The Impact of Changing the RRTMG Cloud Overlap Method on Tropical Cyclone Evolution in HWRF

The representation of sub-grid scale cloud properties, including their vertical correlation or cloud overlap, in dynamical models remains a significant source of uncertainty in weather forecasts that also effects the simulation of tropical cyclones. This project is examining the effect of replacing the default maximum-random cloud overlap assumption in RRTMG with an exponential-random method. The latter approach relaxes the strict application of maximum overlap over continuous layers of fractional cloud by defining an exponential transition from maximum to random overlap as a function of distance through adjacent cloudy layers. The impact on local fluxes and heating rates can be substantial, and these radiative effects can in turn influence the evolution of tropical cyclones as well as their surrounding environment. HWRF forecasts of several recent hurricane cases have been studied to illustrate the effect of the cloud overlap change. Longwave and shortwave radiative heating rates are the primary means by which the radiation code influences the atmosphere, and differences in these fields show that the overlap change significantly alters the radiative forcing. The impacts on the atmospheric state in the vicinity of each tropical cyclone and on the forecast track and intensity over multiple forecast cycles will also be presented.