Impact of Surface Roughness Representation on Hurricane Intensity Forecasts

The impact of surface roughness representation on tropical cyclone (TC) intensity forecasts is investigated using the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®) . The momentum, heat and moisture flux across the ocean-atmosphere interface, one of the important physical processes modulating the tropical cyclone (TC) intensity, depends largely on how the surface roughness length is specified. However, uncertainties exist in representing the roughness lengths under high wind conditions in numerical weather prediction (NWP) models, as evident from the wide range of roughness lengths applied in operational and research models. The surface scheme of COAMPS is modified based on new findings from recent observational studies of surface fluxes under high winds, e.g., the Coupled Boundary Layers Air-Sea Transfer (CBLAST) field campaign. The modified roughness length for momentum reflects the observations that the drag coefficient levels off when the surface wind speed reaches 33 m s-1. The ratio of the enthalpy transfer coefficient to the drag coefficient under high wind conditions is raised from lower than 0.5 to higher values depending on the wind speed. High-resolution COAMPS simulations of eighteen TC cases are performed using the original surface scheme and the newly modified scheme. Our preliminary comparisons suggest that the new scheme helps reduce a negative bias in 10-m maximum wind speed forecasts. The inner core structure and precipitation distribution of selected TC cases will be discussed associated with the changes in the surface scheme.

The surface physics schemes of the Weather Research and Forecasting (WRF) model, now readily available in the COAMPS through a fully interchangeable physical parameterization functionality, provide an opportunity to compare and evaluate various roughness length calculations and their impact on TC intensity forecasts. Results will be shown to illustrate the high sensitivity of TC intensity to surface flux calculations and to examine the impact of various roughness length calculations derived from several different parameterizations on TC simulations.