Longwave Radiative Effects beyond the Initial Intensification Phase of Tropical Cyclones
Yi Dai, Margaret Torn, Ian N. Williams, William D. Collins
Abstract
Abstract The effects of longwave radiation on tropical cyclone intensification, with an emphasis on the mature stage, are explored in an idealized modeling framework. Results show that although the cloud-radiative effect aids in early intensification of the vortex, it does not promote increase in the maximum tangential wind (Vmax) and could even reduce Vmax at the mature stage. At later stages, maximum radiative heating is located outside the eyewall and promotes convection there, and the secondary circulation encourages convergence of absolute angular momentum outside the eyewall instead of near the eyewall region, based on a budget analysis. Clear-sky radiative cooling helps invigorate domainwide convection, also limiting the Vmax increase at later stages. The area-averaged frozen moist static energy (FMSE) variance increases even though Vmax decreases. In this sense, the FMSE variance is similar to the monotonically growing integrated kinetic energy, and is more indicative of the system-scale strength than of Vmax. Sensitivity experiments are performed with random initial perturbations and varied initial soundings. An axisymmetric model with a 10-member ensemble not only confirms the results from three-dimensional simulations, but also demonstrates that the weak radiative heating outside the eyewall is indeed able to slow down Vmax within 1 day.