A numerical modeling investigation of the impact of coastal roughness and moisture availability on the 10-meter horizontal wind field of a landfalling hurricane

Understanding of the structure and evolution of the low-level wind field of a tropical cyclone (TC) is important in issuing accurate forecast of the surface winds and the TC's damage potential. However, the difficulty in obtaining a reading of the 10-meter wind field of an approaching TC, coupled with the strong variability of the 10-meter wind field in landfalling TCs proves to be a great obstacle for the operational forecaster. This study seeks to simplify this problem by isolating environmental moisture availability and the roughness of a coast and investigating their impact on the evolution of the 10-meter wind field of a controlled modeled hurricane.

Six identical hurricanes were forced to make landfall along a flat, east-west oriented coast, covered in a uniform land-use category. The land-use categories varied in their moisture availability and roughness length. The hurricane for the investigation is initialized centered 400 km south of the east-west coastline. The TC moves northeastward at about 4 ms-1 and does not make landfall until the 15th hour of the simulation. Tropical storm (TS) force winds begin to impact the coastline at the 8th hour, but the weakening of the center of the storm does not start until around the 11th hour.

As expected, the greatest winds in the 10-meter wind field remain offshore where the impact of friction is almost insignificant. In addition, the storms interacting with greater roughness levels of the coastline weaken significantly more rapidly than their counterparts under the same moisture constraints. Moreover, as one would expect, of the storms experiencing similar roughness levels of the coastline but different moisture availabilities, the TCs in more moist environments retain stronger winds in the core of the storm than their counterparts in dryer environments; however, TCs in dryer environments had a broader wind field offshore, particularly on the western side.

This study will present further differences and will related the low-level windfield to convective structures in the TCs. Reasons for the differences in the 10-meter wind fields will be proposed at the conference to help provide a better understanding of the low-level wind fields and eventually aid in the better forecasting of TCs.