Frictionally Induced Feedback in a Reduced Dynamical Model of Tropical Cyclone Intensification

Abstract

This study examines the role of frictional feedback in the atmospheric boundary layer during tropical cyclone (TC) development. Using a reduced model of TC dynamics, it is shown that a feedback between frictional convergence and convective heating in the absence of slantwise moist neutrality is capable of producing a stable maximum-intensity limit, even without surface fluxes. However, the efficiency of this frictional-convergence feedback depends crucially on how effectively boundary layer moisture convergence is converted into convective heating, which decreases rapidly as the TC inner core approaches a state of moist neutrality. This decreasing efficiency during TC intensification explains why the effect of the frictional-convergence feedback is generally small compared to that of the wind-induced surface heat exchange (WISHE) feedback under the strict conditions of slantwise moist neutrality. Examination of the reduced TC model with a constant-heating source reveals that TC intensification is not peculiar to any specific feedback mechanism but, rather, is a direct consequence of the inward advection of absolute angular momentum, regardless of feedback mechanism.