using dropsonde data from the three flights. The polar low developed in a cold air outbreak, with temperature differences between the sea surface and 500 hPa of about 45–50°C. Cross-sections show that the horizontal gradients of potential temperature weakened as the polar low matured, suggesting that baroclinic energy conversion took place. Dropsonde data of potential temperature and relative humidity show evidence of a tropopause fold, which is possibly a manifestation of upper-level forcing. This is corroborated by potential vorticity inversion, which shows a dominant role of upper-level forcing throughout the polar low's lifetime. During the cyclogenesis stage the polar low circulation was confined below 700 hPa, with a northerly low-level jet of 26 m s −1 . In the mature stage, its circulation reached up to the tropopause (∼450 hPa), with maximum wind speed between 700 and 900 hPa of about 26–28 m s −1 . At this stage the polar low warm core was about 3 K warmer than surrounding air masses. The deep moist towers at the eye-like structure of the polar low extended up to the tropopause with relative humidity values above 70%, indicating a possibly important role for condensational heating in the development. Estimates of surface fluxes of sensible and latent heat using temperature and moisture from the dropsonde data show latent heat fluxes west of the polar low increasing from 175 to 300 W m −2 as the low matured, while the sensible heat fluxes rose from 200 to 280 W m −2 , suggesting a gradually increasing contribution of surface fluxes to the energetics of the polar low with time. Copyright © 2011 Royal Meteorological Society

Description: We present simulations of London's meteorology using the Met Office Unified Model with a new, sophisticated surface energy-balance scheme to represent the urban surfaces, called MORUSES. Simulations are performed with the urban surfaces represented and with the urban surfaces replaced with grass in order to calculate the urban increment on the local meteorology. The local urban effects were moderated to some extent by the passage of an onshore flow that propagated up the Thames estuary and across the city, cooling London slightly in the afternoon. Validations of screen-level temperature show encouraging agreement to within 1–2 K, when the urban increment is up to 5 K. The model results are then used to examine factors shaping the spatial and temporal structure of London's atmospheric boundary layer. The simulations reconcile the differences in the temporal evolution of the urban heat island (UHI) shown in various studies and demonstrate that the variation of UHI with time depends strongly on the urban fetch. The UHI at a location downwind of the city centre shows a decrease in UHI during the night, while the UHI at the city centre stays constant. Finally, the UHI at a location upwind of the city centre increases continuously. The magnitude of the UHI by the time of the evening transition increases with urban fetch. The urban increments are largest at night, when the boundary layer is shallow. The boundary layer experiences continued warming after sunset, as the heat from the urban fabric is released, and a weakly convective boundary layer develops across the city. The urban land-use fraction is the dominant control on the spatial structure in the sensible heat flux and the resulting urban increment, although even the weak advection present in this case study is sufficient to advect the peak temperature increments downwind of the most built-up areas. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

Description: Warm conveyor belts (WCBs) are key flow structures associated with extratropical cyclones. They transport moist air from the cyclone's warm sector poleward and upward close to the tropopause level, leading to the formation of elongated cloud bands, intense latent heating and surface precipitation. In this study a comprehensive dataset of airborne lidar observations of moisture and wind from different campaigns has been investigated with a trajectory-based approach to identify ‘lucky encounters’ with WCBs. On 19 July 2007, an upstream flight over the Iberian Peninsula during the European THORPEX Regional Campaign (ETReC 2007) in Central Europe intersected two WCBs: one in the upper tropospheric outflow region about 3 days after starting the ascent, and the other one in the boundary layer inflow region over Spain just prior to the strong ascent. Comparison of the lidar humidity measurements with analysis fields from the European Centre for Medium-Range Weather Forecasts (ECMWF) reveals significant positive deviations, equivalent to an overestimation of the modelled humidity, in this low-tropospheric WCB inflow region (of about 1 g kg −1 (14%) on average and with peak deviations up to 7 g kg −1 ). It is noteworthy that this substantial bias occurs in a potentially dynamically highly relevant air mass that will be subsequently lifted within a WCB to the upper troposphere. A Lagrangian moisture source diagnostic reveals that these large moisture deviations occur within air masses that, according to the ECMWF analyses, are coherently transported from the western Mediterranean towards Spain and experience intense moisture uptake over the Ebro valley. It is suggested that inaccuracies in surface evapotranspiration, horizontal moisture advection, and turbulent vertical transport of moisture in the atmospheric boundary layer potentially contribute to the erroneous low-tropospheric humidity in the inflow region of this particular summertime WCB over Spain in the ECMWF analyses. Copyright © 2011 Royal Meteorological Society

Description: The role of the representation of deep convection on key elements of the West African summer monsoon climate is addressed using the Regional Climate Model RegCM3. Two simulations in which a scheme of deep convection is activated and then turned off are performed and intercompared. Results show that the presence of deep convective heating along the intertropical convergence zone sustains increased lower-level baroclinicity favoring intensification of the jet core and leading to a more realistic African easterly jet. In addition, although the isentropic potential vorticity (IPV) is lower when the convection scheme is switched off, African easterly waves (AEWs) are still generated and propagate westwards but they dissipate around the west coast. Substantial differences between the two simulations occur mainly at the 6- to 9-day time-scale over land, when much weaker activity is simulated in the absence of convection. This indicates that orographic friction and low-level large-scale moisture convergence, generating high values of latent heat and IPV, may play the dominant role in the genesis and growth of AEWs and that deep convection acts to strengthen the overall wave activity and to favor their west coast development. Copyright © 2011 Royal Meteorological Society

Description: A unique airborne differential absorption lidar (DIAL) for water vapour observations was developed at the Deutsches Zentrum für Luft- und Raumfahrt (DLR). Installed on board the DLR Falcon 20 aircraft, the system measured a dataset of about 3900 water vapour profiles during the T-PARC field campaign. These high-resolution humidity observations were assimilated into the European Centre for Medium-Range Weather Forecasts (ECMWF) global model using a version of the operational four-dimensional variational data assimilation system. The assimilation system is able to extract the information for DIAL observations, and verification with independent dropsonde observations shows a reduction in the analysis error when DIAL water vapour observations are assimilated. The forecast influence of the humidity observations is found to be small in most cases, but the observations are able to affect the forecast considerably under certain conditions. Systematic errors are investigated by comparison between humidity model fields, DIAL and dropsonde observations. Overall, DIAL observations are roughly 7–10% drier than model fields throughout the troposphere. Comparison with dropsonde observations suggests that the DIAL observations are too dry in the lower troposphere but not above it. Copyright © 2011 Royal Meteorological Society

Description: In this paper we provide an overview of various satellite products over the Sahara Desert that were available during the GERBILS field campaign. Our results indicate that all mid-visible satellite aerosol optical depth (AOD) products match well with AERONET retrievals. For low AOD (AOD 〈 1), the satellite AODs compare well with aircraft AOD values but they tend to underestimate at high AOD values. We then assessed the satellite products in 0.5 × 0.5 degree grids for the entire study region (10–30°N and 20°W–10°E). If we use a multi-angle imaging spectroradiometer (MISR) as a benchmark for AOD retrievals over bright targets, the estimated AOD derived from the ozone-monitoring instrument aerosol index–MISR relationship performs best when compared with MISR for the entire study region. Although differences exist among satellite products, the advancement in satellite retrieval techniques now provide AOD retrievals over bright targets such as deserts, which are useful for numerical modeling simulation comparisons and other studies. Furthermore, the in situ information from aircraft and the ground continue to provide valuable information for validating satellite products and for assessing their strengths and weaknesses. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, theMet Office

Description: The impact of the high-frequency (HF, 〈90 days) variability on low-frequency (LF) interannual sea surface temperature (SST) variations associated with the El Niño-Southern Oscillation (ENSO) is investigated by conducting a series of oceanic general circulation model experiments. Two nonlinear rectification mechanisms are examined. The first is the internal oceanic nonlinear dynamics and the second is the nonlinear rectification of the LF surface wind stress by the HF wind. Numerical simulations show that the latter is dominant in modulating the LF SST variability. The HF wind increases both the amplitude and skewness of the LF wind stress anomaly. As a result, it increases both the amplitude and skewness of the SST anomaly (SSTA) in the eastern equatorial Pacific. For strong El Niño events in 1982/83 and 1997/98, such a nonlinear rectification effect may result in a SSTA increase of 1°C. A mixed-layer heat budget analysis reveals that whereas meridional and vertical advections primarily contribute to the strengthening of the warm and cold episodes, the nonlinear zonal advection is responsible for the increase of the SSTA skewness. Including the nonlinear rectification of the HF wind on both the surface wind stress and heat flux anomalies leads to a positively (negatively) skewed SSTA in the eastern (central) Pacific. Thus the combined dynamic and thermodynamic effect reshapes the ENSO zonal structure in such a way that it makes the maximum SSTA confined further to the eastern equatorial Pacific. Copyright © 2011 Royal Meteorological Society

Description: In situ , satellite and model analyses data in April–July 2006 are used to investigate the links between sea surface temperature (SST) and the marine atmospheric boundary layer (MABL) in the Gulf of Guinea. The study region between 10°W and 6°E is divided into three areas with different characteristics: the North Area (4.5°N to 1°N) with the wettest atmosphere and the warmest SST, the Upwelling Area (1°N to 4°S) with the strongest SST decrease, and the South Area (4°S to 8°S) with a drier atmosphere and a more slowly decreasing SST than in the Upwelling Area. The key zone of the air-sea interactions in this region seems to be the SST front between North and Upwelling Areas. On the one hand, the study of the MABL on either side of the front shows a well-mixed layer between the surface and about 500 m high, sensitive to surface variations, which gets shallower (deeper) when the SST decreases (increases). The MABL height (about 1500 m) follows the same variations but is not exactly collocated with the SST variations. On the other hand, the observation of the MABL across the SST front shows a strengthening of southeasterlies in the South Area coinciding with a strong SST decrease in the Upwelling Area. In the latter, the wind weakens above the colder SST. Besides, in the North Area, the wind strengthens above the warmer SST. However, the wind acceleration spans from the equator to 2°N in April and as far as 4°N in June. A convergence zone is observed in the vicinity of 2°N in April, suggesting a convection activity there, favoured by the SST front. Copyright © 2011 Royal Meteorological Society

Description: This article presents and assesses an algorithm that constructs 3D distributions of cloud from passive satellite imagery and collocated 2D nadir profiles of cloud properties inferred synergistically from lidar, cloud radar and imager data. It effectively widens the active–passive retrieved cross-section (RXS) of cloud properties, thereby enabling computation of radiative fluxes and radiances that can be compared with measured values in an attempt to perform radiative closure experiments that aim to assess the RXS. For this introductory study, A-train data were used to verify the scene-construction algorithm and only 1D radiative transfer calculations were performed. The construction algorithm fills off-RXS recipient pixels by computing sums of squared differences (a cost function F ) between their spectral radiances and those of potential donor pixels/columns on the RXS. Of the RXS pixels with F lower than a certain value, the one with the smallest Euclidean distance to the recipient pixel is designated as the donor, and its retrieved cloud properties and other attributes such as 1D radiative heating rates are consigned to the recipient. It is shown that both the RXS itself and Moderate Resolution Imaging Spectroradiometer (MODIS) imagery can be reconstructed extremely well using just visible and thermal infrared channels. Suitable donors usually lie within 10 km of the recipient. RXSs and their associated radiative heating profiles are reconstructed best for extensive planar clouds and less reliably for broken convective clouds. Domain-average 1D broadband radiative fluxes at the top of the atmosphere (TOA) for (21 km) 2 domains constructed from MODIS, CloudSat and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data agree well with coincidental values derived from Clouds and the Earth's Radiant Energy System (CERES) radiances: differences between modelled and measured reflected shortwave fluxes are within ±10 W m −2 for ∼35% of the several hundred domains constructed for eight orbits. Correspondingly, for outgoing longwave radiation ∼65% are within ±10 W m −2 . Copyright © 2011 Royal Meteorological Society and Crown in the right of Canada

Description: In geophysical data assimilation, the control space is by definition the set of parameters which are estimated through the assimilation of observations. It has recently been proposed to design the discretizations of control space in order to assimilate observations optimally. The present paper describes the embedding of that formalism in a consistent Bayesian framework. General background errors are now accounted for. Scale-dependent errors, such as aggregation errors (that lead to representativeness errors) are consistently introduced. The optimal adaptive discretizations of control space minimize a criterion on a dictionary of grids. New criteria are proposed: degrees of freedom for the signal (DFS) built on the averaging kernel operator, and an observation-dependent criterion. These concepts and results are applied to atmospheric transport of pollutants. The algorithms are tested on the European tracer experiment (ETEX), and on a prototype of CO 2 flux inversion over Europe using a simplified CarboEurope-IP network. New types of adaptive discretization of control space are tested such as quaternary trees or factorised trees. Quaternary trees are proven to be both economical, in terms of storage and CPU time, and efficient on the test cases. This sets the path for the application of this methodology to high-dimensional and noisy geophysical systems. Part II of this article will develop asymptotic solutions for the design of control space representations that are obtained analytically and are contenders to exact numerical optimizations. Copyright © 2011 Royal Meteorological Society

Description: Spatial and temporal characteristics of a nocturnal low-level jet (LLJ) on the east side of the Western Ghat mountain range over India's west coast and processes leading to the formation of the jet are discussed. The boundary-layer jet has a regional scale extent, as revealed by high-resolution Advanced Research Weather Research and Forecasting (ARW) model simulations, and contributes to the formation of ‘atmospheric streams’ of water vapor over the selected land regions. Simulations indicate that the formation of LLJ is mainly attributed to the baroclinicity of the valley atmosphere due to the gently rolling terrain, which is assisted by the persistence of an unstable residual layer above the developing stable boundary layer in the valley and cooling over the slopes. Prior to the formation of LLJ, the boundary layer is dominated by deep roll circulations. The LLJ followed a gust front zone associated with a mountain wave. The low-level flow below the jet is decoupled from the upper-level flow as a result of strong vorticity below the jet and suppression of turbulence at the jet core. A conceptual model for the boundary layer interactions, dynamics of the mountain wave, LLJ, etc. are proposed for Western Ghat region. Copyright © 2011 Royal Meteorological Society

Description: This article examines the first-guess (FG) departures of microwave imager radiances assimilated in all-sky conditions (i.e. clear, cloudy and precipitating). Agreement between FG and observations is good in clear skies, with error standard deviations around 2 K, but in heavy cloud or precipitation errors increase to 20 K. The forecast model is not good at predicting cloud and precipitation with exactly the right intensity or location. This leads to apparently non-Gaussian behaviour, both heteroscedasticity, i.e. an increase in error with cloud amount, and boundedness, i.e. the size of errors is close to the geophysical range of the observations, which runs from clear to fully cloudy. However, the dependence of FG departure standard deviations on the mean cloud amount is predictable. Using this dependence to normalise the FG departures gives an error distribution that is close to Gaussian. Thus if errors are treated correctly, all-sky observations can be assimilated successfully under the assumption of Gaussianity on which assimilation systems are based. This ‘symmetric’ error model can be used to provide a robust threshold quality-control check and to determine the size of observation errors for all-sky assimilation. In practice, however, this ‘observation’ error is being used to account for the model's difficulty in forecasting cloud, which really comes from errors in the background and in the forecast model. Hence in future it will be necessary to improve the representation of background and model error. Separately, symmetric cloud amount is recommended as a predictor for bias correction schemes, avoiding the sampling problems associated with ‘asymmetric’ predictors like the FG cloud amount. Copyright © 2011 Royal Meteorological Society

Description: Laboratory experiments to determine the preferred orientation of free-falling hexagonal prisms were performed at Reynolds numbers appropriate to falling ice crystals in the atmosphere. Hexagonal plates orient with their c axis vertical for aspect ratios 〈 0.9, whilst hexagonal columns fall with their c axis horizontal. A secondary alignment is also observed: regular hexagonal columns fall preferentially with two prism facets aligned vertically and not horizontally – the latter scenario was previously assumed to be responsible for the rare Parry arc. However, if the column is made scalene in its cross-section, it can orient such that a pair of prism facets is horizontal. This finding indicates that the development of scalene crystals may be key to the production of certain ice-crystal optical phenomena. Copyright © 2011 Royal Meteorological Society

Description: Gravity-wave-induced temperature fluctuations predicted by a mountain-wave parametrization scheme are compared with observed stratospheric temperature fluctuations from the High-Resolution Dynamics Limb Sounder (HIRDLS) instrument. The focus of the study is on the southern Andes region during the month of August 2006. The comparison reveals that while the mean amplitude of temperature fluctuations predicted by the parametrization is broadly consistent with those observed by HIRDLS, there are significant differences for individual cases. Ray-tracing calculations performed for these cases suggest that these differences are likely to be associated with horizontal propagation of mountain waves, which is not represented by current column-based parametrization schemes. The results presented in this note suggest that this is a deficiency of mountain-wave parametrization schemes that may affect models at resolutions typical of both climate and numerical weather prediction modelling. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

Description: A new version of the Edwards–Slingo (ES) radiation scheme is developed using the correlated- k distribution (CKD) method. The work is conducted based on the line-by-line radiative transfer scheme GENLN2. A dataset of new ozone absorption cross-section in the ultraviolet spectrum and new oxygen collision-induced continuum data have been implemented in both the GENLN2 and new ES schemes. In order to improve the efficiency of the ES scheme, a new technique is proposed in this work to optimize the k distribution and a new method is used to deal with the gaseous overlapping absorption in a spectral band. The accuracy of the scheme is improved by replacing the scaling function used in the ES scheme with a pre-determined look-up table for consideration of the pressure and temperature dependency of the absorption oefficients. The number of spectral bands and number of absorbing species are both increased in the new scheme for better resolution of the spectral variation of absorbing species, aerosols and clouds. However, this does not increase the computational cost. Instead, it is reduced substantially compared with the previous version of the code. The treatment of transmission in the short-wave spectrum is improved by implementing the absorbing species of CH 4 , N 2 O and O 2 collision-induced continuum absorption which are not included in the ES scheme. New O 3 absorption cross-section data in the ultraviolet spectrum measured by the European Space Agency are used to generate the CKD spectral data in the short-wave spectral bands. These data have a temperature dependency and better spectral resolution. The irradiance and heating rate determined by the new scheme are tested against the same variables determined by GENLN2. It has been shown that the new scheme produces results more accurate than the ES scheme. Copyright © 2011 Royal Meteorological Society

Description: Ice cloud representation in general circulation models remains a challenging task, due to the lack of accurate observations and the complexity of microphysical processes. In this article, we evaluate the ice water content (IWC) and ice cloud fraction statistical distributions from the numerical weather prediction models of the European Centre for Medium-Range Weather Forecasts (ECMWF) and the UK Met Office, exploiting the synergy between the CloudSat radar and CALIPSO lidar. Using the last three weeks of July 2006, we analyse the global ice cloud occurrence as a function of temperature and latitude and show that the models capture the main geographical and temperature-dependent distributions, but overestimate the ice cloud occurrence in the Tropics in the temperature range from −60 °C to −20 °C and in the Antarctic for temperatures higher than −20 °C, but underestimate ice cloud occurrence at very low temperatures. A global statistical comparison of the occurrence of grid-box mean IWC at different temperatures shows that both the mean and range of IWC increases with increasing temperature. Globally, the models capture most of the IWC variability in the temperature range between −60 °C and −5 °C, and also reproduce the observed latitudinal dependencies in the IWC distribution due to different meteorological regimes. Two versions of the ECMWF model are assessed. The recent operational version with a diagnostic representation of precipitating snow and mixed-phase ice cloud fails to represent the IWC distribution in the −20 °C to 0 °C range, but a new version with prognostic variables for liquid water, ice and snow is much closer to the observed distribution. The comparison of models and observations provides a much-needed analysis of the vertical distribution of IWC across the globe, highlighting the ability of the models to reproduce much of the observed variability as well as the deficiencies where further improvements are required. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

Description: Mass-flux convection schemes continue to play key roles in large-scale atmospheric modelling. Currently, however, the specification of detrainment seems a potential weakness. Using cloud-resolving model results from the European Cloud Systems (EUROCS) humidity case, we consider how detrainment adapts to its environment and formulate a minimal-complexity description of partial detrainment in convective cloud fields. This can be viewed as an approximation to the behaviour of a multi-plume scheme. The algorithm is straightforwardly implemented within the Gregory–Rowntree convection scheme as an extension of its original partial-detrainment scheme. Numerical weather prediction tests in the Met Office Unified Model show significant benefits in full-model performance. © 2011 Crown Copyright, the Met Office. Published by JohnWiley & Sons Ltd.

Description: We investigate the issue of variational and sequential data assimilation with nonlinear and non-smooth observation operators using a two-dimensional limited-area shallow-water equation model and its adjoint. The performance of the four-dimensional variational approach (4D-Var: two dimensions plus time) compared with that of the maximum-likelihood ensemble filter (MLEF), a hybrid ensemble/variational method, is tested in the presence of non-smooth observation operators. Following the work of Lewis & Overton and Karmitsa, we investigate minimization of the data-assimilation cost functional using the limited-memory Broyden–Fletcher–Goldfarb–Shanno (L-BFGS) quasi-Newton algorithm originally intended for smooth optimization and the limited-memory bundle method (LMBM) algorithm specifically designed to address large-scale non-smooth minimization problems. Numerical results obtained for the MLEF method show that the LMBM algorithm yields results superior to the L-BFGS method. Results for 4D-Var suggest that L-BFGS performs well when the non-smoothness is not extreme, but fails for non-smooth functions with large Lipschitz constants. The LMBM method is found to be a suitable choice for large-scale non-smooth optimization, although additional work is needed to improve its numerical stability. Finally, the results and methodologies of 4D-Var and MLEF are compared and contrasted. Copyright © 2011 Royal Meteorological Society

Description: The simultaneous presence of convective, symmetric and inertial instability in a prefrontal region of strong vertical wind shear was simulated with the non-hydrostatic model MOLOCH. Model diagnostics of absolute vorticity, pseudo-angular momentum, saturated equivalent potential temperature and vorticity reveal a sequence of events that includes ‘Δ M -adjustment’, followed by slanted ascent in symmetrically unstable regions, becoming saturated in the later stages. An idealized experiment without orography was performed to isolate the presence and role of instabilities characterizing the development. The diagnosed circulation is reminiscent of a wavenumber-two normal mode of dry symmetric instability, while moist symmetric instability is confined to a very limited region, despite the appearance of wider areas of negative moist potential vorticity. The evaluation of moist thermodynamic quantities which give proper account of condensate loading is suggested as a possible resolution of this apparent inconsistency. Copyright © 2011 Royal Meteorological Society

Description: Localization is an essential element of ensemble-based Kalman filters in large-scale systems. Two localization methods are commonly used: covariance localization and domain localization. The former applies a localizing weight to the forecast covariance matrix, while the latter splits the assimilation into local regions in which independent assimilation updates are performed. The domain localization is usually combined with observation localization, which is a weighting of the observation-error covariance matrix, resulting in a similar localization effect to that of covariance localized filters. It is shown that the use of the same localization function in covariance localization and observation localization results in distinct effective localization length-scales in the Kalman gain. In order to improve the performance of observation localization, a regulated localization scheme is introduced. Twin experiments with the Lorenz-96 model demonstrate that the regulated localization can lead to a significant reduction of estimation errors as well as increased stability of the assimilation process. Copyright © 2011 Royal Meteorological Society

Description: In this work the dynamic behaviour of the wind in the nocturnal boundary layer is studied, with a particular focus on systematic behaviour of the near-surface wind. Recently, an extension of the well-known Blackadar model for frictionless inertial oscillations above the nocturnal boundary layer was proposed by Van de Wiel et al. , which accounts for frictional effects within the nocturnal boundary layer. It appears that the nocturnal wind velocity profile tends to perform an inertial oscillation around an equilibrium wind profile, rather than around the geostrophic wind vector (as in the Blackadar model). In the present study we propose the concept of ‘composite hodographs’ to evaluate the ideas and assumptions of the aforementioned analytical model. Composite hodographs are constructed based on a large observational dataset from the Cabauw observatory. For comparison and deeper analysis, this method is also applied to single-column model simulations that represent the same dataset. From this, it is shown that winds in the middle and upper part of the nocturnal boundary layer closely follow the dynamics predicted by the model by Van de Wiel et al. In contrast, the near-surface wind shows more complex behaviour that can be described by two different stages: (1) a decelerating phase where the wind decreases rapidly in magnitude due to enlarged stress divergence in the transition period near sunset (an aspect not included in the analytical model), and (2) a regular type of inertial oscillation, but with relatively small amplitude as compared to the oscillations in the middle and upper parts of the nocturnal boundary layer. Copyright © 2011 Royal Meteorological Society

Description: The limits of predictability of the meridional overturning circulation (MOC) and upper-ocean temperatures due to errors in ocean initial conditions and model parametrizations are investigated in an idealized configuration of an ocean general circulation model (GCM). Singular vectors (optimal perturbations) are calculated using the GCM, its tangent linear and adjoint models to determine an upper bound on the predictability of North Atlantic climate. The maximum growth time-scales of MOC and upper-ocean temperature anomalies, excited by the singular vectors, are 18.5 and 13 years respectively and in part explained by the westward propagation of upper-ocean anomalies against the mean flow. As a result of the linear interference of non-orthogonal eigenmodes of the non-normal dynamics, the ocean dynamics are found to actively participate in the significant growth of the anomalies. An initial density perturbation of merely 0.02 kg m −3 is found to lead to a 1.7 Sv MOC anomaly after 18.5 years. In addition, Northern Hemisphere upper-ocean temperature perturbations can be amplified by a factor of 2 after 13 years. The growth of upper-ocean temperature and MOC anomalies is slower and weaker when excited by the upper-ocean singular vectors than when the deep ocean is perturbed. This leads to the conclusion that predictability experiments perturbing only the atmospheric initial state may overestimate the predictability time. Interestingly, optimal MOC and upper-ocean temperature excitations are only weakly correlated, thus limiting the utility of SST observations to infer MOC variability. The excitation of anomalies in this model might have a crucial impact on the variability and predictability of Atlantic climate. The limit of predictability of the MOC is found to be different from that of the upper-ocean heat content, emphasizing that errors in ocean initial conditions will affect various measures differently and such uncertainties should be carefully considered in decadal prediction experiments. Copyright © 2011 Royal Meteorological Society

Description: Preferred jet stream positions and their link to regional circulation patterns over the winter North Atlantic/European sector are investigated to corroborate findings of multimodal behaviour of the jet positions and to analyse patterns of preferred paths and transition probabilities between jet regimes using ERA-40 data. Besides the multivariate Gaussian mixture model, hierarchical clustering and data image techniques are used for this purpose. The different approaches all yield circulation patterns that correspond to the preferred jet regimes, namely the southern, central and the northern positions associated respectively with the Greenland anticyclone or blocking, and two opposite phases of an East Atlantic-like flow pattern. Growth and decay patterns as well as preferred paths of the system trajectory are studied using the mixture model within the delay space. The analysis shows that the most preferred paths are associated with central to north and north to south jet stream transitions with a typical time-scale of about 5 days, and with life cycles of 1–2 weeks. The transition paths are found to be consistent with transition probabilities. The analysis also shows that wave breaking seems to be the dominant mechanism behind Greenland blocking. Copyright © 2011 Royal Meteorological Society

Description: The transformed Eulerian mean residual circulation is calculated from ERA-Interim for 1989–2009. Known as the Brewer–Dobson circulation, this measures the tropical upwelling of mass from troposphere to stratosphere, the mean meridional mass transport in the stratosphere and the downwelling of mass in the Extratropics. Major features of the Brewer–Dobson circulation, including the seasonal migration of the tropical upwelling toward the summer pole, are well represented. In the tropical lower stratosphere vertical velocities are less noisy than in other reanalyses, though significant tidal variations demonstrate the need for 6-hourly data. Throughout the year tropical lower stratospheric ascent rates are a minimum at the Equator and strongest in the Northern Hemisphere. In each hemisphere the maximum tropical ascent occurs during summer, whereas the strongest circulation and maximum in extratropical descent occur in the winter hemisphere. At 70 hPa the annual mean upwelling mass flux is 5.9 × 10 9 kg s −1 , with the zonal drag from resolved waves and parametrized orographic gravity wave drag (OGWD) providing 70% and 4% of the driving, respectively. Hence it is concluded that the OGWD probably underestimates the momentum deposited above 70 hPa in addition to there being an absence of drag from non-orographic gravity waves. A statistically significant trend of −5% per decade in the upwelling mass flux is considered unreliable because it is inconsistent with the negative temperature trend, assuming a mainly adiabatic temperature response at this level (70 hPa) to the changes in upwelling. Copyright © 2011 British Crown copyright, the Met Office. Published by John Wiley & Sons Ltd.

Description: Single column model simulations using the UK Met Office Unified Model, as used in the Australian Community Climate Earth System Simulator, are presented for the Tropical Warm Pool–International Cloud Experiment (TWP–ICE) field study. Two formulations for the representation of clouds are compared with the extensive observations taken during the campaign, giving insight into the ability of the model to simulate tropical cloud systems. During the active monsoon phase the modelled cloud cover has a stronger dependence on relative humidity than the observations. Observed ice cloud properties during the suppressed monsoon period show that the ice water content is significantly underestimated in the simulations. The profiles of modelled ice fall speeds are faster than those observed in the levels above 12 km, implying that the observations have smaller sized particles in larger concentrations than the models. Both simulations show similar errors in the diurnal cycle of relative humidity during the active monsoon phase, suggesting that the error is less sensitive to the choice of cloud scheme and rather is driven by the convection scheme. However, during the times of suppressed convection the relative humidity error is different between the simulations, with congestus convection drying the environment too much, particularly in the prognostic cloud-scheme simulation. This result shows that the choice of cloud scheme and the way that the cloud and convection schemes interact plays a role in the temperature and moisture errors during the suppressed monsoon phase, which will impact the three-dimensional model simulations of tropical variability. Copyright © 2011 Royal Meteorological Society

Description: This work tackles the problem of the automated detection of the atmospheric boundary layer (BL) height h , from aerosol lidar/ceilometer observations. A new method, the Bayesian selective method (BSM), is presented. It implements a Bayesian statistical inference procedure which combines in a statistically optimal way different sources of information. Firstly, atmospheric stratification boundaries are located from discontinuities in the ceilometer backscattered signal. The BSM then identifies the discontinuity edge that has the highest probability to effectively mark the BL height. Information from the contemporaneous physical boundary layer model simulations and a climatological dataset of BL height evolution are combined in the assimilation framework to assist this choice. The BSM algorithm has been tested for 4 months of continuous ceilometer measurements collected during the BASE:ALFA project, and is shown to realistically diagnose the BL depth evolution in many different weather conditions. A standard one-dimensional processing of the ceilometer signal without the a priori support of the dynamical and climatological BL models often fails to correctly detect h , with the greatest inaccuracies occurring at night-time when residual layers can generate very strong signals, which are then classified by an automated application of the gradient or of the wavelet analysis as the most probable BL height. The BSM approach instead carries information on the low climatological probability to find elevated BL depths at night and penalizes the selection of these points. Moreover, this method is able to correctly convey information along the temporal dimension, thus filling data gaps using earlier and subsequent ceilometer information for the retrieval. Copyright © 2011 Royal Meteorological Society

Description: Retrospective optimal interpolation (ROI) is a method that is used to minimize cost functions with multiple minima without using adjoint models. We address two weaknesses associated with the cost-effective formulation of ROI and offer possible solutions. The first weakness of the cost-effective ROI formulation is that the error tolerance would become large in practical application due to computation costs. When the error tolerance is large, accuracy-saturated modes do not extract information from new incoming observations even though they are likely to be flawed. To address this problem, we modify the existing ‘reduced-resolution’ formulation by using eigen-decomposition of the background error covariance at each analysis step. We refer to the modified algorithm as a ‘reduced-rank’ algorithm. This modification allows us to deal with larger error variances while analysing the same number of control variables, because eigen-decomposition steeply reorders the error variance in descending order. As a result, when the reduced-rank algorithm is applied, the number of analysed control variables becomes smaller than when the reduced-resolution algorithm is used with the same error tolerance. The second weakness is the underestimation of the trailing mode of background error covariance that is projected onto the future observation space. This originates from errors in control variables extracted from analysis procedures. To prevent the occurrence of filter divergence due to this underestimation, we reduce the weighting of the observational increments in the analysis. By implicitly assuming that the rate of being projected onto the future observation space of the trailing eigenmodes is similar to that of the leading eigenmodes, we develop a method that inflates the observation error covariance and consequently improves analysis quality in the Lorenz 40-variable and the 960-variable model experiments. Copyright © 2011 Royal Meteorological Society

Description: We develop a hydrostatic Hamiltonian particle-mesh (HPM) method for efficient long-term numerical integration of the atmosphere. In the HPM method, the hydrostatic approximation is interpreted as a holonomic constraint for the vertical position of particles. This can be viewed as defining a set of vertically buoyant horizontal meshes, with the altitude of each mesh point determined so as to satisfy the hydrostatic balance condition and with particles modelling horizontal advection between the moving meshes. We implement the method in a vertical-slice model and evaluate its performance for the simulation of idealized linear and nonlinear orographic flow in both dry and moist environments. The HPM method is able to capture the basic features of the gravity wave to a degree of accuracy comparable with that reported in the literature. The numerical solution in the moist experiment indicates that the influence of moisture on wave characteristics is represented reasonably well and the reduction of momentum flux is in good agreement with theoretical analysis. Copyright © 2011 Royal Meteorological Society

Description: This study investigates transient events of intense ocean evaporation with an amplitude exceeding 250 W m −2 , a duration of a few days, and a spatial extent of about 10ˆ6 km 2 over the eastern North Atlantic (referred to as ‘evaporation hot spots’) and their impact on southern Alpine heavy precipitation. First, moisture sources for a heavy precipitation event in the Piedmont in November 2002 are studied using a water-tagging simulation with a regional model. The results reveal three main moisture sources: land evapotranspiration, and evaporation from the Mediterranean and the North Atlantic, with the last source contributing the most. This was partly due to an evaporation hot spot that appeared along the western edge of a prominent upper-level trough about two days prior to the onset of heavy precipitation. In the hot spot area strong surface winds induced by the upper-level trough led to intense evaporation of water that was transported around the trough to the Piedmont region during subsequent days, where it contributed to the heavy precipitation. Secondly, analyses by the European Centre for Medium-Range Weather Forecasts (ECMWF) are used to investigate climatologically the potential relationship between eastern North Atlantic evaporation hot spots and southern Alpine precipitation. During a 10-year time period, 42 hot spots have been identified in the eastern North Atlantic. It is shown that they typically occur along the western flank of prominent upper-level troughs, and that the evaporating moisture is transported to Europe within one to four days. A climatological analysis of southern Alpine heavy precipitation events shows that they are frequently preceded by intense North Atlantic evaporation. Hence the climatological analysis further supports the conclusion from the Piedmont 2002 tagging experiment that intense evaporation over the North Atlantic and the subsequent moisture transport, both induced by the upper-level trough, are potential key factors for the development of southern Alpine heavy precipitation events. Copyright © 2011 Royal Meteorological Society

Description: The spectral predictability of the Met Office's Unified Model is examined using identical-twin experiments and the relative kinetic energy. In the troposphere, previous NWP results are recovered, namely the emergence of distinct regimes and the maximisation of the growth rate on synoptic scales at early times; in the middle atmosphere, the predictability decay is slower. This difference is attributed to the increase in the amplitude of gravity waves. The influence of small-scale motions is highlighted: improving their numerical representation by decreasing the timestep enhances predictability. Copyright © 2011 British Crown copyright, the MetOffice. Published by John Wiley & Sons Ltd.

Description: The effect of the relative orientation of the vertical wind shear to the surface wind on tropical cyclogenesis is explored in environments of radiative-convective equilibrium (RCE) through numerical simulation. This study serves as a companion paper to an earlier study on the thermodynamics of genesis in RCE. It is found, when the mean surface wind and shear are aligned, a negative surface wind anomaly arises from the superposition of the mean and vortex surface flows left of the shear vector. The resulting weak surface enthalpy fluxes and up-shear quasi-balanced subsidence leads to dry air being located cyclonically down-wind of the down-shear convective anomaly. Thus convection is inhibited from propagating cyclonically around the core leading to a large down-shear vortex tilt. Conversely, in a counter-aligned orientation, the negative surface wind anomaly and driest air is found right of the shear vector. Hence the driest air rotates into the down-shear flank where it moistened by shear-organized convection. Furthermore, the boundary layer is relatively moist left of shear due to the positive surface wind anomaly, therefore promoting the cyclonic propagation from down-shear and constraining the magnitude of the vortex tilt. Genesis is intimately tied to the magnitude of the tilt and is found to occur once the mid-level vortex has precessed into the up-shear flank. For smaller values of maximum tilt, vortex precession is comparatively rapid, aided by “showerhead” moistening provided by the up-shear advection of frozen condensate aloft. With the up-shear flank pre-moistened, rapid precession of the mid-level vortex, at smaller radii, leads to near saturation on the mesoscale and the onset of rapid intensification. When the magnitude of the tilt is quite large, precession is much slower and the showerhead effect is significantly reduced until just prior to the emergence of the mid-level vortex in the up-shear flank. Copyright © 2011 Royal Meteorological Society

Description: Relative humidity (RH) measurements, as derived from wet-bulb and dry-bulb thermometers operated as a psychrometer within a thermometer screen, have limited accuracy because of natural ventilation variations. Standard RH calculations generally assume a fixed screen psychrometer coefficient, but this is too small during poor ventilation. By comparing a reference humidity probe—exposed within a screen containing a psychrometer—with wind-speed measurements under controlled conditions, a wind-speed correction for the screen psychrometer coefficient has been derived and is applicable when 2-metre wind speeds fall below 3 m s −1 . Applying this to hourly-averaged data reduced the mean moist RH bias of the psychrometer (over the reference probe) from 1.2% to 0.4%, and reduced the interquartile range of the RH differences from 2.0% to 0.8%. This correction is particularly amenable to automatic measurement systems. Copyright © 2011 Royal Meteorological Society

Description: In early January 2011, southern China experienced another severe wintertime storm as in the winter of 2008. The storm consisted of a narrow east-west-oriented freezing rain band over central Guizhou with an extensive area of snow to the north and a rain swath to the south. This study investigates this event using conventional surface and radiosonde data as well as final (FNL) analyses data from the US National Center for Atmospheric Research/National Centers for Environmental Prediction(NCAR/NCEP). The results show that forcing by geostrophic and diabatic heating built up a positive direct secondary circulation perpendicular to the quasi-stationary front beneath 700 hPa to maintain the surface cold layer and warm layer aloft through surface cold advection and warm advection aloft. In addition, turbulence induced by strong wind shear in the middle and lower stratiform clouds with a low concentration of ice nuclei plays an important role in the growth of cloud drops and the enhancement of supercool raindrop precipitation over Guizhou. Copyright © 2011 Royal Meteorological Society

Description: An unusually deep (961 hPa) hurricane-like polar low over the Barents Sea during 18–21 December 2002 is studied by a series of fine-mesh (3 km) experiments using the Weather Research and Forecasting (WRF) model. The simulated polar low was similar to hurricanes and similar previous case-studies in that it had a clear, calm and warm eye structure surrounded by moist convection organized in spiral cloud bands, and the highest surface wind speeds were found in the eye wall. The proximity to the sea ice and the high surface wind speeds (about 25 m s −1 ) during the deepening stage triggered extremely high surface sensible and latent heat fluxes at the eye wall of about 1200 and 400 W m −2 , respectively. As the polar low moved eastward and weakened, maximum surface sensible and latent heat fluxes dropped to about 600 and 300 W m −2 , respectively. Two types of sensitivity experiments were designed to analyse the physical properties of the polar low. Firstly, physical processes such as condensational heating and sensible and/or latent heat fluxes were switched off–on throughout the simulation. In the second type, these processes were turned off–on after the polar low had reached its peak intensity, which minimized the deformation of the polar-low environment, making it suitable to study the direct effect of physical processes on the mature vortex. The experiments suggest that the deepening stage of the polar low was dominated by baroclinic growth and that upper-level potential vorticity forcing contributed throughout its life cycle. After the deepening stage, the baroclinicity vanished and the polar low was fuelled by surface sensible heat fluxes while latent heat fluxes played a minor role. Condensational heating was not essential for the energetics of the polar low. Surprisingly, in experiments where condensational heating was turned off throughout the simulation, the polar low intensified. Copyright © 2012 Royal Meteorological Society