ECMWF Workshop on Ocean-Atmosphere Interaction, 10 - 12 November 2008 101 Atmosphere-Ocean Coupling in Tropical Cyclones Isaac Ginis Graduate School of Oceanography University of Rhode Island, Narragansett, RI USA 1. Introduction The advent of numerical weather prediction tropical cyclone (TC) models has demonstrably improved the forecasts of TCs over the last decades. But to establish useful warning systems for TCs, it is necessary to accurately predict both storm track and intensity. Whereas TC tracks are determined almost exclusively by their large-scale atmospheric environment, storm intensity is influenced to a greater degree by smaller-scale features in both the atmosphere and ocean. The factors that control the intensity of TCs are still poorly understood, leading to limited reliability in forecasts of TC intensity evolution. Variability in TC intensity originates from two sources: internal variability and environmental interactions. There are three critical aspects of the environmental interactions: 1) the dynamical and microphysical processes near and at the sea surface that influence the turbulent exchange of heat and momentum between the ocean and atmosphere, 2) vertical and horizontal transport of momentum and heat in the atmospheric boundary layer and 3) the turbulent entrainment of relatively cold water through the seasonal thermocline, which affects the sea surface temperature and thereby influences storm intensity. Three-dimensional, coupled atmosphere-ocean research and operational models have been developed to simulate and predict the mutual response of a TC and the ocean (Bender and Ginis 2000, Bao et al., 2000, Bender et al., 2007, Chen et al., 2007, Surgi 2007). One such coupled model, the GFDL/URI hurricane-ocean prediction system, has been used operationally at the NOAA’s National Centers for Environmental Prediction (NCEP) since 2001. The GFDL/URI model has demonstrated steady improvements in TC intensity prediction over the last several years (Bender et al. 2007). Another fully coupled model, the Hurricane Weather Research and Forecast (HWRF) model became operational at NCEP in 2007 (Surgi 2007). Expert reports commissioned by NOAA, the U.S. National Science Board and the American Geophysical Union have concluded that further advances in TC intensity forecasts and impacts projection require novel theoretical concepts and the next generation very high resolution coupled atmosphere-wave-ocean numerical models with improved boundary layer and surface flux parameterizations, tested against high-quality observations. Below I briefly summarize the results of the most recent efforts of our research group at URI and our collaborators to advance our understanding and parameterization of air-sea fluxes in tropical cyclone conditions and improving the ocean model initialization as a route toward skillful prediction of tropical cyclone intensity and structure. 2. Role of surface waves in air-sea momentum fluxes under tropical cyclones Recent observations from the Coupled Boundary Layer and Air-Sea Transfer (CBLAST) field program that was sponsored by the U.S. Office of Naval Research (Black et al., 2007) and the analysis of GPS drop sondes by Powell et al. (2003, 2007) have shown that the drag coefficient varies widely under TCs. However, even though this was one of the main foci of the CBLAST program, it was a daunting task to measure surface fluxes in wind speeds exceeding 30 m s -1 . Very few measurements exist for higher wind speeds and this remains an area of significant uncertainty. Our approach to this problem is based on