A numerical modelling study of the physical mechanisms causing radiation to accelerate tropical cyclogenesisand cause diurnal cycles

Abstract. Numerical modeling studies indicate that radiative forcing can significantly accelerate tropical cyclogenesis. The primary mechanism appears to be nocturnal differential radiative forcing between a developing tropical disturbance and its relatively clear-sky surroundings. This generates weak ascent in the system core, which promotes enhanced convective activity. The goal of this study is to examine this hypothesis in more detail and in doing so shed light on the particular physical mechanisms that are responsible for the accelerated development of the system. In order to clarify the effects of radiation the radiative forcing occurring in a full physics simulation is imposed as a forcing term on the thermodynamic equation in a simulation without microphysics, surface fluxes or radiation included. This gives insight into the radiatively induced circulations and the resultant changes to the temperature and moisture profiles in the system core that can influence convective development. Simulations with separate environment and core radiative forcings support the hypothesis that differential radiative forcing due to nocturnal longwave cooling in the environment is the main factor responsible for accelerating the rate of tropical cyclogenesis. Simple idealized cloud experiments indicate that both cooling and moistening caused by the induced ascent significantly influence convective development, with the cooling having the largest impact. Diurnal cycles of Convective Available Potential Energy (CAPE), outgoing longwave radiation, deep convection and upper level outflows are examined and their relation to the radiative forcing is discussed.