Sensitivity of three Mediterranean heavy rain events to two different sea surface fluxes parameterizations in high-resolution numerical modeling

Abstract

The southeast of France is prone to heavy rain events during the fall season. For these extreme precipitating events, the Mediterranean Sea supplies heat and moisture to the atmospheric boundary layer through the turbulent heat fluxes. In turn the low-level jet, which frequently accompanies these events, induces a stress on the oceanic mixed layer. The aim of this study is to examine the sensitivity of short-range (24 h) high-resolution (2-3 km) forecasts to the sea surface fluxes parameterization for three representative torrential rainfall events. We consider in particular two sea surface fluxes bulk parameterizations: the original MESO-NH model parameterization based on the Louis (1979) formulation and the iterative Coupled Ocean-Atmosphere Response Experiment (COARE) bulk algorithm from Fairall et al. (2003). Results show that the two parameterizations produce very different air-sea fluxes values, especially for strong winds; the largest differences are therefore found under the low-level jets, where the COARE parameterization reduces the wind stress and the latent heat flux. The evaporation reduction results in a decrease of the low-level humidity transport over the Mediterranean Sea in every case studied so that the convective activity, still intense, results in lower rainfall amounts. The impact of surface fluxes parameterization on the ocean mixed layer modeling is also examined by driving a one-dimensional ocean model with the same set of atmospheric parameters but using the two parameterizations. Results show a significant impact on the mixed layer depth. Copyright 2008 by the American Geophysical Union.