The Effect of Surface Heat Fluxes in the Outer Region of Typhoon Megi(2016)

Locations of outer rainbands and the embedded convective activities can affect tropical cyclone (TC) size; it has been suggested that outer rainbands which are more vigorous or located farther away from the eyewall tend to favor the enlargement of TC size. Furthermore, the removal of the surface heat fluxes in the whole outer core region suppresses outer rainband activities, resulting in the reduction of TC size. This study attempts to further understand the impact of surface heat fluxes in one certain radial interval in the outer core region on TC size and on the outer rainband activities.

The Advanced Research Weather Research and Forecasting (WRF) Model (ARW; version 3.5.1 is used in this study to conduct simulations of Typhoon Megi (2016). Several experiments are carried out, including a control (CTRL) simulation and sensitivity experiments with surface heat fluxes suppressed in varying degrees and in different radial intervals of the vortex. In the sensitivity simulations, the surface wind speed is capped by a number of specified values in the calculation of the surface heat fluxes. The constrained area is a ring structure, ranging from 3 to 4 times of the radius of maximum wind. The TC size in the sensitivity simulations is larger than CTRL, and the radar reflectivity shows less active convection on the inner side of the capped region, yet more active convection in the outer side. Through the stream function analysis, the upward motion in the inner edge of the constrained ring and the inflow around the ring region are weaker while the upward motion in the outer edge of the ring is stronger. Thus, the deceleration in the boundary layer inflow on the outer edge leads to the convergence at lower levels, favoring the development of convection. Equivalent potential temperature below 1 km height in the inner side of the constrained region is lower than that in CTRL, indicating a more moist static stable environment. Different from the effect of removing surface heat fluxes in the whole outer core region, the suppression of surface heat fluxes in the specified ring region leads to an increase in TC size. Sensitivity experiments to different constrained radial intervals in the outer core region and further analyses are still under investigation.