Description: In July and August 2010 floods of unprecedented impact afflicted Pakistan. The floods resulted from a series of intense multi-day precipitation events in July and early August. At the same time a series of blocking anticyclones dominated the upper-level flow over western Russia and breaking waves i.e. equatorward extrusions of stratospheric high potential vorticity (PV) air formed along the downstream flank of the blocks. Previous studies suggested that these extratropical upper-level breaking waves were crucial for instigating the precipitation events in Pakistan. Here a detailed analysis is provided of the extratropical forcing of the precipitation. Piecewise PV inversion is used to quantify the extratropical upper-level forcing associated with the wave breaking and trajectories are calculated to study the pathways and source regions of the moisture that precipitated over Pakistan. Limited-area model simulations are carried out to complement the Lagrangian analysis. The precipitation events over Pakistan resulted from a combination of favourable boundary conditions with strong extratropical and monsoonal forcing factors. Above-normal sea-surface temperatures in the Indian Ocean led to an elevated lower-tropospheric moisture content. Surface monsoonal depressions ensured the transport of moist air from the ocean towards northeastern Pakistan. Along this pathway the air parcel humidity increased substantially (60–90% of precipitated moisture) via evapotranspiration from the land surface. Extratropical breaking waves influenced the surface wind field substantially by enhancing the wind component directed towards the mountains which reinforced the precipitation. Copyright © 2010 Royal Meteorological Society

Description: The recent studies of the sensitivity of tropical-cyclone intensification to the surface drag coefficient in a three-dimensional model by Montgomery et al. , and Smith et al. , are extended to include perturbations of the surface drag coefficient in one of four boundary-layer parametrization schemes: the bulk scheme, the Blackadar scheme, the MRF scheme, and the Gayno–Seaman scheme. The schemes are slightly modified to have the same drag coefficient formulation and the same constant exchange coefficients for sensible heat and moisture. We find that the intensification rate and mature intensity are essentially unaltered when the drag coefficient is perturbed randomly by variations of up to 60%. The results, in conjunction with an analysis of coherent drag coefficient variations for a moving vortex, question the notion that coupled wind–wave mod els are necessary to accurately forecast tropical-cyclone intensification and mature intensity. Copyright © 2012 Royal Meteorological Society

Description: Although mineral aerosol (dust) particles are irregular in shape, they are treated as homogeneous spheres in climate model radiative transfer calculations. Here, we test the effect of dust particle non-sphericity in the ECHAM5.5-HAM2 global aerosol–climate model. The short-wave optical properties of the two insoluble dust modes in HAM2 are modelled using an ensemble of spheroids that has been optimized to reproduce the optical properties of dust-like aerosols, thereby providing a significant improvement over spheres. First, the direct radiative effects (DRE) of dust non-sphericity were evaluated diagnostically, by comparing spheroids with both volume-equivalent and volume-to-area (V/A) equivalent spheres. In the volume-equivalent case, the short-wave DRE of insoluble dust at the surface and at the top of the atmosphere (TOA) was slightly smaller (typically by 3–4%) for spheroidal than for spherical dust particles. This rather small difference stems from compensating non-sphericity effects on the dust optical thickness and asymmetry parameter. In the V/A-equivalent case, the difference in optical thickness was virtually eliminated and the DRE at the TOA (surface) was ∼20% (∼13%) smaller for spheroids than for spheres, due to a larger asymmetry parameter. Even then, however, the global-mean DRE of non-sphericity was only 0.055 W m −2 at the TOA and 0.070 W m −2 at the surface. Subsequently, the effects of dust non-sphericity were tested interactively in simulations in which ECHAM5.5-HAM2 was coupled to a mixed-layer ocean model. Consistent with the rather small radiative effects noted above, the climatic differences from simulations with spherical dust optics were generally negligible. Copyright © 2012 Royal Meteorological Society

Description: This article addresses the effects of cloud condensation nuclei on the evolution of an intense tropical convective system, known as Hector , using data taken from the ACTIVE and TWP-ICE field campaigns, which were conducted in 2005 and 2006. The Hector thunderstorms were observed in a variety of aerosol conditions so the data serve as an ideal dataset to test whether aerosols have a significant impact on the evolution of convective clouds and precipitation. We find evidence for an aerosol effect on the storm's properties, which are reproduced with a state-of-the-art mesoscale cloud-resolving model. Including the measured aerosol concentration within the model is shown to improve the fractions skill-score metric for every case presented in the article, thus giving us confidence that the deep convection observed during the period was indeed influenced by the aerosol entering the storm's inflow. However, we do not find a general relationship for the way aerosols affect properties such as cloud-top height, precipitation or radiative properties, as has been suggested in previous work. The reasons for this appear to be because of the nonlinearity of interactions between neighbouring cells and because of the variability in the meteorological profiles of temperature, wind and humidity. Copyright © 2013 Royal Meteorological Society

Description: A new incremental four-dimensional variational (4D-Var) data assimilation algorithm is introduced. The algorithm does not require the computationally expensive integrations with the nonlinear model in the outer loops. Nonlinearity is accounted for by modifying the linearization trajectory of the observation operator based on integrations with the tangent linear (TL) model. This allows us to update the linearization trajectory of the observation operator in the inner loops at negligible computational cost. As a result the distinction between inner and outer loops is no longer necessary. The key idea on which the proposed 4D-Var method is based is that by using Gaussian quadrature it is possible to get an exact correspondence between the nonlinear time evolution of perturbations and the time evolution in the TL model. It is shown that J -point Gaussian quadrature can be used to derive the exact adjoint-based observation impact equations and furthermore that it is straightforward to account for the effect of multiple outer loops in these equations if the proposed 4D-Var method is used. The method is illustrated using a three-level quasi-geostrophic model and the Lorenz (1996) model. Copyright © 2012 Royal Meteorological Society

Description: The Met Office is currently developing an ensemble-based data-assimilation system for use in numerical weather prediction (NWP). In an effort to inform those developments, a simple model has been used to compare a number of different flavours of ensemble Kalman filter (EnKF). As in many previous studies, deterministic (square-root) filters are seen to perform better than stochastic (perturbed-observations) filters. Two previously proposed methods for mitigating the effects of perturbing the observations were tested. These modified stochastic filters improved performance, but were still worse than the deterministic filters. Very little difference was seen between the performance of the various deterministic filters tested. The typical approach to localization in the Local Ensemble Transform Kalman Filter (LETKF) is based solely on the distance between each observation and the model point being updated. It takes no account of the inter-observation distance and we argue that this is incorrect. However, the difference between covariance localization and observation localization is only relevant when the assimilation is not weak and hence it did not affect performance in these tests. If a hybrid covariance matrix is used to update the ‘deterministic’ forecast around which the ensemble is centred, then using the same covariance matrix when updating the perturbations is seen to be beneficial in a stochastic filter. This is not the case in the square-root filter, due to restrictions imposed by its formulation. Tests with a nonlinear observation operator indicate better behaviour when observations are assimilated simultaneously rather than sequentially. An ensemble of variational minimizations has some advantages in this situation. These results, along with other considerations, have led the Met Office to develop a 4-dimensional ensemble-variational (4D-En-Var) system for data assimilation and ensemble generation. Copyright © 2012 British Crown Copyright, the Met Office. Published by John Wiley & Sons, Ltd

Description: We apply a recently developed method, the Ensemble Prediction and Parameter Estimation System (EPPES), to demonstrate how numerical weather prediction (NWP) model closure parameters can be optimized. As proof of concept, we tune the medium-range forecast skill of the ECMWF model HAMburg version (ECHAM5) atmospheric general circulation model using an ensemble prediction system (EPS) emulator. Initial state uncertainty is represented in the EPS emulator by applying the initial state perturbations generated at the European Centre for Medium-range Weather Forecasts (ECMWF). Model uncertainty is represented in the emulator via parameter variations at the initial time. We vary four closure parameters related to parametrizations of subgrid-scale physical processes of clouds and precipitation. With this set-up, we generate ensembles of 10-day global forecasts with the ECHAM5 model at T42L31 resolution twice a day over a period of three months. The cost function in the optimization is formulated in terms of standard forecast skill scores, verified against the ECMWF operational analyses. A summarizing conclusion of the experiments is that the EPPES method is able to find ECHAM5 model closure parameter values that correspond to smaller values of the cost function. The forecast skill score improvements verify positively in dependent and independent samples. The main reason is the reduced temperature bias in the tropical lower troposphere. Moreover, the optimization improved the top-of-atmosphere radiation flux climatology of the ECHAM5 model, as verified against the Clouds and the Earth's Radiant Energy System (CERES) radiation data over a 6-year period, while the simulated tropical cloud cover was reduced, thereby increasing a negative bias as verified against the International Satellite Cloud Climatology Project (ISCCP) data. Copyright © 2012 Royal Meteorological Society

Description: This article presents observations and model simulations of the low-level nocturnal structure of the atmosphere over West Africa. The measurements are taken from the dry-season Special Observing Period (SOP-0) of the African Monsoon Multidisciplinary Analysis (AMMA), at Niamey, Niger, on 23 and 24 January 2006. During this time, mesoscale structures in the atmospheric aerosol loadings were observed. The available observations indicate that these mesoscale features at Niamey are consistent with the passage of gravity currents or bores in the northerly Harmattan winds. Model simulations at resolutions down to 1 km indicate that the mesoscale structures are caused by nocturnal frontogenesis in the baroclinic zone to the south of the Sahara in the winter months. This frontogenesis is a continental-scale phenomenon, which has significant implications for the uplift and transport of dust and biomass-burning aerosols in the region. An accompanying frontogenetic feature appears further south in the model simulations, associated with the winter intertropical front. The frontogenesis is possibly linked with the development of structures showing characteristics of canonical mesoscale phenomena, including internal bores and gravity currents. Representation of these features in the UK Met Office Unified Model at different resolutions (from 12 km to 1 km) is discussed. Copyright © 2012 Royal Meteorological Society and British Crown Copyright, the Met Office

Description: Kinetic energy (KE) spectra are applied to evaluate two convection-permitting models: the AROME numerical weather prediction operational model and the Meso-NH research model, that share the same physics and differ only in the dynamics (semi-Lagrangian semi-implicit versus Eulerian explicit schemes). A first analysis of AROME spectra for winter and summer seasons shows that the model-derived spectra match the observational spectra well, including the transition between k −3 and k −5/3 regimes. The vertical distribution of the spectra is coherent with previous observational and numerical studies and the diurnal cycle has a strong impact on the amount of KE in the mesoscale during summer. A comparative analysis of KE spectra for both models is then performed on a real case of individual convective cells that developed over plains, during the afternoon of 11 April 2007, characterized by a strong cold air outflow in the low levels. AROME spectra are characterized by a coarser effective resolution than Meso-NH, even without explicit diffusion, revealing the impact of the implicit diffusion of the semi-implicit semi-Lagrangian scheme used in AROME. For large time steps, the damping increases and can be attributed preferentially to the SI part of the SISL (semi-implicit semi-Lagrangian) formulation. Adiabatic runs show that the transition to a shallow mesoscale regime is still apparent even if the mesoscale KE variance strongly depends on the presence of the physical processes. Effective resolution of Meso-NH remains around 4–6Δ x for horizontal grid spacings between 2.5 km and 250 m. The effects of subgrid mixing schemes are also investigated with Meso-NH at 500 m horizontal grid spacing in the grey zone for turbulence, illustrating the difficulty in finding a good equilibrium between resolved and subgrid mixing at this scale. Copyright © 2012 Royal Meteorological Society

Description: The radiation scheme in the Met Office Unified Model (UM) is extended to allow the calculation of short-wave radiances. Comparisons between short-wave reflectances simulated with the global forecasting configuration of the UM and observations from SEVIRI (Spin Enhanced Visible and Infrared Imager) on board Meteosat 8 are presented. One of the main purposes for such a comparison is model validation, since it allows us to directly compare model and satellite reflectances, without having to make use of retrieval systems. Eliminating the retrieval algorithm has the advantage that we can be certain that any discrepancies between the modelled and the measured radiances are due to inaccuracies in the model, rather than differences in the assumptions made both in the model and the retrieval algorithm. In principle, this should allow us to detect weaknesses in the model parametrizations. In particular, we compare the four short-wave channels of SEVIRI to the corresponding radiance simulations obtained from the UM over oceanic regions. Generally good agreement between the simulated and observed reflectances was obtained in the visible channels, but greater disagreement was found in the NIR 1.6 and IR 3.9 channels, hinting at deficiencies in the model's representation of particle size. Sensitivity studies are carried out to explore these sensitivities. Copyright © 2012 British Crown Copyright, the Met Office. Published by John Wiley & Sons, Ltd