Ocean-atmosphere interactions and coupling associated with Mediterranean heavy rainfall events

The Mediterranean coastal areas are prone to torrential rainfall in autumn. The Mediterranean Sea is the major source of heat and moisture for these extreme meteorological events. Generally, an unstable air mass is transported by a strong low-level wind from the sea towards mountainous regions. The convection is then triggered and could stay several hours over the same area, producing locally large rainfall totals and sometimes flood damages. The sensitivity of these extreme events to the sea surface conditions has been examined based on high-resolution atmospheric simulations (2-3 km) of three heavy rainfall events over Southern France. Various SST fields have been used in Meso-NH simulations (optimal interpolation of in-situ observations, satellite SST, empirical increase/decrease of SST). We also evaluated the sensitivity to the air-sea fluxes parameterizations by introducing in the Meso-NH surface scheme (SURFEX) two new iterative bulk sea surface parameterizations: the UNITFP parameterization including a multi-campaign calibration of the exchange coefficients and the COARE 3.0 algorithm (Fairall et al., 2003). The main results are: first, the significant impact on the atmospheric convection (intensity and location) of the overall SST value beneath the low-level jet; secondly a strong decrease of the wind stress and of the evaporation simulated when the bulk iterative parameterizations are used instead of the standard Meso-NH parameterization. As a second part of this study, an air-sea coupled modelling between the Meso-NH model and a 1D oceanic model in TKE equations is currently developed in order to study the impact of heavy precipitation and associated strong wind stress on the Mediterranean Sea mixed layer. Some insights about this coupling development and about its impacts on the same torrential rainfall events simulations will be presented during the conference. This study constitutes a preliminary work for the future HYMEX experimental project focused on the hydrological cycle of the Mediterranean basin.