The Role of WISHE in the Rapid Intensification of Tropical Cyclones

Rapid intensification (RI) is one of the important processes in Tropical Cyclone (TC) intensity evolution. This study examines the role of surface heat fluxes, particularly in relation to the wind-induced surface heat exchange (WISHE) mechanism, in the RI of TCs. The Advanced Research Weather Research and Forecasting model is used in this study to conduct simulations associated with Typhoon Megi (2010), including the control experiment and several sensitivity experiments. Sensitivity experiments with capped surface fluxes reducing the WISHE feedback exhibit delayed RI and weaker peak intensity. Suppressing the WISHE mechanism alters the thermodynamic environment and convective-scale processes. Reduction in convective activity are identified, resulting in a weaker secondary circulation and reduced diabatic heating. Potential temperature budget analyses demonstrate that the vertical extent of the warm core becomes shallower, as a result of weaker downward motion in the eye. Within the inner core region, shallower inflow reduces the transport of angular momentum from outer radii, leading to slower spin-up of the tangential circulation. In addition, the axisymmetrization of convection within the TC inner core is delayed in experiments with stronger suppression of surface heat fluxes, partly explaining the delayed onset of RI. In other words, the vortices can intensify to a certain strength quickly with WISHE feedback. When the vortex becomes enough strong and axisymmetric (e.g., establishment of convection ring), it can significantly develop.

Further ensemble simulations with perturbed initial vortex show similar results when WISHE is suppressed at different thresholds, while the size and convection of the perturbed vortices in the early stage would also affect the onset time of RI. A vortex with more axisymmetric vertical velocity and larger wind field is found to intensify early. In all, this study highlights the crucial role of the WISHE feedback in RI.