After six years of scientific, technical developments and meteorological validation, the Application of Research to Operations at Mesoscale (AROME-France) convective-scale model became operational at Meteo-France at the end of 2008. This paper presents the main characteristics of this new numerical weather prediction system: the nonhydrostatic dynamical model core, detailed moist physics, and the as- sociated three-dimensional variational data assimilation (3D-Var) scheme. Dynamics options settings and variables are explained. The physical parameterizations are depicted as well as their mutual interactions. The scale-specific features of the 3D-Var scheme are shown. The performance of the forecast model is evaluated using objective scores and case studies that highlight its benefits and weaknesses.

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The AROME-France Convective-Scale Operational Model

Introduction to Mathematical Statistics. By R.V. Hogg and A. T. Craig. Pp. ix, 245. 47s. 1959. (The Macmillan Company, New York)

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A Description of the Advanced Research WRF Model Version 4

. SURFEX is a new externalized land and ocean surface platform that describes the surface fluxes and the evolution of four types of surfaces: nature, town, inland water and ocean. It is mostly based on pre-existing, well-validated scientific models that are continuously improved. The motivation for the building of SURFEX is to use strictly identical scientific models in a high range of applications in order to mutualise the research and development efforts. SURFEX can be run in offline mode (0-D or 2-D runs) or in coupled mode (from mesoscale models to numerical weather prediction and climate models). An assimilation mode is included for numerical weather prediction and monitoring. In addition to momentum, heat and water fluxes, SURFEX is able to simulate fluxes of carbon dioxide, chemical species, continental aerosols, sea salt and snow particles. The main principles of the organisation of the surface are described first. Then, a survey is made of the scientific module (including the coupling strategy). Finally, the main applications of the code are summarised. The validation work undertaken shows that replacing the pre-existing surface models by SURFEX in these applications is usually associated with improved skill, as the numerous scientific developments contained in this community code are used to good advantage.

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The SURFEXv7.2 land and ocean surface platform for coupled or offline simulation of earth surface variables and fluxes

. Detailed studies of snow cover processes require models that offer a fine description of the snow cover properties. The detailed snowpack model Crocus is such a scheme, and has been run operationally for avalanche forecasting over the French mountains for 20 yr. It is also used for climate or hydrological studies. To extend its potential applications, Crocus has been recently integrated within the framework of the externalized surface module SURFEX. SURFEX computes the exchanges of energy and mass between different types of surface and the atmosphere. It includes in particular the land surface scheme ISBA (Interactions between Soil, Biosphere, and Atmosphere). It allows Crocus to be run either in stand-alone mode, using a time series of forcing meteorological data or in fully coupled mode (explicit or fully implicit numerics) with atmospheric models ranging from meso-scale models to general circulation models. This approach also ensures a full coupling between the snow cover and the soil beneath. Several applications of this new simulation platform are presented. They range from a 1-D stand-alone simulation (Col de Porte, France) to fully-distributed simulations in complex terrain over a whole mountain range (Massif des Grandes Rousses, France), or in coupled mode such as a surface energy balance and boundary layer simulation over the East Antarctic Ice Sheet (Dome C).

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The detailed snowpack scheme Crocus and its implementation in SURFEX v7.2

Fornumericalweatherpredictionmodelsandmodelsresolvingdeepconvection, shallowconvectiveascentsaresubgridprocessesthatarenotparameterizedbyclassicallocal turbulent schemes. The mass flux formulation of convective mixing is now largely accepted as an efficient approach for parameterizing the contribution of larger plumes in convec- tive dry and cloudy boundary layers. We propose a new formulation of the EDMF scheme (for Eddy Diffusivity\Mass Flux) based on a single updraft that improves the representa- tion of dry thermals and shallow convective clouds and conserves a correct representation of stratocumulus in mesoscale models. The definition of entrainment and detrainment in the dry part of the updraft is original, and is specified as proportional to the ratio of buoy- ancy to vertical velocity. In the cloudy part of the updraft, the classical buoyancy sorting approach is chosen. The main closure of the scheme is based on the mass flux near the surface, which is proportional to the sub-cloud layer convective velocity scale w∗. The link with the prognostic grid-scale cloud content and cloud cover and the projection on the non- conservative variables is processed by the cloud scheme. The validation of this new for- mulation using large-eddy simulations focused on showing the robustness of the scheme to represent three different boundary layer regimes. For dry convective cases, this parameteri- zation enables a correct representation of the countergradient zone where the mass flux part represents the top entrainment (IHOP case). It can also handle the diurnal cycle of boundary- layer cumulus clouds (EUROCS\ARM) and conserve a realistic evolution of stratocumulus (EUROCS\FIRE).

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A Parameterization of Dry Thermals and Shallow Cumuli for Mesoscale Numerical Weather Prediction

In numerical weather prediction and climate models, planetary boundary-layer (PBL) clouds are linked to subgrid-scale processes such as shallow convection. A comprehensive statistical analysis of large-eddy simulations (LES), obtained for warm PBL cloud cases, is carried out in order to characterize the distributions of the horizontal subgrid cloud variability. The production of subgrid clouds is mainly associated with the variability of the total water content. Nevertheless, in the case of PBL clouds, the temperature variability cannot be completely discarded and the saturation deficit, which summarizes both temperature and total water fluctuations, provides a better representation of the cloud variability than the total water content. The probability density functions (PDFs) of LES saturation deficit generally have the shape of a main asymmetric bell-shaped curve with a more or less distinct secondary maximum specific to each type of PBL clouds. Unimodal theoretical PDFs, even those with a flexible skewness, are not sufficient to correctly fit the LES distributions, especially the long tail that appears for cumulus clouds. They do not provide a unified approach for all cloud types. The cloud fraction and the mean cloud water content, diagnosed from these unimodal PDFs, are largely underestimated. The use of a double Gaussian distribution allows correction of these errors on cloud fields and provides a better estimation of the cloud-base and cloud-top heights. Eventually, insights for the design of a subgrid statistical cloud scheme are provided, in particular a new formulation for the weight of the two Gaussian distributions and for the standard deviation of the convective distribution.

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Evaluation of Statistical Distributions for the Parametrization of Subgrid Boundary-Layer Clouds

The performance in term of tropical cyclone track and intensity prediction of the new coupled ocean‐atmosphere system based on the operational atmospheric model AROME‐Indian Ocean and the ocean model NEMO is assessed against that of the current operational configuration in the case of seven recent tropical cyclones. Five different configurations of the forecast system are evaluated: two with the coupled system, two with an ocean mixed layer parameterization and one with a constant sea surface temperature. For each ocean‐atmosphere coupling option, one is initialized directly with the MERCATOR‐Ocean PSY4 product as in the current operational configuration and the other with the ocean state that is cycled in the AROME‐NEMO coupled suite since a few days before the cyclone intensification. The results show that the coupling with NEMO generally improves the intensity of cyclones in AROME‐IO, reducing the mean intensity bias of the 72 hr forecast of about 10 hPa. However, the impact is especially significant when the TCs encounter a slow propagation phase. For short‐term forecasts (less than 36 hr), the presence of a cooling in the initial state that has been triggered by the AROME high‐resolution cyclonic winds in a previous coupled forecast already improves the tropical cyclone intensity bias of 2–3 hPa for both coupled or uncoupled configurations.

Evaluation of a Mesoscale Coupled Ocean‐Atmosphere Configuration for Tropical Cyclone Forecasting in the South West Indian Ocean Basin

. A widely applicable parameterization of turbulent heat and momentum fluxes at sea has been developed for the SURFEX v8.1 surface model. This wave-age dependent stress parameterization (WASP) combines a close fit to available in situ observations at sea up to wind speed of 60 m s − 1 with the possibility of activating the impact of wave growth on the wind stress. It aims in particular at representing the effect of surface processes that depend on the surface wind according to the state of the art. It can be used with the different atmospheric models coupled with the surface model SURFEX, including the 5 CNRM-CM climate model, the operational (numerical weather prediction) systems in use at Météo-France and the research model Meso-NH. Designed to be used in coupled or forced mode with a wave model, it can also be used in atmosphere-only configuration. It has been validated in several case studies covering different surface conditions known to be sensitive to the representation of surface turbulent fluxes: i) the impact of a Sea Surface Temperature (SST) front on low-level flow by weak wind; ii) the simulation of a Mediterranean heavy precipitating event where waves are known to influence the low-level wind 10 and displace precipitation; iii) several tropical cyclones; and iv) a climate run over 35 years. It shows skills comparable to or better than the different parameterizations in use

Sylvie Malardel

Papers

The AROME-France Convective-Scale Operational Model

A Parameterization of Dry Thermals and Shallow Cumuli for Mesoscale Numerical Weather Prediction

Evaluation of Statistical Distributions for the Parametrization of Subgrid Boundary-Layer Clouds

Evaluation of a Mesoscale Coupled Ocean‐Atmosphere Configuration for Tropical Cyclone Forecasting in the South West Indian Ocean Basin

The wave-age dependent stress parameterization (WASP) for momentum and heat turbulent fluxes at sea in SURFEX v8.1.