Improvement of Global Forecast of Tropical Cyclone Intensity by Spray Heat Flux and Surface Roughness

Abstract

In global forecasting systems, tropical cyclone (TC) intensity is usually underestimated, which is one of the major challenges for TC forecasting. One possible reason is the deficiency of physical parameterizations associated with ocean surface waves. To improve the TC intensity forecast, effects of two powerful wave‐related processes, the spray‐mediated heat flux and surface roughness, are considered in a global coupled ocean‐atmosphere‐wave system (CFSv2.0‐WW3). Serial 24 hr forecasting experiments for all TCs from May to October 2019 are conducted, and comparisons are made against the International Best Track TC data. The results show that for strong TCs the underestimation of TC intensity in CFSv2.0‐WW3 is significantly improved by the modified surface roughness parameterization and the accelerated spray heat flux parameterization based on Gaussian Quadrature. For major hurricanes with initial wind speeds above 30 m/s, the error of TC maximum wind speed decreases by about 34.4%. To understand the associated dynamic and thermodynamic processes, a series of 120 hr simulations for major hurricane Hagibis is conducted. The effect of spray‐mediated heat flux accounts for 91.7% of the TC intensity improvement. The increased heat fluxes offset the negative contributions of vertical advection and ventilation, leading to enhanced entropy. The low‐level pressure is then decreased, resulting in enhanced convergence of absolute vorticity, which overpowers the negative effects of frictional dissipation and vertical momentum advection. The study indicates that the application of these two wave‐related parameterizations can potentially improve the global forecast and reanalysis of TC intensity.