Development of a High-Resolution Ocean-Waves-Atmosphere Coupled System for Tropical Storms Modeling

Although tropical cyclone (TC) track forecasts have been improved for the past few decades, TC intensity forecasts, in particular, rapid intensity changes, are still a difficult task. To investigate the tropical cyclone intensity change and structure and better describe the associated precipitation, we developed an ocean-waves-atmosphere coupling system. An ocean-atmosphere coupling is essential to reproduce the air-sea interactions which are fundamental for tropical cyclones. The importance of oceanic waves has also been underlined: they are involved in air-sea exchanges and play an important role through the emission of sea sprays, the main source of cloud condensation nuclei (CCN) in a clean maritime environment. The French mesoscale model Meso-NH has thus been coupled to the oceanic model CROCO (Coastal and Regional Ocean Community model) and to the oceanic waves model WaveWatch3. A parameterization of the emission of sea salt aerosol that depends on the wind speed, significant wave height, and sea surface temperature and salinity is also introduced. The importance of this coupling to produce consistent sea salt aerosols emissions will be illustrated with the simulation of tropical cyclone Bejisa that developed in the south-west Indian ocean in December 2013-January 2014.Simultaneously, an original coupling between an aerosol scheme (Organic Inorganic Log-normal Aerosol Model, ORILAM) and a 2-moment bulk microphysics scheme (Liquid Ice Multiple Aerosols, LIMA) has been realized in Meso-NH to fully represent the aerosol-microphysics-radiation interactions in tropical cyclones. The MACC (Monitoring Atmospheric Composition and Climate project) analysis from the European Center for Medium-Range Weather Forecast are used to initialize and feed the lateral boundary conditions for the aerosol fields, and sea salt aerosols are emitted by high wind and waves at each time step. A first evaluation of this coupling is performed through the simulation of tropical cyclone Dumile (January 2013) in the south-west Indian Ocean. The importance of explicitly taking account of sea salt aerosol emissions associated with high winds and waves in tropical cyclones is shown to be a critical point for simulating long-lasting systems that need to generate their own CCN. Finally, the full coupling will be presented along with some ongoing additional development (coupling with an electrification and lightning scheme, additional secondary ice production parameterizations, inclusion of several ice crystal shapes…) done in the framework of the ReNovRisk program.