An observational study of tropical cyclone spin-up in Supertyphoon Jangmi and Hurricane Georges

Abstract

An observational study of tropical cyclone spin-up is performed using dropsondes and satellite imagery from Supertyphoon Jangmi and Hurricane Georges. Additionally, ELDORA data are analyzed in Tropical Storm Jangmi. The dropsonde analysis shows that the peak tangential wind occurs persistently within the boundary layer and suggests that significant supergradient winds are present there. However, as a result of data limitations arising from asymmetries and irregular placement of dropsondes in both time and space, a strongly conclusive statement about the presence of supergradient winds in Tropical Storm Jangmi is not possible. A dense deployment of dropsondes across the RMW is recommended to resolve the pressure gradient with sufficient accuracy to validate that supergradient winds exist at the tropical storm stage. An examination of ELDORA data in Tropical Storm Jangmi indicates that multiple rotating updrafts are present near the eye underneath cold cloud tops of -65C. Analysis of the infrared satellite imagery from both storms suggests that rotating updrafts are omnipresent before and during rapid intensification. The findings of this study support a new theoretical model positing that spin-up of the maximum winds occurs within the boundary layer and rotating deep convective cells are predominant during tropical cyclone intensification.