British Crown Copyright Research Articles

Horizontal grids for global weather and climate prediction models: a review

AbstractA latitude–longitude grid is used by almost all operational atmospheric forecast models, and many research models. However, it is expected that the advantages of a latitude–longitude grid will become outweighed on massively parallel computers by data‐communication bottlenecks. There is therefore renewed interest in quasi‐uniform alternatives. This review surveys and assesses previously proposed horizontal grids for modelling the atmosphere over the sphere. Aspects of numerical accuracy likely to be affected by grid structure are discussed; particular attention is paid to computational modes and grid imprinting. Computational modes are potentially very serious, since they may be excited in realistic applications by boundary conditions, nonlinearity, physical forcing, and data assimilation. The geometry of polyhedra is reviewed due to its relation to numerical degrees of freedom, and hence to numerical wave dispersion and the possible existence of computational modes.All grids proposed to date have known problems or issues that merit further investigation. Orthogonal logically rectangular grids may be generated using conformal maps, but these suffer from singularities and resolution clustering. Resolution clustering may be avoided by using overset grids, but there are potential issues associated with the overlap regions. Alternatively, resolution clustering may be avoided, whilst retaining a logically rectangular grid, by giving up orthogonality; however, existing numerical schemes exploit orthogonality to obtain various properties thought to be important for accuracy, and it is not yet known whether these can also be obtained on non‐orthogonal grids. Quasi‐uniformity and orthogonality can be obtained without resolution clustering or overlaps by using non‐quadrilateral grid cells, such as triangles, or pentagons and hexagons. However, when a staggered placement of variables is used to minimise dispersion errors for fast waves, non‐quadrilateral grids support computational modes.In view of the lack of a single ideal grid, several topics meriting further investigation are identified. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

A new floating model level scheme for the assimilation of boundary‐layer top inversions: the univariate assimilation of temperature

AbstractThe assimilation of observations with a forecast is often heavily influenced by the description of the error covariances associated with the forecast. When a temperature inversion is present at the top of the boundary layer (BL), a significant part of the forecast error may be described as a vertical positional error (as opposed to amplitude error normally dealt with in data assimilation). In these cases, failing to account for positional error explicitly is shown to result in an analysis for which the inversion structure is erroneously weakened and degraded.In this article, a new assimilation scheme is proposed to explicitly include the positional error associated with an inversion. This is done through the introduction of an extra control variable to allow position errors in the a priori to be treated simultaneously with the usual amplitude errors. This new scheme, referred to as the ‘floating BL scheme’, is applied to the one‐dimensional (vertical) variational assimilation of temperature. The floating BL scheme is tested with a series of idealised experiments and with real data from radiosondes.For each idealised experiment, the floating BL scheme gives an analysis which has the inversion structure and position in agreement with the truth, and outperforms the assimilation which accounts only for forecast amplitude error. When the floating BL scheme is used to assimilate a large sample of radiosonde data, its ability to give an analysis with an inversion height in better agreement with that observed is confirmed. However, it is found that the use of Gaussian statistics is an inappropriate description of the error statistics of the extra control variable. This problem is alleviated by incorporating a non‐Gaussian description of the new control variable in the new scheme. Anticipated challenges in implementing the scheme operationally are discussed towards the end of the article. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

Evaluation of two cloud parametrization schemes using ARM and Cloud‐Net observations

AbstractGround‐based remote‐sensing observations from Atmospheric Radiation Measurement (ARM) and Cloud‐Net sites are used to evaluate the clouds predicted by a weather forecasting and climate model. By evaluating the cloud predictions using separate measures for the errors in frequency of occurrence, amount when present, and timing, we provide a detailed assessment of the model performance, which is relevant to weather and climate time‐scales. Importantly, this methodology will be of great use when attempting to develop a cloud parametrization scheme, as it provides a clearer picture of the current deficiencies in the predicted clouds.Using the Met Office Unified Model, it is shown that when cloud fractions produced by a diagnostic and a prognostic cloud scheme are compared, the prognostic cloud scheme shows improvements to the biases in frequency of occurrence of low, medium and high cloud and to the frequency distributions of cloud amount when cloud is present. The mean cloud profiles are generally improved, although it is shown that in some cases the diagnostic scheme produced misleadingly good mean profiles as a result of compensating errors in frequency of occurrence and amount when present. Some biases remain when using the prognostic scheme, notably the underprediction of mean ice cloud fraction due to the amount when present being too low, and the overprediction of mean liquid cloud fraction due to the frequency of occurrence being too high. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

The Brewer–Dobson circulation inferred from ERA‐Interim

AbstractThe 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 × 109 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.

Case‐to‐case variability of predictability of deep convection in a mesoscale model

AbstractSuccessful quantitative precipitation forecasts under convectively unstable conditions depend on the ability of the model to capture the location, timing and intensity of convection. Ensemble forecasts of two mesoscale convective outbreaks over the UK are examined with a view to understanding the nature and extent of their predictability. In addition to a control forecast, twelve ensemble members are run for each case with the same boundary conditions but with perturbations added to the boundary layer. The intention is to introduce perturbations of appropriate magnitude and scale so that the large‐scale behaviour of the simulations is not changed. In one case, convection is in statistical equilibrium with the large‐scale flow. This places a constraint on the total precipitation, but the location and intensity of individual storms varied. In contrast, the other case was characterised by a large‐scale capping inversion. As a result, the location of individual storms was fixed, but their intensities and the total precipitation varied strongly. The ensemble shows case‐to‐case variability in the nature of predictability of convection in a mesoscale model, and provides additional useful information for quantitative precipitation forecasting. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

Uncertainty in carbon exchange modelling in a forest canopy due to kB−1 parametrizations

AbstractThe difference in turbulent transfer efficiency between momentum and scalars, represented by kB−1, has been the subject of considerable interest in micrometeorology, and various parametrizations have been proposed to address this issue. The simple kB−1 parametrizations that are based on either empirical formulations or K‐theory are still popularly used in the land surface models despite their theoretical deficiencies. Moreover, the impact of the uncertainty in this parameter on modelling surface carbon exchange has not been previously estimated. In this study, we examined the uncertainties of the simulated gross primary productivity (GPP), ecosystem respiration (RE), and net ecosystem exchange (NEE ≡ RE–GPP) for a forest canopy due to kB−1 parametrizations. The tested parametrizations included not only the schemes that set kB−1 as a constant or as a function of only the Reynolds number (popularly used in the land surface model), but also the schemes that express kB−1 as a function of plant phenology derived from the Lagrangian theory. Except for parametrizations that produced aerodynamic resistance as large as canopy resistance over a tall forest canopy, the variabilities of GPP and RE induced by kB−1 parametrizations were less than 2% of the annual GPP and RE, respectively. Nevertheless, the model produced approximately 10% variability of NEE values with changes in the kB−1 parametrizations (203±24 g C m−2year−1) because the simulated RE was less sensitive to the kB−1 parametrizations than the simulated GPP due to the negative feedback among kB−1, temperature, and RE. Our findings reveal that kB−1 parametrizations should be suitably applied in land surface modelling for better simulation of the global carbon cycle. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

Objective detection of sting jets in low‐resolution datasets

AbstractSting jets are transient coherent mesoscale strong wind features that can cause damaging surface wind gusts in extratropical cyclones. Currently, we have only limited knowledge of their climatological characteristics. Numerical weather prediction models require enough resolution to represent slantwise motions with horizontal scales of tens of kilometres and vertical scales of just a few hundred metres to represent sting jets. Hence, the climatological characteristics of sting jets and the associated extratropical cyclones cannot be determined by searching for sting jets in low‐resolution datasets such as reanalyses.A diagnostic is presented and evaluated for the detection in low‐resolution datasets of atmospheric regions from which sting jets may originate. Previous studies have shown that conditional symmetric instability (CSI) is present in all storms studied with sting jets, while other rapidly developing storms of a similar character but no CSI do not develop sting jets. Therefore, it is assumed that the release of CSI is needed for sting jets to develop. While this instability will not be released in a physically realistic way in low‐resolution models (and hence sting jets are unlikely to occur), it is hypothesized that the signature of this instability (combined with other criteria that restrict analysis to moist mid‐tropospheric regions in the neighbourhood of a secondary cold front) can be used to identify cyclones in which sting jets occurred in reality. The diagnostic is evaluated, and appropriate parameter thresholds defined, by applying it to three case studies simulated using two resolutions (with CSI‐release resolved in only the higher‐resolution simulation). Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

A long‐term assessment of precipitation forecast skill using the Fractions Skill Score

AbstractThe Fractions Skill Score (FSS) is a spatial verification metric routinely computed in the operational verification suite. It enables the comparison of forecasts of different resolutions against a common spatial truth (radar rainfall analyses) in such a way that high‐resolution forecasts are not penalized for representativeness errors that arise from the ‘double penalty’ problem. Officially Met Office model precipitation forecast accuracy is monitored using the Equitable Threat Score (ETS) at gauge locations. These precipitation scores form part of a basket of measures assessing six surface parameters known as the UK index, which forms the basis for making decisions regarding model upgrades (especially over the UK). It is used to monitor the impact of continuous model improvements. This framework and the methodology underlying it, is less appropriate for high‐resolution forecasts for reasons as described above. For precipitation forecasts in particular, a new framework for long‐term monitoring is necessary and the FSS provides such a potential framework.This paper provides an objective critique of FSS results to date. It has been shown that the ‘convection‐permitting’ (4 km) Unified Model (MetUM) forecasts are better than the 12 km MetUM (significant at the 5% level). The scale at which the models have sufficient practical skill is typically 10 km better for the high‐resolution forecasts, and are better at forecasting afternoon convection exceeding 4 mm (6 h)−1. The use of frequency (percentile) thresholds is recommended because of the implicit bias removal this approach provides, as any rain in a forecast period is treated as ‘the event of interest’. © 2011 British Crown Copyright, the Met Office. Published by John Wiley & Sons Ltd.

Variational assimilation of cloud fraction in the operational Met Office Unified Model

AbstractShort‐range forecasts of cloud cover are considered an important part of providing a public weather service in the UK. These require accurate initial conditions. Satellite imagery and surface cloud reports provide observations of cloud but using these to initialize forecast models is not easy. Cloud data are not well suited to variational analysis schemes. This paper describes how the Met Office variational analysis has been adapted to use such data. Results show that cloud observations can be usefully assimilated. Copyright © 2011 British Crown copyright, the Met Office. Published by John Wiley & Sons Ltd.

Comparison of cloud products within IASI footprints for the assimilation of cloudy radiances

AbstractThis article compares different methods of deriving cloud properties in the footprint of the Infrared Atmospheric Sounding Interferometer (IASI), onboard the European MetOp satellite. Cloud properties produced by ten operational schemes are assessed and an intercomparison of the products for a 12 h global acquisition is presented. Clouds cover a large part of the Earth, contaminating most of the radiance data. The estimation of cloud top height and effective amount within the sounder footprint is an important step towards the direct assimilation of cloud‐affected radiances. This study first examines the capability of all the schemes to detect and characterize the clouds for all complex situations and provides some indications of confidence in the data. Then the dataset is restricted to thick overcast single layers and the comparison shows a significant agreement between all the schemes. The impact of the retrieved cloud properties on the residuals between calculated cloudy radiances and observations is estimated in the long‐wave part of the spectrum. Copyright © 2011 Royal Meteorological Society, Crown in the right of Canada, and British Crown copyright, the Met Office

Assessing implicit large eddy simulation for two‐dimensional flow

AbstractNumerical models of the atmosphere cannot resolve all relevant scales; the effects of unresolved scales on resolved scales must be represented by a subgrid model or parametrization. When the unresolved scales are similar in character to the resolved scales (as in three‐dimensional or layerwise two‐dimensional turbulence) the problem is essentially one of large eddy simulation. In this situation, one approach to subgrid modelling is implicit large eddy simulation (ILES), where the truncation errors of the numerical model attempt to act as the subgrid model. ILES has been shown to have some success for three‐dimensional turbulence, but the validity of the approach has not previously been examined for two‐dimensional or layerwise two‐dimensional flow, which is the regime relevant to weather and climate modelling. Two‐dimensional turbulence differs qualitatively from three‐dimensional turbulence in several ways, most notably in having upscale energy and downscale enstrophy transfers. The question is of practical importance since many atmospheric models in effect use the ILES approach, for example through the use of a semi‐Lagrangian advection scheme. In this paper a number of candidate numerical schemes are tested to determine whether their truncation errors can approximate the subgrid terms of the barotropic vorticity equation. Results show that some schemes can implicitly model the effects of the subgrid term associated with the stretching and thinning of vorticity filaments to unresolvable scales; the subgrid term is then diffusive and is associated with the downscale enstrophy transfer. Conservation of vorticity, by using a flux form scheme rather than advective form for advection of vorticity, was found to improve performance of a candidate ILES scheme. Some effects of the subgrid terms could not be captured by any of the schemes tested, whether using an implicit or a simple explicit subgrid model: none of the schemes tested is able to capture the upscale transfer of energy from unresolved to resolved scales. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

Doppler radar observations of non‐occluding, cyclic vortex genesis within a long‐lived tornadic storm over southern England

AbstractOn 3 November 2009 a long‐lived tornadic storm developed close to the apex of a small frontal wave moving eastwards over southern England. Ten tornadoes and several instances of non‐tornadic wind damage occurred along a narrow path extending for over 120 km. The storm passed within 5 km of two of the radars in the Met Office's network, providing an opportunity for detailed observation. Initial development was in the form of an intense segment of line convection at the northern flank of the frontal wave and associated deepening surface mesolow (diameter 60–70 km). The line evolved rapidly; the southern portion bowed out to form a transient ‘S’‐shaped echo before dissipating. The northern section persisted, leaving a hook‐shaped storm which became quasi‐steady in overall echo shape.Radars revealed the cyclic development of five discrete low‐level mesovortices (diameter 1–4 km) between 1130 and 1330 UTC, along a zone of intense cyclonic vertical vorticity at the southeastern edge of the storm. These vortices intensified rapidly (typically from initial detection to maximum observed differential velocity in ∼10 min), some attaining low‐level differential velocities exceeding 40 m s−1. However, significant differential velocities (>20 m s−1) were not observed above 2.5 km above ground level, and no persistent mid‐level mesocyclone was present. The cyclic behaviour was unusual in that vortices were ‘non‐occluding’, moving southeastwards relative to the storm (to the right of the storm motion vector), away from the core. This is in contrast to the more commonly observed evolution of vortex occlusion (by storm downdraughts) and subsequent leftward and rearward storm‐relative movement during decay. Copyright © 2011 British Crown copyright, the Met Office. Published by JohnWiley & Sons Ltd.

Satellite cloud and precipitation assimilation at operational NWP centres

AbstractThe status of current efforts to assimilate cloud‐ and precipitation‐affected satellite data is summarised with special focus on infrared and microwave radiance data obtained from operational Earth observation satellites. All global centres pursue efforts to enhance infrared radiance data usage due to the limited availability of temperature observations in cloudy regions where forecast skill is estimated to strongly depend on the initial conditions. Most systems focus on the sharpening of weighting functions at cloud top providing high vertical resolution temperature increments to the analysis, mainly in areas of persistent high and low cloud cover. Microwave radiance assimilation produces impact on the deeper atmospheric moisture structures as well as cloud microphysics and, through control variable and background‐error formulation, also on temperature but to lesser extent than infrared data. Examples of how the impacts of these two observation types are combined are shown for subtropical low‐level cloud regimes. The overall impact of assimilating such data on forecast skill is measurably positive despite the fact that the employed assimilation systems have been constructed and optimized for clear‐sky data. This leads to the conclusion that a better understanding and modelling of model processes in cloud‐affected areas and data assimilation system enhancements through inclusion of moist processes and their error characterization will contribute substantially to future forecast improvement. Copyright © 2011 Royal Meteorological Society, Crown in the right of Canada, and British Crown copyright, the Met Office

A quasi‐spheroidal system for modelling global atmospheres: geodetic coordinates

AbstractIn global atmospheric models, the geopotentials of apparent gravity are usually represented as spheres following minor but systematic approximations, including neglect of the ellipticity of the Figure of the Earth and of the latitude variation of the magnitude of apparent gravity, g. Improved representation using families of simple spheroids encounters various problems. Confocal oblate spheroids are technically convenient but qualitatively unsuitable as regards the implied latitude variation of g and vice versa for similar oblate spheroids. Representation of geopotentials by the quasi‐spheroidal surfaces of the geodetic coordinate system (used in aircraft navigation and satellite geodesy) is considered here. A single reference spheroid represents the Figure of the Earth, and exterior geopotentials are represented by surfaces of constant perpendicular distance from the reference spheroid. These surfaces are not precise spheroids, and latitude variation of g is not represented. The (orthogonal) coordinates are longitude, geographic latitude and perpendicular height above the reference spheroid. The geodetic system has many desirable features, including ease of transformation to Cartesian coordinates and hence derivation of the metric factors (not required for geodesy or navigation). Transformation from Cartesian coordinates is not analytically straightforward, but is amenable to perturbation or efficient numerical solution. A comparison is made with precisely spheroidal coordinate systems. Copyright © 2011 British Crown copyright, the Met Office. Published by John Wiley & Sons Ltd.

Comparing the UK Met Office Climate Prediction System DePreSys with idealized predictability in the HadCM3 model

AbstractThe initial condition effect on climate prediction skill over a 2‐year hindcast time‐scale has been assessed from ensemble HadCM3 climate model runs using anomaly initialization over the period 1990–2001, and making comparisons with runs without initialization (equivalent to climatological conditions), and to anomaly persistence. It is shown that the assimilation improves the prediction skill in the first year globally, and in a number of limited areas out into the second year. Skill in hindcasting surface air temperature anomalies is most marked over ocean areas, and is coincident with areas of high sea surface temperature and ocean heat content skill. Skill improvement over land areas is much more limited but is still detectable in some cases. We found little difference in the skill of hindcasts using three different sets of ocean initial conditions, and we obtained the best results by combining these to form a grand ensemble hindcast set.Results are also compared with the idealized predictability studies of Collins (Clim. Dynam. 2002; 19: 671–692), which used the same model. The maximum lead time for which initialization gives enhanced skill over runs without initialization varies in different regions but is very similar to lead times found in the idealized studies, therefore strongly supporting the process representation in the model as well as its use for operational predictions. The limited 12‐year period of the study, however, means that the regional details of model skill should probably be further assessed under a wider range of observational conditions. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

Performance of different forecast systems in an exceptional storm in the Western Mediterranean Sea

AbstractWe consider an exceptional storm—‘Klaus’ (January 2009)—its evolution on the Western Mediterranean Sea, and how the associated wind and wave conditions were modelled by seven of the major systems presently operational in this area. We intercompare the model results and then verify them and the related model ensemble versus the available measured data.Working with short‐term forecasts (24 h) only, as expected, each model correctly anticipates the incoming of an exceptional storm. However, even at such limited range, we have found substantial differences among the results of the different models. The differences concern the time the storm should have entered the Western Mediterranean Sea, the peak values of wind speed and significant wave height, the general distribution of the fields, and the locations where the maxima were achieved.We have compared the model results versus the available measured data, wind from scatterometer, waves from altimeter, plus a few buoy data. We have found some inconsistencies in the results, model wind data being on average larger than the measured one, while the opposite was true for wave heights. However, the limited amount of data available and its different times and positions, at and off the centre of the storm, impede the drawing of any definite conclusion in this respect.On the whole we feel that our results, although related to a single storm, cast doubts on the reliability of a single forecast system to provide sufficiently reliable and accurate forecasts in case of an incoming exceptional storm. The results, both for wind and waves, have improved using an ensemble of the seven considered models. This suggests that there is no relevant systematic error in the used models except, as possibly suggested by our results, in the case of wave generation under very strong wind and very young sea conditions. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

Evaluation of ice cloud representation in the ECMWF and UK Met Office models using CloudSat and CALIPSO data

AbstractIce 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

Simulating climate change in UK cities using a regional climate model, HadRM3

AbstractA regional climate model (RCM) at 25 km resolution, coupled to a simple urban land‐surface scheme has been used to assess the sensitivity of UK urban climates to large‐scale greenhouse gas (GHG) induced climate change, local forcing from urban land use, and anthropogenic heat flux (AHF) resulting from energy use in urban areas. GHG‐induced climate change is similar over both urban and rural land surfaces in the model, suggesting that under a changing climate the relative magnitude of urban heat islands in the UK would remain the same. However, indirect effects from urbanisation or changes in energy consumption within UK cities would contribute to further localized anthropogenic climate change that can be quantified by this model. The presence of the urban heat island also has a significant impact on the frequency of extreme high temperatures both in present and future climates, and will exacerbate the impact of climate change on the urban population.The simple urban land‐surface exchange scheme is a cost effective additional component of the RCM. It is shown to provide a realistic city‐scale urban heat island responding to the dynamical weather of the model at every model time‐step, with a magnitude and variability comparable to observed values. The climate model provides long integrations to derive large samples of heat islands under a full range of simulated dynamical conditions, and in this study sampling more than 100 years of climate change. Copyright © 2011 Royal Meteorological Society and British Crown copyright, the Met Office

Ensemble predictability of an isolated mountain thunderstorm in a high‐resolution model

AbstractConvective‐scale ensemble simulations with perturbed initial conditions are performed to investigate the physical mechanisms, sensitivities and predictability of convective precipitation for a well‐observed event from the Convective and Orographically induced Precipitation Study (COPS). On this day an isolated thunderstorm developed over the Black Forest mountains in southwest Germany that was initiated by thermally driven low‐level convergence. With the default ensemble configuration, none of the members simulated any deep convection. A soil moisture and atmospheric moisture deficiency was found to be responsible for a lack of boundary layer moisture in the analysis. Correction of this moisture bias yielded an ensemble in which some members produced a realistic convective storm. In contrast, other members only produced shallow convection. A robust feature of the members that simulated the storm was that an impinging upper‐level disturbance to the west had progressed further east towards the COPS region. This led to increased westerly winds aloft, which when mixed down into the boundary layer acted to feed a deeper layer of moist valley air into the convergent mountain circulation. The simulations reveal a very strong sensitivity to uncertainties in the large‐scale flow as well as the boundary layer structure and stability. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

Physico‐chemical and optical properties of Sahelian and Saharan mineral dust: in situ measurements during the GERBILS campaign

AbstractThis paper presents new results on the composition, size and shape of mineral dust particles from African sources which were obtained by analysis of bulk filter samples collected in June 2007 onboard the BAe‐146 research aircraft of the Facility for Airborne Atmospheric Measurements (FAAM). The aircraft was operated over Mauritania, Mali and Niger during the Geostationary Earth Radiation Budget Intercomparisons of Longwave and Shortwave radiation (GERBILS) campaign. Dust sampled during the campaign originated from various sources, including locally in the Sahel as a result of large‐scale convective activity.Regardless of origin, clays (illite, kaolinite) dominated the total volume (79–90%); the remainder was composed of quartz, calcium‐rich minerals (calcite, dolomite, gypsum) and alkali feldspars. Iron oxides, measured using a selective chemical extraction method, accounted for 1–3% of the total dust mass. The dependence of particle number size and shape distribution on the origin of dust seems minor too, although our results might be slightly misleading due to the fact that those kinds of data have been gathered on flights when dust had comparable origins and residence time.Mineral dust is only weakly absorbing in the mid‐visible wavelengths (single scattering albedo ω0 > 0.95 at 550 nm), and ω0 measured values can be reproduced by measuring the bulk fractions of the major minerals, i.e. clays, quartz, calcite and iron oxides. At this wavelength, knowledge of the nature of clays and iron oxides, or the state of mixing of the minerals, does not induce significant differences in the results. A more precise description of the nature of clays and iron oxides is necessary at lower wavelengths owing to larger differences in their spectral optical properties. In particular, knowledge of the nature of the dominant clay is important for determining light scattering in the backward hemisphere. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office