Influences of Vertical Mixing in PBL on the Evolution of Landfalling Hurricanes

Vertical mixing in the planetary boundary layer (PBL) plays an important role in hurricane evolution. Most previous investigations on this subject have emphasized hurricane evolution over the ocean only, instead of over land. In this study, the effects of vertical mixing on hurricane simulations over land are examined using the Hurricane Weather Research and Forecasting (HWRF) model with several landfalling hurricane cases.

           First, the effect of vertical mixing strength on the evolution of landfalling hurricanes is examined.  It is found that strong vertical mixing in the PBL has a great positive impact on numerical simulations of hurricane over land with respect to track, intensity and quantitative precipitation forecasts (QPFs). The weak vertical mixing, however, leads to better hurricane simulations over the ocean but too strong hurricane over land. Results suggest that different strengths of vertical mixing should be used over ocean and land respectively in numerical hurricane models to better portray the interactions between the hurricane boundary layer and the ocean/land surface.

Second, the parameterization scheme of eddy diffusivity for both momentum (K_m) and thermal (K_h) is modified in the HWRF model with extensive consideration of the vertical transport of surface heat and moisture fluxes. Results show that numerical simulations of landfalling hurricanes are very sensitive to changes in the K_m parameter. Compared with the hurricane simulations with the original K_m parameter, those with the modified K_m parameter lead to improved simulations of hurricane track, intensity, and quantitative precipitation against observations during and after landfall. Further diagnosis shows that the modified K_m parameter enhances the response of the hurricane boundary layer to the land surface. It makes the interactions between landfalling hurricanes, especially their inner core and secondary circulation, and the land surface more efficient, thus leading to the reasonable evolution of hurricane vortices over land. 

The implication of overall results from this study for future model improvements is discussed. Details will be presented.