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3rd HyMeX workshop 1-4 June 2009 Heraklion (Gournes), Crete-Greece

A high-resolution data assimilation system for Mediterranean high-impact weather events: AROME West-MED.

Fanny Duffourg (Météo-France, CNRM/GAME); Geneviève Jaubert, Pierre Brousseau, Véronique Ducrocq

The Western Mediterranean region is regularly affected by high-impact weather events such as Heavy Precipitating Events (HPE) and flash-flooding, cyclogenesis and strong winds. The contribution of fine-scale processes has been identified as a reason for the still low ability to predict such high-impact weather events. Recently developed high-resolution deterministic modelling suites, with their kilometric grid-mesh and explicit treatment of atmospheric deep convection, improve significantly the representation of high-impact weather systems.

The new French operational limited-area high-resolution modelling system AROME (2.5 km) has been adapted over a large domain covering Western Mediterranean from Portugal to Sicily and from Atlas to the Northern Alps. Such domain allows to better represent the influence of the main mountain ranges on the atmospheric circulation as well as the contribution of the Sea to humidity and stability of the lower layers.

The AROME West-MED data assimilation and forward model characteristics is presented in the poster. The main geometrical, dynamic and physical parameterisations characteristics remain the same as those of AROME-France. They mix a non-hydrostatic extension of the ALADIN NWP operational system dynamics and the Meso-NH research model physics. As AROME-France, AROME West-MED is hourly coupled with the ALADIN-France forecasts. Future work will test a coupling with the upcoming 15-km horizontal resolution version of ARPEGE or other larger scale models.

Special care has been taken for the data assimilation system as the representation of mesoscale structures in the initial conditions is a key ingredient for accurate forecasts. AROME West-MED data assimilation system adopts the AROME-France 3-hour cycled 3D-Var one, inherited from ALADIN. Model errors statistics have been recalculated with the new domain. Conventional observations (surface observations from land stations, ships and buoys, vertical soundings from radiosondes, pilot balloons, dropsondes and aircrafts measurements) are assimilated together with winds from wind profilers, Atmospheric Motion Vectors and scatterometers, doppler winds from radars, satellite radiances and ground-based GPS measurements.

The simulation of the 1st November 2008 HPE with AROME West-MED illustrates the ability of this new modelling system to reproduce Mediterranean high-impact weather events. In comparison with AROME-France, AROME West-MED clearly benefits from the inclusion of the Mediterranean Sea in the new domain and appears well adapted for Mediterranean high-impact weather events study.

It is foreseen that this system will run in real-time during the HyMeX SOPs. Up to this time, studies will be carried out to improve the AROME West-MED configuration especially by fostering the integration of new research developments (surface data assimilation, additional observations assimilation, new land cover data, etc.). The possibility of a two-way coupling of AROME West-MED with a 1D ocean model according to Lebeaupin et al. [2009] will also be investigated.