Application of a Hurricane Boundary Layer Model for Improved Surface Wind Forecast

Wednesday 5 May 2021 - 14:00 to 15:00
Virtual / VC
Mansur Ali Jisan

Hurricane damaging winds are one of the many reasons behind the catastrophe caused by hurricanes during landfall. In most cases damaging wind occurs within the first few hours of landfall when the land roughness changes abruptly from the ocean towards the land.This study investigates the impact of land-roughness on the hurricane’s near-surface wind structure during its landfall using a physics-based Hurricane Boundary Layer (HBL) model. The HBL model utilizes the physical balances in the dynamic equations to determine how the near-surface winds respond to local variability in the surface conditions during hurricane landfall. At the upper boundary, the spatial distribution of the gradient wind is prescribed and the pressure gradient force, derived from the gradient balance equation, is assumed vertically uniform. The horizontal and vertical resolution of the model is 1 km and 30 meters, respectively. Surface roughness in the model is specified based on a 0.5-kilometer resolution Moderate Resolution Imaging Spectroradiometer (MODIS) global land cover climatology. A hurricane vortex specification procedure is developed using the National Hurricane Center’s (NHC) forecast data to realistically represent the storm structure at the top of the boundary layer. We used two radial wind profile parametrizations of the tangential wind: Modified Rankine (MR) and an exponential decay-based wind profile functions. Finally, a new tangential wind profile parametrization was applied by combining the two wind decay formulations.

The HBL model was applied for two recent hurricanes that made landfall on the United States coast: Hurricane Irma (2017) and Hurricane Florence (2018). In simulating the wind during landfall, the wind structure was compared with and without the influence of land-roughness. These comparisons reveal significant changes in wind speed and spatial distribution due to land-roughness impact.The model simulated results were validated in terms of storm structure and time-series distribution of wind speed. The storm structure was validated against the NHC forecast products of Radius of Maximum Wind, Radius of the 18 m/s wind, and 26 m/s wind. Timeseries comparisons and the radial distribution of the wind field were validated against available observations from the aircraft Stepped Frequency Microwave Radiometer (SFMR), National Data Buoy Center’s buoys, and land-based stations. The calculated Root Mean Squared Error (RMSE) is within 2-5 m/s, suggesting reasonable agreements with observations. Radial wind structure sensitivity analysis suggests a reasonable agreement with SFMR based observations for an MR-based tangential parametrization combined with an exponential decay function.Findings from this research will help improve our understanding and prediction of hurricane wind speed and structure during landfall. It will also help coastal communities and emergency managers to better prepare for the risks presented by future hurricane landfalls.

Seminar category: