Potential Role Of Irreversible Moist Processes In Modulating Tropical Cyclone Surface Wind Structure

Tropical cyclone (TC) wind structure is important for its intensity change and induced damage, but its modulating factors remain to be explored. A heat-engine-based surface wind structure parameter alpha, reflecting TC's relative compactness, is introduced and derived based on an entropy budget framework. We found that alpha is modulated by three key parameters: the thermodynamic efficiency E-PI in potential intensity theory, the Carnot efficiency E-C of the system, and the degree of irreversibility alpha (irr) of the system. A higher alpha (irr) contributes to a larger alpha and a lower heat engine efficiency. An expression linking TC intensity and compactness also emerges under this framework. Idealized simulations of a typical moist TC (CTL), a dry (DRY) TC, and a moist reversible TC (REV; in which hydrometeors do not fall out) evinced that the significantly higher alpha (irr) in CTL, due to irreversible entropy productions from precipitation dissipation, water vapor diffusion, and irreversible phase changes, contributes to its much larger compactness compared to DRY and REV. The study illustrates the importance of irreversible entropy production processes in modulating TC surface wind field. Simple estimate suggests that alpha will increase due to a hypothesized increased alpha (irr) with warming because of increased water content. This indicates that TCs will become more compact in a warmer climate.