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

Modelling of the Mediterranean circulation using atmospheric fields at different space-time resolutions

Cindy Lebeaupin Brossier (LMD/IPSL); Karine Béranger (ENSTA-LOCEAN/IPSL), Philippe Drobinski (LMD/IPSL)

In the framework of the MORCE-MED project, a two-way ocean-atmosphere coupling is developed between the Weather Research and Forecasting (WRF) atmospheric model and the NEMO ocean model over the Mediterranean basin. The future ocean-atmosphere coupled system is part of the future regional numerical platform including also the modelling of the continental superficial layers, atmospheric chemistry and marine biogeochemistry. Forced by global reanalyses or by the global climatic numerical system outputs, the whole regional coupled model could contribute to the HyMex project, in particular to study the trends of the air-sea interactions, i. e. the different terms of the water cycle budget in the present climate and in climate projections. The observational phases of the HyMeX project will also constitute a unique opportunity to validate our regional coupled system.
Before applying the full two-way interactive coupling between the ocean and atmospheric regional models for long-term simulations over the Mediterranean, the forcing mode is considered through a sensitivity study. The downscaling of the NCEP reanalyses over the full Mediterranean basin with a 20-km resolution has been done with the WRF model between August 1998 and July 1999. The daily atmospheric fields obtained are then used to drive the NEMO model (with a 6-8 km resolution) over the Mediterranean Sea in a perpetual mode during a spinup of 8 years. Then, three experiments are done for a period of 4 years. The first experiment (or control experiment) is the continuity of the spinup. In the second experiment, a higher temporal resolution is used and the frequency of the forcing is 3 hours, which allows a good representation of the diurnal cycle and of the extreme air-sea exchanges that occur with a short duration during severe meteorological events. In the third experiment, a finer spatial resolution of the forcing is applied over the Gulf of Lions area in order to approach the ocean model resolution and to well represent the channelling of the Mistral and Tramontane.
The preliminary analyses that compare the general Mediterranean circulation as the characteristics of the mixed layer, of the deep convection and of the upwellings in the control experiment and in the two sensitivity experiments, will be presented during the workshop. We aim also to quantify the benefit of increasing the resolutions of the atmospheric forcing by a comparison of our experiments to observations and climatologies.