Wave coupling impact on tropical cyclone simulations in global models

As global atmospheric climate models move to higher horizontal resolution (25 – 50 km), it can be used as a convenient tool to study the impact of global warming on tropical cyclone statistics; the causes of year-to-year variability in global tropical cyclone activity; the relationship between tropical cyclone frequency/intensity and climate change, etc. However, the current global models still have difficulties in simulating the intensity of the storms. Besides the relatively course resolution in the climate models compare to the regional models, the wind speed dependent only air-sea fluxes parameterization could also be one of the reasons. The air-sea momentum flux is usually represented by the drag coefficient, Cd, or the sea surface roughness, zo. Field observations have shown large scattering in Cd especially under high wind condition. People argue this large scattering is caused by the spatial and time variation in the wave field. In this talk, we will present the frame-work of a coupled global atmosphere-wave model using the GFDL's prototype Global Cloud-Resolving Model (HiRAM) and WaveWatch III developed at NCEP/EMC. In the coupled model, the sea surface roughness, zo, is parameterized as a function of wave age and wind speed. Preliminary results on real case tropical cyclone simulations will be presented and the impact of wave coupling on tropical cyclone predictions will be discussed.