Effects of wind-evaporation feedback in outer regions on tropical cyclone development

This study examined the roles of wind-evaporation feedback in the tropical cyclone (TC) intensification, with special attention devoted to the feedback in weak wind areas (domains where the 10-m wind speed is smaller than 5, 10, and 15 m s⁻¹). This was done by setting lower limits of the 10-m wind speed in the calculation from the underlying ocean in a nonhydrostatic cloud-resolving model. As a result, the surface evaporation is enhanced in outer regions of a TC where the actual wind speed is smaller than the prescribed lower limit(s). Results show that increasing the lower limit reduces the radial water vapor contrast in the lower troposphere (below 100 m) and suppresses the TC size and intensity at the mature stage by 30 – 33 % and 5 – 14 %, respectively, compared to the control run with all standard model settings. The increased evaporation enhances the outer convective activity and reduces the radial pressure gradient in the lower troposphere. As a result, the inflow and the inward advection of angular momentum are reduced and the enhanced convection in the outer region suppresses eyewall updraft and thus reduces the secondary circulation and finally the TC intensity. Moreover, the outer region convection suppresses the rainband activity, within a radius of 300 km from the TC center. The contribution of the wind-evaporation feedback to the enhancement of the radial contrast of water vapor in the lower troposphere is a fundamentally important element for TC intensification, suggesting that the understanding of TC development process can be improved by elucidating the role of the weak wind area.