Driving Forces of Extreme Updrafts Associated with Convective Bursts in the Eyewall of a Simulated Tropical Cyclone

Many studies have indicated that convective bursts (CBs) are closely related to TC intensification, but few studies have been conducted on the mechanisms that control the formation and evolution of CBs. In this study, the 1-min output data of a simulated TC are used to understand the convective extreme updrafts of CBs in the TC eyewall. Three different reference states including the local square-area mean (LAM), the lower wavenumber-components mean (WNM), and the arc-area mean (ArcM) are used to calculate driving forces of convective extreme updrafts. In contrast to the WNM and ArcM reference states, the LAM reference state struggles to capture realistic basic state structures. The LAM reference state can be modified through the inclusion of the mean hydrometeor mixing ratio in the basic state to produce physically consistent forcing in relation to the simulated convective extreme updrafts. The simulated convective extreme updrafts in the TC eyewall exhibit two peaks at middle and upper levels, respectively, since the effect of hydrometeor loading decelerates the air parcels between the updraft maxima. While the positive buoyancy makes air parcels in the CBs accelerate at middle levels, in agreement with previous studies, it is found that, at the upper levels, both the positive buoyancy and the upward vertical perturbation pressure gradient force accelerate the air parcels. This study suggests that the vertical perturbation pressure gradient force also plays an important role in the formation of CBs in the TC eyewall.