Examining the Impact of Different Vertical Mixing Strengths on Hurricanes Evolution over Land

This study evaluates the effects of different vertical mixing strengths in planetary boundary layer (PBL) on hurricanes track, intensity, and structure changes during their evolution over land. The NCEP Hurricane Weather Research and Forecasting (HWRF) model is employed. A series of sensitivity experiments with different eddy diffusivity for momentum (Km) over land were conducted for Hurricanes Dennis (2005), Katrina (2005) and Rita (2005). Results show that, with stronger vertical mixing, the environment intrusion effect is enhanced because the exchange of heat/moisture between land surface and atmosphere becomes more effective. As a result, the model tends to produce weaker hurricane vortices during their evolution over land, with weaker intensity, warm core, and more loose rainband structures. In contrast, the weaker vertical mixing leads stronger hurricane intensity over land. With a set of reasonable vertical mixing parameter, significant improvements on track, intensity, quantitative precipitation forecasts, as well as the storm structures, can be achieved with the HWRF model. Further diagnoses compared the impact of vertical mixing in PBL on hurricane evolution over land and ocean. It is found that the stronger vertical mixing in PBL is benefit to hurricanes decay over land but produces much stronger storm structures over ocean. Meanwhile, a weaker vertical mixing in PBL, however, is benefit to hurricane evolution over ocean but prohibits hurricanes decay over land. Overall, the impact of vertical mixing in PBL is more important for hurricane evolution over land, with the extensively consideration of surface heat and moisture fluxes in PBL parameterizations.