Role of the Sea State on the Air-Sea Exchanges during a Mediterranean Heavy Precipitation Event in a Kilometer-Scale Forecast System

During late summer and fall, the Mediterranean area is prone to heavy precipitation events (HPEs) characterized by large rainfall amounts in a short period (typically more than 100 mm in less than 24 hours) which often lead to devastating flash-floods. For these events, the Mediterranean Sea is a source of heat and moisture that feed the mesoscale convective systems. The air-sea exchanges are favored by a strong marine low-level jet (>20 m/s) blowing over a warm and thin ocean mixed layer. Frequently, such events are also associated with a sea surface elevation, submersion and rough sea (waves significant height above 6 meters), which increase the coastal flooding risk.

The international HyMeX (Hydrological cycle in Mediterranean Experiment) program investigates the Mediterranean Sea hydrological cycle with an emphasis on the severe events that largely contribute to it. A large part of HyMeX is devoted to the improvement of the prediction skill of high-impact weather events in the area, with notably the development of integrated (multi components) forecast systems. Within this framework, the coupling between the Météo-France kilometer-scale AROME Numerical Weather Prediction model, the NEMO ocean model and the WaveWatchIII model is currently developed at CNRM.

As a preliminary work to the implementation of the ocean-atmosphere-waves coupled forecast system, this study aims to investigate the representation of air-sea exchanges that occur during such events with a focus on the role of the sea state on the turbulent fluxes. For that purpose, an innovative turbulent flux parametrization, named WASP (Wave-Age dependent Stress Parametrization), which takes into account directly the impact of the sea state, has been developed, introduced in the surface scheme (SURFEX) of AROME and tested for a HPE occurring between 12 and 14 October 2016 in South-Eastern France. Preliminary results coming from a set of numerical sensitivity experiments in the forced or coupled mode will be presented and discussed during the conference.