Evaluation of drag coefficients under medicane conditions: Coupling waves, sea spray and surface friction

This work represents an initial step toward the coupled ocean-atmosphere-aerosols modeling of severe storms in the Mediterranean basin. This is realized extending the classical Charnock formulation of surface roughness over sea and consequently the determination of air-sea momentum fluxes in numerical models. To improve the forecast of severe Mediterranean storm systems, like the so-called Medicanes, it is essential to consider the wave-state and sea spray effects on the sea surface roughness explicitly. This is implemented in the present work by coupling the third-generation WAVEWATCH III model (WWIII) and the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) model configured with the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) aerosol package. The communication between the two models is realized through an offline coupling scheme, which utilizes a regridding algorithm between the numerical domains of the two models. In this context, the wave peak frequency is passed from WWIII to WRF-Chem model permitting the calculation of the wave-age and the sea spray source functions as well as the modification of the overseas drag coefficients. Two different surface roughness parameterizations are tested and compared with ERA5 reanalysis data from the ECMWF - Copernicus Climate Change Service. A single case study is performed considering the cyclone Rolf that occurred between 5 and 9 November 2011 in the western Mediterranean basin. It is considered one of the longest-lasting and most intense tropical-like cyclones in the Mediterranean with more than 48-h occurrence of tropical-like features and a wind speed peak reaching 30 m s−1 in its mature phase. The role of sea spray is evaluated considering two basic strategies, namely the limited-saturation layer approach and the “sea spray force” on the momentum balance. It is demonstrated that (i) the formulation based on the limited-saturation layer approach is better suited for describing the drag in the cyclonic region, where velocities are highest and consequently there is a very high concentration of sea-spray spume droplets; (ii) the momentum drag level-off at the highest wind speed. Our results give a contribution on the understanding of the air-sea flux parameterizations for a range of wind speed above 25–35 m s−1, indicating that sea-spray generation should be accounted for in coupled numerical models for ocean and atmosphere.