EQECAT's Time-Stepping United States Mainland Tropical Cyclone Wind Field Model

EQECAT's USWIND™ model projects property loss costs from landfalling and by-passing tropical cyclones. One of the major components of the model—the hazard module—uses a detailed high-resolution time-stepping wind field model to simulate the evolution of surface winds as a storm threatens land. The over-water (marine) wind speed at the 10-meter level is modified to account for the effects of terrain roughness. The impact of terrain roughness on surface wind speed is modeled based on high-resolution land use/land cover (LULC) data for 16 cardinal wind directions and local fetch of the surface.

The time-stepping model simulates the evolution of a surface wind field as a tropical cyclone traverses in time over water or land. In effect, the wind speed and direction are modeled for short time-steps during the period when the storm affects land. As the simulation progress over time, the peak wind speed affecting a site will vary with respect to the storm's proximity and the local wind direction's fetch over the terrain. The local effect is captured through the modeling of directionally varying frictional factors. The peak wind affecting a site, as the tropical cyclone impacts the land, is stored for a given storm depending on the local terrain of the site. A short time-step of wind field simulation captures the impact of changing terrain as a tropical cyclone transitions from smooth to rough terrain and vice versa. The changing land roughness is modeled in terms of roughness lengths from 16 LULC classes.

The modeled wind fields are validated using observed tropical cyclone wind speed data obtained from the NHC reports and other official sources. New events are reviewed, validated, and added to the catalog in the hazard model. Comparisons of modeled and observed wind fields of historical events impacting the United States coastline from Texas to Maine are made to validate the EQECAT wind field model. Validations show that the time-stepping wind field model estimates the tropical cyclone wind fields reasonably well with the observed winds.