ALBERT

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: A simple self-contained theory is proposed for describing the life cycles of convective systems as a discharge–recharge process. A closed description is derived for the dynamics of an ensemble of convective plumes based on an energy cycle. The system consists of prognostic equations for the cloud work function and the convective kinetic energy. The system can be closed by introducing a functional relationship between the convective kinetic energy and the cloud-base mass flux. The behaviour of this system is considered under a bulk simplification. Previous cloud-resolving models as well as bulk statistical theories for ensemble convective systems suggest that a plausible relationship would be to assume that the convective kinetic energy is linearly proportional to the cloud-base mass flux. As a result, the system reduces to a nonlinear dynamical system with two dependent variables, the cloud-base mass flux and the cloud work function. The fully nonlinear solution of this system always represents a periodic cycle regardless of the initial condition under constant large-scale forcing. Importantly, the inclusion of energy dissipation in this model does not in itself lead the system into equilibrium. Copyright © 2011 Royal Meteorological Society

Description: During the African Monsoon Multidisciplinary Analysis (AMMA) campaign in 2006, extended surface and boundary-layer measurements were performed to study the influence of soil-moisture patterns on the generation of thermally forced circulations and triggering of deep convection. However, not all processes involved in the triggering of a mesoscale convective system (MCS) could be identified in previous studies. Therefore, COSMO (Consortium for Small-scale Modeling) simulations were carried out investigating possible trigger mechanisms. On 31 July 2006, an MCS was initiated and on 1 August, soil-moisture inhomogeneities resulted in a thermally forced circulation with an associated convergence zone, but no deep convection was triggered. It was found that the MCS on 31 July was influenced by a cyclonic vortex and favoured by the superposition of two convergence zones of different origins. Initiation of the MCS occurred in the simulation when moist monsoon air was transported to the north, associated with a cold pool ahead of another MCS, and reached the convergence zone. On 1 August, the simulation reproduced the thermally forced circulation caused by the soil-moisture pattern, which had been produced by the precipitation of the MCS. However, due to low humidity in the boundary layer and low convective available potential energy, the lifting along the convergence zone did not trigger deep convection. Copyright © 2011 Royal Meteorological Society

Description: The structure and propagation of oceanic Yanai waves (also known as mixed Rossby–gravity waves) are investigated by analysis of satellite-derived sea-surface-height (SSH) data. Significant spectral peaks along the dispersion curves of first and second baroclinic mode Yanai waves are identified by the wavenumber–frequency spectral analysis of SSH for the period 10–17 days. The spatial structure and propagation of these modes are described by an analysis of SSH time series filtered in the frequency–wavenumber domain that includes strong signals along the dispersion curves but excludes those of 17-day tropical instability waves. The difference in meridional structure between the first and second baroclinic modes, which is consistent with theory, is evident. The slow eastward propagation of first and second baroclinic mode Yanai wave packets, which is consistent with the group velocity derived from theory, is also isolated from the SSH data. Year-to-year variability of Yanai wave activity is compared with that of the El Niño–Southern Oscillation (ENSO), showing a moderate correlation with ENSO Modoki. © Published in 2011 by John Wiley & Sons Ltd.

Description: An ensemble method combined with a four-dimensional variational data assimilation system is used to provide monthly estimates of the background error covariance matrix for global stratospheric and upper tropospheric ozone for the year 2008. The system is composed of the Mocage chemical transport model and the Valentina assimilation system. The ensemble was obtained from parallel analyses of perturbed data from the Microwave Limb Sounder (MLS) instrument. The monthly estimates of background error covariances have then been introduced in the assimilation suite. To assess the separate contribution of each of its components, a number of analyses were realized, using only some estimated components of the background error covariances and a basic model for the others. The evaluation is realized by comparing the analyses with independent ozone profiles (from ozonesondes) and total ozone columns (from the Ozone Monitoring Instrument). It demonstrates that using the estimated statistics compared to basic models for the background error covariance matrix globally slightly improves the analysis quality; however, using the estimated statistics more largely improves the analysis quality for special situations encountered in April and October. In these situations, the most important parameter for the analysis quality is the use of estimated correlations. Copyright © 2011 Royal Meteorological Society

Description: In the present study, the synoptic conditions that favour the occurrence of aerosol episodes over the broader Mediterranean basin are investigated. For this purpose daily satellite measurements of aerosol optical depth (AOD) at 550 nm from MODIS-Terra (Collection 5) for the period from 1 March 2000 to 28 February 2007 were used together with mean-sea-level pressure and geopotential height at 700 hPa obtained from the NCEP/NCAR Reanalysis Project. An objective and dynamic algorithm was developed in order to determine strong and extreme aerosol episodes in the study area. According to the applied methodology and the relevant criteria, 322 days with aerosol episodes have been identified. The atmospheric circulation of each day was objectively classified by the implementation of Factor Analysis and Cluster Analysis, resulting in eight representative synoptic conditions (clusters). The aerosol episode days (AEDs) are most frequently observed during the dry period of the year, and especially in July–August (108 days or 33.5%) and April–May (106 days or 33%) and give rise to desert dust, anthropogenic pollution and biomass-burning episodes. The most frequent synoptic conditions are those of Clusters 4 (30.1%) and 5 (21%), inducing episodes that occur primarily in the western and secondarily in the central Mediterranean. In the former case, which is observed in summer, Europe is under the influence of the extended subtropical anticyclone of the Atlantic (Azores), while the eastern Mediterranean is under the influence of the extended thermal low of south-west Asia. In the latter case, in spring and summer, anticyclonic conditions prevail over central Europe and over the east Atlantic Ocean, and low pressures at the northwestern parts of the broader Mediterranean basin and across the Sahara. During the aerosol episode days of the various clusters, the mean regional AOD can reach values up to 0.8 ± 0.2 and 2.3 ± 1.3 for strong and extreme episodes, respectively. Copyright © 2011 Royal Meteorological Society

Description: Short-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.

Description: Atmospheric chemistry and air-quality numerical models are driven by uncertain forcing fields: emissions, boundary conditions, wind fields, vertical turbulent diffusivity, kinetic chemical rates, etc. Data assimilation can help to assess these parameters or fields of parameters. Because such parameters are often much more uncertain than the fields diagnosed in meteorology and oceanography, data assimilation is much more of an inverse modelling challenge in this context. In this article these ideas are experimented with by revisiting the Chernobyl accident dispersion event over Europe. A fast four-dimensional variational scheme (4D-Var) is developed, which seems appropriate for the retrieval of large parameter fields from large observation sets and the retrieval of parameters that are nonlinearly related to concentrations. The 4D-Var, and especially an approximate adjoint of the transport model, is tested and validated using several advection schemes that are influential on the forward simulation as well as on the data-assimilation results. Firstly, the inverse modelling system is applied to the assessment of the dry and wet deposition parameters. It is then applied to the retrieval of the emission field alone, the joint optimization of removal-process parameters and source parameters and the optimization of larger parameter fields such as horizontal and vertical diffusivities or the dry-deposition velocity field. The physical parameters used so far in the literature for the Chernobyl dispersion simulation are partly supported by this study. The crucial question of deciding whether such an inversion is merely a tuning of parameters or a retrieval of physically meaningful quantities is discussed. Even though inversion of parameter fields may fail to determine physical values for the parameters, it achieves statistical adaptation that partially corrects for model errors and, using the inverted parameter fields, leads to considerable improvement in the simulation scores. Copyright © 2011 Royal Meteorological Society

Description: Taking the spherical geopotential approximation as a starting point, previous work has revealed a quartet of dynamically consistent models of the global atmosphere according to whether approximations of shallow and quasi-hydrostatic type are or are not made. The generalization of this quartet of consistent models is sought within the framework of geopotential surfaces that are zonally symmetric but not assumed to be spherical. The distinction between approximations of shallow and quasi-hydrostatic type is more subtle than in the spherical case: consistent quasi-hydrostatic approximation requires the simultaneous omission of two metric terms that vanish identically in the spherical case (and indeed whenever the vertical metric factor is independent of the meridional coordinate). These two metric terms are not eliminated by the formal process of metric factor approximation by which consistent models of shallow type may be generated. Consistently approximated, hydrostatic, shallow forms are obtained by applying quasi-hydrostatic and shallow approximations successively, and are comparable with the well-known spherical geopotential forms as regards the number and nature of the terms present. The formulation of shallow-water models in non-spherical geopotential coordinates is considered in conclusion. Copyright © 2011 British Crown copyright, the Met Office. Published by John Wiley & Sons Ltd.

Description: A new approach for calculating downwelling surface long-wave radiation (DSLW) under all sky conditions is presented. The DSLW model (DSLW/UMD) is driven with a synthesis of the latest 1° resolution Moderate-resolution Imaging Spectroradiometer (MODIS) level-3 cloud parameters and information from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim model. The DSLW/UMD's clear-sky contribution is based on the Rapid Radiative Transfer Model (RRTM) and is globally applicable, while a statistical cloud structure model and parametrization determine the cloud contribution to DSLW. Daily averaged estimates of DSLW for 2003–2007 along with four commonly used methods based on radiative transfer (RT) theory are compared against ground measurements from the Baseline Surface Radiation Network (BSRN). It is shown that for all four years, the proposed approach performs as well as or better than the available RT-based models when evaluated against BSRN measurements. Correlation coefficients, absolute value of bias (W m −2 ), and RMSE (W m −2 ) are 0.973 to 0.974, −0.07 to 1.9, and 16.82 to 17.35, respectively. When stratified into Tropical, Midlatitude, and Polar latitudinal belts, largest discrepancies against ground observations are found at high latitudes. Copyright © 2011 Royal Meteorological Society

Description: A single-Doppler wind retrieval method called the Ground-Based Velocity-Track Display technique (GBVTD) has been developed in recent years to retrieve horizontal circulations of tropical cyclones. The technique is able to retrieve axisymmetric tangential and radial winds, asymmetric tangential winds for wave numbers 1–3, and along-beam mean winds in tropical cyclones. It has been successfully applied to tropical cyclone monitoring and warning. This study explores, for the first time, the assimilation of GBVTD-retrieved winds into a tropical cyclone prediction model, and examines its impact relative to that of directly assimilated radial velocity data. super typhoon Saomai (2006), the most intense landfalling typhoon ever recorded in China, is chosen as the test case, and data from the coastal operational radar at Wenzhou, China, are used. The ARPS 3DVAR system is used to assimilate either the radial velocity data directly or the GBVTD-retrieved winds, at 30 min intervals for 2 hours. The assimilation of the GBVTD-retrieved winds results in much improved structure and intensity analyses of Saomai compared to those in the Japan Meteorological Agency mesoscale reanalysis and compared to the analysis assimilating radial velocity ( V r ) data directly. The ability of the GBVTD method in providing wind information covering the full circle of the inner-core circulation is the primary reason for its superior performance over direct assimilation of V r data; for the latter, the azimuthal data coverage is often incomplete. With the improved initial conditions, the subsequent forecasts of typhoon intensity, track and precipitation are also improved. The improvements to both track and intensity predictions persist over a 12-hour forecast period, which is mostly after landfall. Subjective and quantitative evaluations of the precipitation and circulation patterns show consistent results. A further sensitivity experiment shows that the axisymmetric wind component in the GBVTD retrieval has the dominant impact on the prediction. Copyright © 2011 Royal Meteorological Society

Description: Atmospheric tracers are often observed to be functionally related, and these relations can be physically or chemically significant. It is therefore highly desirable that the transport schemes used in chemistry and chemistry-climate models should not disrupt such functional relations in unphysical ways through numerical mixing or, indeed, unmixing. Here, diagnostics are proposed that quantify numerical mixing by a transport scheme for a single tracer, two tracers that are nonlinearly related, and three (or more) tracers that add up to a constant. For the two-tracer test, the question of how physically reasonable the numerical mixing is can be addressed by using scatter/correlation plots. Truncation errors will, in general, result in scatter points deviating from the preexisting functional curve and thereby introduce numerical mixing between the tracers. The proposed diagnostics quantify the mixing in terms of the normalized distances between the pre-existing functional curve and scatter points, and divide it into three categories: real mixing and two types of spurious numerical unmixing. For the three-tracer test, we quantify, in terms of standard error norms, how nearly a transport scheme can preserve the sum by transporting the individual tracers. The mixing diagnostics do not require the knowledge of the analytical solution to the transport problem for the individual tracers. However, using an idealized flow field and spatial distributions facilitates the use of the mixing diagnostics in transport scheme development. Hence we propose to exercise the new mixing diagnostics using an idealized but highly deformational analytical flow field. Example results using the CSLAM (Conservative Semi-LAgrangian Multi-tracer) scheme are presented. Copyright © 2011 Royal Meteorological Society

Description: Ground-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

Description: An extensive off-line evaluation of the Noah/Single Layer Urban Canopy Model (Noah/SLUCM) urban land-surface model is presented using data from 15 sites to assess (1) the ability of the scheme to reproduce the surface energy balance observed in a range of urban environments, including seasonal changes, and (2) the impact of increasing complexity of input parameter information. Model performance is found to be most dependent on representation of vegetated surface area cover; refinement of other parameter values leads to smaller improvements. Model biases in net all-wave radiation and trade-offs between turbulent heat fluxes are highlighted using an optimization algorithm. Here we use the Urban Zones to characterize Energy partitioning (UZE) as the basis to assign default SLUCM parameter values. A methodology (FRAISE) to assign sites (or areas) to one of these categories based on surface characteristics is evaluated. Using three urban sites from the Basel Urban Boundary Layer Experiment (BUBBLE) dataset, an independent evaluation of the model performance with the parameter values representative of each class is performed. The scheme copes well with both seasonal changes in the surface characteristics and intra-urban heterogeneities in energy flux partitioning, with RMSE performance comparable to similar state-of-the-art models for all fluxes, sites and seasons. The potential of the methodology for high-resolution atmospheric modelling application using the Weather Research and Forecasting (WRF) model is highlighted. This analysis supports the recommendations that (1) three classes are appropriate to characterize the urban environment, and (2) that the parameter values identified should be adopted as default values in WRF. Copyright © 2011 Royal Meteorological Society

Description: The 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

Description: A 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

Description: Two methods for assimilating radar reflectivity into the COSMO numerical weather prediction (NWP) model were compared to precipitation forecasts. The first method assimilated observed radar reflectivity, and the second one assimilated observed and extrapolated radar reflectivity. The assimilation technique was based on the correction of the model's water vapour mixing ratio. The extrapolation was performed by the COTREC method and was 1 hour long. The model's horizontal resolution was 2.8 km. The comparison of methods was based on verification of the observed and forecast hourly precipitation. The comparison was performed for the 1 st , 2 nd and 3 rd hours of each forecast. On the whole, 45 forecasts from nine days of convective precipitation were evaluated for each hour. The evaluation included subjective verification and the following objective skill scores: Fractions Skill Scores, SAL and a measure based on a categorical-probabilistic approach. The results confirmed that assimilation complemented by the extrapolated data improves the accuracy of precipitation forecasts. The improvement was obvious in a majority of the single forecasts studied, and it is confirmed by all evaluation techniques. COSMO forecasts that used the extrapolation showed reasonable competence in forecasting for the first and the second hours. Copyright © 2011 Royal Meteorological Society

Description: Cirrus observations taken during EMERALD1 clearly showed vertical structure in ice crystal habit and size distribution. In this study, we use these observations along with radiative and cloud modelling to assess the importance of this observed vertical structure in size and habit on radiative properties and cloud evolution. We show that neglecting the vertical structure results in changes to the solar upwelling and downwelling fluxes of 10 and 8 W m −2 , respectively. If further simplifications are made and aggregate and rosettes are treated more simplistically (as columns) then the upwelling and downwelling fluxes are altered by about 16 and 12 W m −2 , respectively. It was noted that the effects of simplifying the vertical structure and habits act in competing ways in terms of the fluxes and therefore emphasize the need to consider both effects in order to improve modelling rather than considering either in isolation. Often, general circulation models have highly parametrized treatments of cloud properties. We illustrate that crude assumptions about ice crystal effective sizes used in models (from the literature) can result in fluxes being in error by 100 W m −2 or more compared to using the observations. The above results are contrasted with effects of ice crystal roughness (altering the asymmetry). We show that moderate estimates of roughness lead to changes in the upward and downward fluxes of about 10–15 W m −2 . This shows that vertical structure, habits and knowledge of size distribution are key factors and on a par with such effects as ice particle roughness, which is considered to have a highly uncertain but important cirrus radiative effect. Copyright © 2011 Royal Meteorological Society

Description: Data assimilation refers to the problem of finding trajectories of a prescribed dynamical model in such a way that the output of the model (usually some function of the model states) follows a given time series of observations. Typically though, these two requirements cannot both be met at the same time–tracking the observations is not possible without the trajectory deviating from the proposed model equations, while adherence to the model requires deviations from the observations. Thus, data assimilation faces a trade-off. In this contribution, the sensitivity of the data assimilation with respect to perturbations in the observations is identified as the parameter which controls the trade-off. A relation between the sensitivity and the out-of-sample error is established, which allows the latter to be calculated under operational conditions. A minimum out-of-sample error is proposed as a criterion to set an appropriate sensitivity and to settle the discussed trade-off. Two approaches to data assimilation are considered, namely variational data assimilation and Newtonian nudging, also known as synchronization. Numerical examples demonstrate the feasibility of the approach. Copyright © 2011 Royal Meteorological Society

Description: The purpose of adaptive observation strategies is to design optimal observation networks in a prognostic way. The implementation of such strategies is based on adaptive observation numerical techniques that provide guidance on where to deploy future additional observations. Most advanced techniques account for the dynamical aspects of the atmosphere and the data assimilation system (DAS). This study aims to assess the influence of the model trajectory on the Kalman Filter Sensitivity (KFS) method used at Météo-France. KFS is an adjoint-based method identifying sensitive areas by means of a forecast score variance. Targeted observations are designed to reduce this score variance. In its first version, KFS was not able to deal with trajectory uncertainties. An ensemble-based approach is undertaken to investigate if it is possible to account for these uncertainties. We assess the robustness of the method regarding trajectory errors and propose a practical solution. To avoid high computational costs, a simplified framework is used. We perform experiments with a two-layer quasi-geostrophic model in an incremental 4D-Var system. KFS is computed to target a potential vorticity anomaly interacting with a simplified jet stream. Numerical experiments show that the impact of additional observations depends equally on the suboptimal formulation of the DAS components and the trajectory errors. When the KFS sensitivity fields are constrained by the DAS components, the sensitivity values are determined by the selection of the reference trajectory. A targeting strategy based on the ensemble mean sensitivity field is proposed. Experiments show this to be more efficient than a random targeting strategy. Copyright © 2011 Royal Meteorological Society

Description: The 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 −2 year −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

Description: The idea is proposed of an analysis ensemble of deterministic, model-independent analyses. The ensemble is based on random perturbations of irregularly distributed observations. The purpose of implementing an analysis ensemble is to define uncertainties in analysis fields due to their observational background and errors. As one possible application, the uncertainty information could, in future, be used to define confidence intervals for verification measures depending on the reference data. The analysis system VERA and a high-resolution Central European observation network are used as a testbed for the development of the methodology. Several approaches for defining weights for the perturbation fields are investigated and compared. Basic weights are determined by a sophisticated data quality control scheme producing error estimates for observations. These estimates can be combined with additional information attempting to include more explicitly the spatial structure of the observed fields in the ensemble. The information is provided by either a principal component analysis of a time series of analysis fields or a 2D-discrete wavelet transform. Strengths and weaknesses of the different adjustments for ensemble analysis perturbations are discussed. It is shown that perturbations provided by the wavelet-based approach lead to useful results for several meteorological parameters tested. Copyright © 2011 Royal Meteorological Society

Description: We present a novel cyclone identification and tracking method that explicitly recognizes ‘multicentre cyclones’ (MCCs), defined as a cyclonic system with two or three sea-level pressure minima within its outermost contour. The method allows for the recognition of cyclone merger and splitting events in a natural way, and provides a consistent measure of the cyclone extent. Using the ERA-Interim reanalysis dataset, we compute a climatology using this method and show that MCCs occur in about 32 of all cyclone tracks and are much more prevalent in more intense storms. We also show that the method permits reconnection of tracks that would have been spuriously split using a conventional method. We present spatial maps of cyclone mergers, splitting, genesis and lysis using the method and also compute statistics of precipitation falling within cyclones, showing that it is strongly concentrated in the most intense cyclones. Copyright © 2011 Royal Meteorological Society

Description: Regional numerical weather prediction models are now routinely run at near kilometre-scale resolutions where surface features are well resolved and mean grid-box slopes are highly significant. The resolved topography has an important effect on the radiative transfer at the surface which is not generally represented using fast two-stream radiation codes. This paper presents a parametrisation of surface radiative transfer for resolved topography suitable for use with a two-stream code. For the short-wave bands, the first-order effects are the interaction of direct solar radiation with surface slopes and shading by surrounding terrain. In the infrared (long-wave bands), the first-order effect is the transfer of radiation between visible points on the terrain making use of the sky-view factor for a sloping surface and the enhanced area of the slope. Energy is conserved over an extended region where the mean flux into the surface will be independent of resolution. These effects are included in the radiation scheme of the Met Office Unified Model and case-studies are run over the UK using horizontal grid resolutions from 100 m to 1.5 km. The scheme adds fine detail to the forecast which is shown to have a significant impact on forecast skill. Under clear-sky conditions, short-wave surface effects can lead to temperature differences of up to 2.5 K, and long-wave effects to differences of up to 1 K. The magnitude of these effects are expected to be much greater for more complex terrain. A consistent extension to the scheme is outlined to take account of second-order effects using short-wave diffuse albedo and long-wave surface emissivity. These lead to a net change in the surface flux due to multiple surface reflections and should be considered in conjunction with a treatment of sub-grid terrain complexity. Copyright © 2011 British Crown copyright, the Met Office. Published by John Wiley & Sons Ltd.

Description: Three models with different discretizations for the shallow-water equations on a sphere are presented and compared using selected test cases. The first model is based on the global latitude–longitude grid system with a uniform Arakawa C grid and a two-time-level Crank–Nicolson iterative semi-Lagrangian method with an advecting wind interpolated in time. The second model uses the overset Yin–Yang grid, which is singularity-free and has quasi-uniform resolution. The local solver on each of the two component grids is based on the same time and space discretizations as in the first model. The positive-definite Helmholtz problem in the second model is solved using an optimized Schwarz-type domain-decomposition method with specific Robin or higher-order transmission conditions. The first and second models are obtained through the barotropic option incorporated into the Global Environmental Multiscale model used operationally at the Canadian Meteorological Center. The third model is discretized using the finite-volume methodology on a geodesic icosahedral grid. The time integration is performed with a fourth-order Runge–Kutta scheme. The tests employed to compare the three models are passive advection of a cosine bell, steady-state geostrophic flow, flow over an idealized mountain, a Rossby–Haurwitz wave, real-case 500 mb flow and evolution of a growing barotropic wave. When no analytic solution is available for a specific test, we compare the results with a high-resolution solution obtained from the first model in which all horizontal operations are evaluated in spectral space. © 2011 Crown in the right of Canada. Published by JohnWiley & Sons Ltd.

Description: A new tropopause definition involving a flow-dependent blending of the traditional thermal tropopause with one based on potential vorticity has been developed and applied to the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses (ERA), ERA-40 and ERA-Interim. Global and regional trends in tropopause characteristics for annual and solsticial seasonal means are presented here, with emphasis on significant results for the newer ERA-Interim data for 1989–2007. The global-mean tropopause is rising at a rate of 47 m per decade, with pressure falling at 1.0 hPa per decade and temperature falling at 0.18 K per decade. The Antarctic tropopause shows decreasing heights, warming and increasing westerly winds. The Arctic tropopause also shows a warming, but with decreasing westerly winds. In the Tropics the trends are small, but at the latitudes of the subtropical jets they are almost double the global values. It is found that these changes are mainly concentrated in the eastern hemisphere. Previous and new metrics for the rate of broadening of the Tropics, based on both height and wind, give trends in the range 0.9–2.2° per decade. For ERA-40 the global height and pressure trends for the period 1979–2001 are similar: 39 m per decade and −0.8 hPa per decade. These values are smaller than those found from the thermal tropopause definition with this dataset, as was used in most previous studies. Copyright © 2011 Royal Meteorological Society

Description: A new tropopause definition, based on a flow-dependent blending of the traditional thermal tropopause with one based on potential vorticity, has been developed. The benefits of such a blending algorithm are most apparent in regions with synoptic-scale fluctuations between tropical and extratropical air masses. The properties of the local air mass determine the relative contributions to the location of the blended tropopause, rather than this being determined by a specified function of latitude. Global climatologies of tropopause height, temperature, potential temperature and zonal wind, based on European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA) ERA-Interim data, are presented for the period 1989–2007. Features of the seasonal-mean tropopause are discussed on a global scale, alongside a focus on selected monthly climatologies for the two high-latitude regions and the tropical belt. The height differences between climatologies based on ERA-Interim and ERA-40 data are also presented. Key spatial and temporal features seen in earlier climatologies, based mainly on the World Meteorological Organization thermal tropopause definition, are reproduced with the new definition. Tropopause temperatures are consistent with those from earlier climatologies, despite some differences in height in the extratropics. Copyright © 2011 Royal Meteorological Society

Description: In the companion paper (Järvinen et al. , 2011. Q. J. R. Meteorol. Soc. DOI: 10.1002/qj.923) we suggested a concept to estimate numerical weather prediction model closure parameters on-line, coupled with an operational ensemble prediction system. In this paper, such a method is developed and demonstrated in low-order numerical tests. The method utilizes the massive amount of operational model predictions to make statistical inference about the underlying probability distributions of the closure parameters. This is achieved with practically no additional computations. The only required change to the ensemble prediction system is to allow perturbation of the model closure parameters for different ensemble members. Otherwise, the method is straightforward. The parametric uncertainty is presented using a hierarchical statistical model. Proposed parameter values are resampled based on their respective likelihood function values, as evaluated against verifying observations. Update formulas are used to feed back the parametric information to the evolving proposal distribution. The method for ensemble prediction and parameter estimation system (EPPES) is demonstrated using a stochastic version of the Lorenz-95 model. The numerical tests show that the EPPES method is capable of detecting unknown and wrongly specified parameter values, and lead to optimal forecast skill in an independent test run. Potential show-stoppers are discussed. Our current research is directed towards a realistic ensemble prediction system. Copyright © 2011 Royal Meteorological Society

Description: This 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

Description: We study the global atmospheric budgets of mass, moisture, energy and angular momentum in the latest reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF), ERA-Interim, for the period 1989–2008 and compare with ERA-40. Most of the measures we use indicate that the ERA-Interim reanalysis is superior in quality to ERA-40. In ERA-Interim the standard deviation of the monthly mean global dry mass of 0.7 kg m −2 (0.007%) is slightly worse than in ERA-40, and long time-scale variations in dry mass originate predominately in the surface pressure field. The divergent winds are improved in ERA-Interim: the global standard deviation of the time-averaged dry mass budget residual is 10 kg m −2 day −1 and the quality of the cross-equatorial mass fluxes is improved. The temporal variations in the global evaporation minus precipitation ( E − P ) are too large but the global moisture budget residual is 0.003 kg m −2 day −1 with a spatial standard deviation of 0.3 kg m −2 day −1 . Both the E − P over ocean and P − E over land are about 15% larger than the 1.1 Tg s −1 transport of water from ocean to land. The top of atmosphere (TOA) net energy losses are improved, with a value of 1 W m −2 , but the meridional gradient of the TOA net energy flux is smaller than that from the Clouds and the Earth's Radiant Energy System (CERES) data. At the surface the global energy losses are worse, with a value of 7 W m −2 . Over land however, the energy loss is only 0.5 W m −2 . The downwelling thermal radiation at the surface in ERA-Interim of 341 W m −2 is towards the higher end of previous estimates. The global mass-adjusted energy budget residual is 8 W m −2 with a spatial standard deviation of 11 W m −2 , and the mass-adjusted atmospheric energy transport from low to high latitudes (the sum for the two hemispheres) is 9.5 PW. Copyright © 2011 Royal Meteorological Society

Description: The statistically steady humidity distribution resulting from an interaction of advection, modelled as an uncorrelated random walk of moist parcels on an isentropic surface, and a vapour sink, modelled as immediate condensation whenever the specific humidity exceeds a specified saturation humidity, is explored with theory and simulation. A source supplies moisture at the deep-tropical southern boundary of the domain and the saturation humidity is specified as a monotonically decreasing function of distance from the boundary. The boundary source balances the interior condensation sink, so that a stationary spatially inhomogeneous humidity distribution emerges. An exact solution of the Fokker–Planck equation delivers a simple expression for the resulting probability density function ( PDF ) of the water-vapour field and also the relative humidity. This solution agrees completely with a numerical simulation of the process, and the humidity PDF exhibits several features of interest, such as bimodality close to the source and unimodality further from the source. The PDF s of specific and relative humidity are broad and non-Gaussian. The domain-averaged relative humidity PDF is bimodal with distinct moist and dry peaks, a feature which we show agrees with middleworld isentropic PDF s derived from the ERA interim dataset. Copyright © 2011 Royal Meteorological Society

Description: According to a classical synoptic concept, Soudano-Saharan depressions (SSDs) are surface lows that track westward over tropical West Africa, curve anticyclonically across the Sahara and may then transform into eastward-moving Mediterranean cyclones. Occurrence frequency and track location undergo a marked seasonal cycle. Interactions between troughs in the upper-level westerlies and mid-level African easterly waves have been suggested as a mechanism for their formation. SSDs have been reported to be associated with dust-storms and precipitation over northern Africa. This paper presents the first-ever systematic investigation of SSDs using re-analysis and satellite data. Depressions are identified and tracked objectively based on closed contours in 0000 UTC fields of 925 hPa geopotential height from the European Centre for Medium-Range Weather Forecasts ERA-Interim re-analysis (1989–2008). To classify as potential SSDs, tracks must: (i) start to the south of 20°N; (ii) intersect 15°–30°N, 10°W–30°E; (iii) cover a meridional distance of at least 10° latitude; and (iv) have a minimum lifetime of 24 hours. Even with a relatively low threshold of 4 gpm, only 50 potential SSDs are found (annual average 2.5, monthly range 0–6). Lagrangian and Eulerian composite analyses reveal that the identified systems: (i) are mostly shallow lee troughs of the central Saharan and Atlas Mountains during the warm season without a well-defined cyclonic wind field; (ii) do not show the seasonal track variation described in the literature; (iii) mostly occur in association with high-pressure anomalies over the Mediterranean Sea; and (iv) are not associated with significant increases in dustiness and precipitation. These results strongly suggest that the disturbances described as SSDs do not manifest themselves as traceable low-level depressions, calling for a fundamental revision of the classical concept in the literature. Copyright © 2011 Royal Meteorological Society

Description: The accuracy of deterministic and probabilistic forecasts of storms in the Adriatic Sea that lead to the flooding of Venice is discussed. We consider ECMWF state-of-the-art high-resolution single deterministic and lower-resolution ensemble-based forecasts of meteorological and sea states (waves) for five storms that affected Venice between 1966 and 2008. Notwithstanding the complicated local orographic situation, it is shown that ECMWF single, deterministic forecasts provide accurate information up to 3–4 days in advance. This range is further extended to between 4 and 6 days if ensemble-based, probabilistic forecasts are considered. The assessment of the quality of Ensemble Prediction System (EPS) probabilistic forecasts during the winters of 2008–2009 and 2009–2010 over the Adriatic and the Mediterranean Seas, and the North Atlantic Ocean, is also discussed to provide a proper statistical evaluation of the accuracy of EPS-based probabilistic forecasts of the wind over the sea. Average results indicate that EPS probabilistic forecasts over these areas are skilful for the whole forecast range considered in this study. Copyright © 2011 Royal Meteorological Society

Description: The equatorial counterpart of the quasi-geostrophic model that has recently been derived and is referred to as the equatorial balanced model is compared to the shallow water equations. This comparison allows for the assessment of the accuracy with which the equatorial balanced model approximates the low-frequency, or balanced, component of the dynamics described by the shallow water equations. In order to be able to consider fully nonlinear dynamics, numerical implementations of both models are developed for a zonally periodic channel on the equatorial β -plane using the Contour-Advective Semi-Lagragian (CASL) algorithm. The CASL algorithm is chosen because it has beneficial qualities compared to other algorithms by taking full advantage of the property that both models materially conserve potential vorticity. The particular dynamics chosen for the assessment is that of an initially unstable westward flow that freely evolves. The assessment also shows that the accuracy of the equatorial balanced model is comparable to that of the quasi-geostrophic model outside the equatorial region and to that of one of the most accurate balanced models with its balance relations linearized in the equatorial region. Copyright © 2011 Royal Meteorological Society

Description: This work is a first step in the direction of implementing a high-order finite-element discretization in the vertical in the non-hydrostatic version of the HARMONIE model. The present dynamical core of the HARMONIE model is shared with the ECMWF and the ALADIN models and uses a horizontal spectral discretization and a semi-implicit semi-Lagrangian time stepping scheme, all of which are maintained in this work. Trying to implement a finite-element discretization in the non-hydrostatic version of the HARMONIE model has been found to be very difficult due to the set of prognostic variables used and the mass-based vertical coordinate. A different set of prognostic variables and a hybrid vertical coordinate based on height are tested here on a vertical slice non-hydrostatic kernel. A stability analysis of the linear model has been done. To evaluate the model stability and accuracy, a set of test cases from the literature are presented in the linear and nonlinear regimes, with and without orography. An iterative centred-implicit scheme can be applied to avoid instability related to steep orography, although this reduces the efficiency of the model. The novel aspects with respect to existing non-hydrostatic model kernels are the use of cubic finite elements in the vertical discretization, the use of a height-based vertical coordinate in conjunction with a spectral discretization in the horizontal, and the coordinate-independent formulation of each element of the model including the semi-Lagrangian advection. Copyright © 2011 Royal Meteorological Society

Description: Successful 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

Description: Experience from operational trials of assimilation of radar data in kilometre-scale numerical weather prediction models (operating without cumulus parametrization) shows that the positive impact of the radar data on convective precipitation forecasts typically decays within a few hours, although certain cases show much longer impact time-scales. In this work the impact time of radar data assimilation is related to characteristics of the meteorological environment. Three cases of convection over southern Germany with different synoptic conditions are investigated (one case with two data assimilation cut-off times), each with an ensemble of ten forecasts at 2.8 km horizontal resolution based on different initial and boundary conditions from a global forecast ensemble. Control forecasts are compared with forecasts where radar rainfall data are assimilated using latent heat nudging. The impact time of the radar data on total precipitation is quantified, and found to correlate well with a convective time-scale that measures the rate at which convection is responding to changes in large-scale forcing. Short impact times were associated with short convective time-scales that are characteristic of equilibrium convection. In this regime the statistical properties of the convection are constrained by the large-scale forcing, and effects of the radar data are lost within a few hours as the convection rapidly returns to equilibrium. When the convective time-scale was large (non-equilibrium conditions), the impact of the radar data was longer since convective systems were triggered by the latent heat nudging and were able to persist for many hours in the very unstable conditions present in these cases. The impact of the assimilated radar data on the location of precipitation was assessed using the equitable threat score (ETS) and the displacement and amplitude score (DAS). The impact times for these measures were consistently shorter than for total precipitation, but again shortest for the equilibrium conditions. Copyright © 2011 Royal Meteorological Society

Description: This study focuses on the radiative impacts of dust on the thermodynamics and dynamics of a major Saharan dust-storm that occurred between 7 and 15 March 2006. The implementation of a dust module within the convective-scale operational model AROME permitted a 10-day simulation at high resolution (5 km) over a large domain (6.5–30°N; 3°W–33°E). The model is able to represent most of the life cycle of this dust-storm and of the principal dust sources. According to the comparison with a complete and comprehensive set of measurements obtained within the framework of the AMMA field campaign in 2006, the simulation succeeds in representing the main thermodynamic and radiative features of the storm both qualitatively and quantitatively. As compared with a simulation without the dust, the coupling with the dust module provides signatures of the storm that are in better agreement with radiation and temperature observations. The intense heating in the core of the dust plume (more than 2°C) resulted in a large vertical dipole of potential vorticity anomaly with higher values at low levels and weaker values at mid levels, either in the lower and the upper part of the dust plume respectively. This shows that the coupling with dust reinforces the intensity of the cold extratropical outbreak with more ascent at the cold front, more cyclonic convergence near the surface, and more anticyclonic divergence aloft. Also, the subsidence is increased at the rear of the outbreak with more northerly winds. A major result is the acceleration of the easterly along-front flow (∼3 m s −1 ) at low levels and in the African Easterly Jet, increasing the emission of dust and its westward long-range transport. Copyright © 2011 Royal Meteorological Society

Description: This article assesses current issues related to scattering radiative transfer (RT) in data assimilation (DA) and proposes possible ways to solve or mitigate these issues. Emphasis is put not so much on fundamental issues related to RT but on the practical application within a framework of operational numerical weather prediction and DA with their tight constraints on computational efficiency. In particular, three potentially critical open issues are studied: firstly, the trade-off between speed and accuracy in RT schemes for DA. A numerically efficient method is proposed to determine beforehand whether scattering needs to be accounted for. Secondly, the impact of spectrally highly variable gaseous absorption coefficients within a given instrument's bandpass and its implications on scattering RT is studied. Results of this second part are also put in context with uncertainties caused by the lack of knowledge of scattering optical properties. Finally, model errors due to, for example, the assumption of plane-parallel RT are studied. It is argued that errors caused by plane-parallel RT will likely continue to dominate the error budget both in terms of biases and random errors. Copyright © 2011 Royal Meteorological Society

Description: The radiative properties of mineral dust aerosol during the GERB Intercomparison of Long-wave and Short-wave (GERBILS) are presented. GERBILS consisted of aircraft flights over land areas between Mauritania and Niger during June 2007. During one case of large aerosol optical depth (AOD=1.0 at 0.55 μ m), a short-wave spectrometer measured sky radiances versus scattering angle that are compared to modelled data. The modelling used phase functions of spheres, spheroids and irregular-shaped particles using T-matrix and ray-tracing methods. Irregular particles provided the most satisfactory solution. In another case of full short-wave and long-wave radiative closure, measurements and modelled clear sky conditions allowed calculation of the direct radiative effect (DRE) at high and low level. The modelled AOD (0.92) required to simulate the measured spectral irradiances agrees with the aircraft AOD (0.79) within measurement uncertainty. The simulated irradiances are less sensitive to particle shape than radiances. However, it is shown through modelling of the surface and top-of-atmosphere (TOA) DRE over all daylight hours that significant differences exist at TOA due to variation in the asymmetry parameter. The TOA short-wave diurnally averaged DRE was modelled as between 0 and –20 W m −2 depending on particle shape. A long-wave interferometer measured downwelling and upwelling radiances to derive surface emissivity across the window region. Measured nadir brightness temperatures from high level show signature of dust. A drop in brightness temperature of 14K was determined using modelled pristine-sky spectra. The modelled outgoing long-wave DRE due to dust from this case was +14 W m −2 averaged over 24 h, or +17 W m −2 per unit AOD. Modelling studies illustrate the sensitivity to aerosol refractive index and size distribution for both short-wave and long-wave DREs. Considering the full spectrum, a refractive index dataset from the literature has been selected that best represents the Saharan dust encountered during GERBILS. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

Description: Downscaled rainfall projections for the Indian summer monsoon are generated using a non-homogeneous hidden Markov model (NHMM) and information from both a dense observational dataset and an ensemble of general circulation models (GCMs). The projections are conditioned on two types of GCM information, corresponding approximately to dynamic and thermodynamic components of precipitation change. These have opposing effects, with a weakening circulation compensating not quite half of the regional precipitation increase that might otherwise be expected. GCM information is taken at the largest spatial scales consistent with regional physics and modelling constraints, while the NHMM produces a disaggregation consistent with the observed fine-scale spatiotemporal variability. Projections are generated using multimodel mean predictors, with intermodel dispersion providing a measure of the uncertainty due to GCM differences. The downscaled simulations exhibit small increases in the number of dry days, in the average length of dry spells, in mean daily intensity and in the exceedance frequency of nearly all daily rainfall percentiles. Copyright © 2011 Royal Meteorological Society

Description: We discuss how synoptic-scale variability controls the transport of atmospheric water vapour by midlatitude cyclones. Idealized simulations are used to investigate quantitatively which factors determine the magnitude of cyclone moisture transport. It is demonstrated that large-scale ascent on the warm conveyor belt and shallow cumulus convection are equally important for ventilating moisture from the boundary layer into the free troposphere, and that ventilated moisture can be transported large distances eastwards and polewards by the cyclone, before being returned to the surface as precipitation. The initial relative humidity is shown to have little effect on the ability of the cyclone to transport moisture, whilst the absolute temperature and meridional temperature gradient provide much stronger controls. Scaling arguments are presented to quantify the dependence of moisture transport on large-scale and boundary-layer parameters. It is shown that ventilation by shallow convection and warm-conveyor-belt advection vary in the same way with changes to large-scale parameters. However, shallow convective ventilation has a much stronger dependence on boundary-layer parameters than warm-conveyor-belt ventilation. Copyright © 2011 Royal Meteorological Society

Description: We apply a cloud-resolving model with explicit aerosol and ice microphysics and Lagrangian ice particle tracking to simulate the evolution of a cirrus cloud field observed during the US Atmospheric Radiation Measurement Program Intensive Operational Period in March 2000. This comprehensive data set includes remote sensing, radiosonde, and aircraft measurements of a midlatitude cirrus cloud system, supported by estimates of the dynamical cloud forcing. The dataset allows us to evaluate and study in great detail the process-oriented representation of the microphysical processes relevant to the formation and evolution of deep, stratiform cirrus (in particular ice crystal sedimentation and aggregation). The suite of explicitly resolved physical processes in our model enables us to better understand the sensitivity of the simulated cirrus properties on a large number of microphysical and environmental parameters. The evolution of the domain-integrated cloud optical depth is largely dominated by homogeneous freezing processes. We find that the evolution of the observed cirrus cloud system is most dependent on updraught speed and ice supersaturation and that homogeneous freezing leads to a total, cloud-averaged ice crystal concentration of 0.1 cm −3 of air. It is not necessary to invoke heterogeneous ice nuclei to explain most of the data, but we cannot rule out that a small concentration (up to 0.002 cm −3 ) of such particles may have affected the cirrus cloud field in nature. Cloud-averaged ice particle size distributions are bimodal, separating two distinct growth regimes in the developed cloud. The small mode (ice particle sizes below a few 100 μ m) forms by homogeneous freezing of supercooled aerosol droplets and grows by deposition of water molecules from the gas phase. The large mode (sizes up to several 1000 μ m) forms and grows by aggregation. We demonstrate that the formation of the largest crystals by aggregation in deep cirrus is controlled in part by the nucleation of new ice crystals in dynamically active, highly supersaturated upper cloud regions. Furthermore, a pronounced increase in the number of aggregation events is predicted in sublimation zones. The combined effect of sublimation and sedimentation leads to the formation of a very thin (vertical extension ∼ 100 m) sublimation microlayer mainly composed of aggregated ice crystals, containing relatively high total ice crystal number concentrations (∼ 0.02 cm −3 ) comparable to those generated locally by homogeneous freezing in the upper cloud layers. Copyright © 2011 Royal Meteorological Society

Description: Two atmospheric dispersion schemes, the transport index and the Bultynck–Malet scheme, have been analysed and combined by means of a three-year database obtained at a rural site comprising CO 2 surface concentrations and meteorological variables (wind and temperature profiles) from a RASS sodar. The most noticeable feature of the transport index is its ability to split the influence of local effects and transport, whereas the Bultynck–Malet scheme is based on seven stability classes and requires temperature gradient and wind data not too close to the surface. The transport index was obtained in the layer from 40 to 100 m. Medians of the transport index were below 600 m, particularly at night, revealing that local effects prevailed during this period. CO 2 concentrations were parametrized as a function of distance, and a boundary of the transport index was calculated to establish two regions, the inner region affected by local effects and the outer, affected by transport. Dependence of this boundary on height was also analysed. The original Bultynck–Malet scheme was adapted to the database by modifying the limit of high winds. As a result, only a few observations were transferred to more stable classes. A successful relationship between CO 2 concentrations and stability classes was obtained and a detailed analysis revealed the noticeable influence of temperature gradient on the two most stable classes. Finally, the relationship between both dispersion schemes enabled us to propose an extremely simplified scheme retaining the two most stable Bultynck–Malet classes and grouping the remaining observations into a third class with lower concentrations. Copyright © 2011 Royal Meteorological Society

Description: A moist entropy potential temperature–denoted by θ s –is defined analytically in terms of the specific entropy for moist air. The expression for θ s is valid for a general mixing of dry air, water vapour and possible condensed water species. It displays the same conservative properties as the moist entropy, even for varying dry air or total water content. The moist formulation for θ s is equal to the dry formulation θ if dry air is considered, and it displays new properties valid for the moist air cases, both saturated or unsaturated ones. Exact and approximate versions of θ s are evaluated for several stratocumulus cases, in particular by using the aircraft observation datasets from the FIRE-I experiment. It appears that there is no (or only a small) jump in θ s at the top of the planetary boundary layer (PBL). The mixing in moist entropy is almost complete in the PBL, with the same values observed in the clear air and the cloudy regions, including the very top of the entrainment region. The Randall–Deardorff Cloud-Top Entrainment Instability analysis may be interpreted as a mixing in moist entropy criterion. The iso- θ s lines are plotted on skew T –log p and conserved variable diagrams. All these properties could suggest some hints on the use of moist entropy (or θ s ) in cloud modelling or in mixing processes, with the marine stratocumulus considered as a paradigm of moist turbulence. Copyright © 2011 Royal Meteorological Society

Description: A frequent problem in forecasting fog or icy roads in a numerical weather prediction (NWP) system is attributed to the misinterpretation of the boundary-layer structure in the assimilation procedure. Case-studies showed that the misinterpretation of temperature inversions and stratocumulus layers in the assimilation is due to inappropriate background-error covariances. This paper looks at the application of static adaptive mesh methods in the Met Office variational assimilation system to modify the background-error correlations in the boundary layer when temperature inversions or stratocumulus layers are present in the background state. Results show improvements in the analysis root mean square errors with respect to radiosonde observations and surface observations and improvements in forecast errors in 2 m temperature in the presence of low clouds. This enhancement in 2 m temperature forecast is attributed to reduced background vertical correlations and increased temperature background-error variances in the assimilation due to the movement of the grid near the surface. Copyright © 2011 British Crown copyright, the Met Office. Published by JohnWiley & Sons, Ltd.

Description: An alternative formulation of the extended Kalman filter for state and parameter estimation is presented, referred to as Short-Time Augmented Extended Kalman Filter (ST-AEKF). In this algorithm, the evolution of the model error generated by the uncertain parameters is described using a truncated short-time Taylor expansion within the assimilation interval. This allows for a simplification of the forward propagation of the augmented error covariance matrix with respect to the classical state augmented approach. The algorithm is illustrated in the case of a scalar unstable dynamics and is then more extensively analyzed in the context of the Lorenz 36-variable model. The results demonstrate the ability of the ST-AEKF to provide accurate estimate of both the system's state and parameters with a skill comparable to that of the full state augmented approach and in some cases close to the EKF in a perfect model scenario. The performance of the filter is analyzed for different initial parametric errors and assimilation intervals, and for the estimates of one or more model parameters. The filter accuracy is sensitive to the nature of the estimated parameter but more importantly to the assimilation interval, a feature connected to the short-time approximation on which the filter formulation relies. The conditions and the context of applications of the present approach are also discussed. Copyright © 2011 Royal Meteorological Society

Description: A new probabilistic forecast product, the Probability of RETurn (PRET), is introduced. PRET, the probability of occurrence of an event that corresponds to a specific return period, is computed from forecasts given by the ECMWF Ensemble Prediction System. It has been designed to provide easy-to-interpret and valuable information on the intensity and rarity of the expected severe weather, especially when the ensemble-based forecast distribution falls outside the model climate distribution. PRET definition relies on the Generalized Extreme Value family of distributions, which has been applied to study the statistics of the extremes in the model forecasts and observed datasets, and to estimate the levels corresponding to return periods not included in the datasets. PRET forecasts for the 2-metre maximum and minimum temperatures over Europe have been generated for six summer and six winter seasons (2003 to 2009). Case-studies have been used to illustrate that the new product is easier to interpret than products that are now commonly used, such as probability forecasts and maps of Extreme Forecast Indices. Average diagnostics of PRET forecasts indicate that the skill in predicting extremely hot temperatures in the warm season is higher than the skill in predicting extremely cold temperatures in the cold season. Copyright © 2011 Royal Meteorological Society

Description: Current operational numerical weather prediction models potentially have sufficient spatial and temporal resolution to resolve much of the inertia-gravity wave spectrum (e.g. horizontal wavelengths in the region of 100–1000 km typically, and even longer zonal wavelengths in the Tropics). The effects of gravity wave dynamics are usually considered to be subgrid-scale and parametrizable, either as stationary, mountain waves or travelling, non-orographic waves. Simple techniques for isolating these waves in both the Met Office and ECMWF forecast models are presented and their associated wave fluxes (including three-dimensional Eliassen–Palm fluxes) are computed for August 2006. In addition, a limited-area configuration of the Met Office Unified model with 4 km horizontal resolution is run over a region that includes the southern half of the Andes mountain range. Spectral decompositions of the wave energy and momentum fluxes show a distinct peak in power at horizontal wavelengths of about 400 km, irrespective of whether a 4 or 40 km grid is used, although about one half of the power resides in wavelengths shorter than 200 km. The computed Eliassen–Palm fluxes are shown to be similar to the net parametrized gravity wave drag, although the latter are too widespread and uniform over the oceans, reflecting the absence of geographical source variation. Comparison of the time mean temperature amplitude of the waves in the mid-stratosphere with estimates derived from limb-sounder satellite measurements from the HIRDLS project suggests that forecast models are capable of capturing the correct overall strength and distribution of gravity wave activity. Copyright © 2011 British Crown copyright, the Met Office. Published by John Wiley & Sons Ltd.

Description: We recall a theoretical analysis of the equivalence between the Kalman filter and the four-dimensional variational (4D-Var) approach to solve data-assimilation problems. This result is then extended to cover the comparison of the singular evolutive extended Kalman (SEEK) filter with a reduced variant of the 4D-Var algorithm. We next concentrate on the solution of the 4D-Var, which is usually computed with a (truncated) Gauss–Newton algorithm using a preconditioned conjugate-gradient-like (CG) method. Motivated by the equivalence of the above-mentioned algorithms, we explore techniques used in the SEEK filter and based on empirical orthogonal functions (EOFs) as an attempt to accelerate the Gauss–Newton method further. This leads to the development of an appropriate starting point for the CG method, together with that of a powerful limited-memory preconditioner (LMP), as shown by preliminary numerical experiments performed on a shallow-water model. Copyright © 2011 Royal Meteorological Society

Description: The Madden–Julian Oscillation (MJO) is the most prominent form of tropical intraseasonal variability in the climate system. Observations suggest that warming in the tropical Indian and Pacific Oceans in recent decades may have contributed to increased trends in the annual number of MJO events. A stochastic model is used to project changes in MJO activity under a global warming scenario. The mean number of events per year may rise from ∼3.9 (1948–2008) to ∼5.7 (2049–2099) and the probability of very active years (5 or more events) may significantly increase from 0.51 ± 0.01 (1990–2008) to 0.75 ± 0.01 (2010–2027) and 0.92 ± 0.01 (2094–2099). Copyright © 2011 Royal Meteorological Society

Description: The Ensemble Transform Kalman Filter (ETKF) is one candidate strategy for targeting observations around tropical cyclones. The characteristics of target areas identified by a revised version of the ETKF are investigated, using the THORPEX Interactive Grand Global Ensemble. In order to emphasize sensitivity to the environmental flow, the axisymmetric circulation associated with the tropical cyclone is removed from each ensemble member. The guidance is found to differ markedly from the ensemble variance, and the guidance products based on ensembles from different operational centres often disagree. For the pre-recurvature stage of typhoon Sinlaku (2008), the targets correspond to asymmetries close to the typhoon, neighbouring features including the adjacent subtropical ridge and a nearby upper-level trough, and far-field features including the monsoon trough and areas of flow convergence. During and after recurvature, targets exist in the midlatitude jet upstream and downstream of the typhoon. The midlatitude targets are discernible for forecasts beyond 3 days. The sensitivity to hypothetical observations of wind is more pronounced than those of temperature or specific humidity. The targets based on upper-level observations correspond to the outflow of the tropical cyclone and nearby and remote jets, while the lower-level targets correspond to confluent flow near the cyclone. Multiple remote targets are evident, particularly for longer forecast times, and some of these may be spurious. When ensemble forecasts initialized at different times are used for the same case, the characteristics of the guidance differ, although not dramatically. Future priorities include the evaluation of the effects of assimilating observations sampled in target areas versus those in non-target areas, and the improvement of the data assimilation methodology assumed in the ETKF. Copyright © 2011 Royal Meteorological Society

Description: Ensemble of data assimilations (EDA) methods have been shown to be able to provide flow-dependent estimates of analysis and background error statistics. For this reason, they potentially present a way to overcome one of the main limitations of current variational data assimilation systems. However, the limited number of ensemble members which can be realistically run in an operational context and the stochastic nature of the EDA approach lead to high levels of sampling noise in the relevant ensemble statistics. To answer this problem, an ‘objective filtering’ technique of the sample ensemble variances proposed by Raynaud et al. in 2009 has been implemented at ECMWF. In this article we present a comparison of the ability of ensemble data assimilation systems of different sizes (10 to 50 members) to represent flow-dependent background-error variances. In particular, the ensemble-based variances are examined in the cases of the severe storm Klaus (24 January 2009) over France and the Atlantic tropical hurricane Ike (1–14 September 2008). Our results show that, while a relatively small ensemble (10 members) can be sufficient to resolve the larger-scale error structures connected to an extratropical cyclogenesis, a larger ensemble is beneficial to resolve more localised anomalies like those connected with a hurricane. In this sense, the objective filtering technique provides a useful indication of the spatial scales the ensemble is able to resolve in a statistically robust way. The day-to-day variability of the ensemble statistics and how this affects the objective spatial filtering procedure are also examined. Our conclusion is that a time-independent implementation of the filter based on a climatology of truncation wave numbers results in more robust ensemble statistics estimates, and ultimately in improved forecast skill scores. Copyright © 2011 Royal Meteorological Society

Description: Systematic natural ventilation effects on measured temperatures within a standard large wooden thermometer screen are investigated under summer conditions, using well-calibrated platinum resistance thermometers. Under low ventilation (2 m wind speed u 2 〈 1.1 m s −1 ), the screen slightly underestimates daytime air temperature but overestimates air temperature nocturnally by 0.2°C. The screen's lag time L lengthens with decreasing wind speed, following an inverse power law relationship between L and u 2 . For u 2 〉 2 m s −1 , L ∼ 2.5 min, increasing, when calm, to at least 15 min. Spectral response properties of the screen to air temperature fluctuations vary with wind speed because of the lag changes. Ventilation effects are particularly apparent at the higher (〉25° C ) temperatures, both through the lag effect and from solar heating. For sites where wind speed decreases with increasing daytime temperature, thermometer screen temperatures may consequently show larger uncertainties at the higher temperatures. Under strong direct beam solar radiation (〉850 W m −2 ) the radiation effect is likely to be 〈0.4° C . Copyright © 2011 Royal Meteorological Society

Description: Forty years after a similar retrospective on ozone research at Oxford by Dobson in 1968, a review and synthesis is presented of the scientific and technological advances that were possible during the period from 1970 to 2010 as a result of the deployment of a number of innovative instruments on meteorological satellites by a research group in the Atmospheric Physics department at Oxford University. Copyright © 2011 Royal Meteorological Society

Description: Singular-vector(SV)-based selective satellite data thinning is applied to the Southern Hemisphere (SH) extratropics to reduce analysis uncertainty and forecast error. For two seasons, the European Centre for Medium-Range Weather Forecasts (ECMWF) four-dimensional variational data assimilation system has been run in five different configurations with different satellite data coverage: two reference experiments used low-density and high-density coverage over the globe; in the SH two SV-based selective thinning experiments used low-density data everywhere apart from targeted regions; and one random-based thinning experiment used low-density data everywhere apart from randomly defined regions. The SV-based target regions have been defined either by daily operational SVs computed for the ECMWF Ensemble Prediction System, or by the previous year's mean seasonal distribution. Results indicate that the impact of the additional data largely depends on the season. Overall, forecast errors grow faster in the SH cold season than in the warm season. In the SH cold season, the general impact of adding data is smaller and the relative difference between the impact of the individual targeting experiments is smaller as well. In the cold season, the data assimilation system failed to extract the meteorological signal carried by the extra satellite data in sensitive regions. In the SH warm season, all experiments with more data produce a statistically more significant and longer-lasting positive impact on forecast skill. In this season, the SV-based targeting experiment performs best and as well as the reference experiment in which the data density is increased globally. Copyright © 2011 Royal Meteorological Society