BOUNDARY LAYER STRUCTURE IN TYPHOON SAOMAI （2006）： UNDERSTANDING THE EFFECTS OF EXCHANGE COEFFICIENT

Recent studies have shown that surface fluxes and exchange coefficients are particularly important to models attempting to simulate the evolution and maintenance of hurricanes or typhoons. By using an advanced research version of the Weather Research and Forecasting （ARW） modeling system, this work aims to study the impact of modified exchange coefficient on the intensity and structures of typhoon Saomai （2006） over the western North Pacific. Numerical experiments with the modified and unmodified exchange coefficients are used to investigate the intensity and structure of the storm, especially the structures of the boundary layer within the storm. Results show that, compared to the unmodified experiment, the simulated typhoon in the modified experiment has a bigger deepening rate after 30-h and is the same as the observation in the last 12-h. The roughness is leveled off when wind speed is greater than 30 m/s. The momentum exchange coefficient （CD） and enthalpy exchange coefficient （CK） are leveled off too, and CD is decreased more than CK when wind speed is greater than 30 m/s. More sensible heat flux and less latent heat flux are produced. In the lower level, the modified experiment has slightly stronger outflow, stronger vertical gradient of equivalent potential temperature and substantially higher maximum tangential winds than the unmodified experiment has. The modified experiment generates larger wind speed and water vapor tendencies and transports more air of high equivalent potential temperature to the eyewall in the boundary layer. It induces more and strong convection in the eyewall, thereby leading to a stronger storm.