An Observational Study of Tropical Cyclone Spin-Up in Supertyphoon Jangmi (2008) and Hurricane Georges (1998)

An observational study of tropical cyclone spin-up is performed using dropsondes and satellite imagery from Supertyphoon Jangmi and Hurricane Georges. A gradient wind analysis is conducted also for both storms and ELDORA data are analyzed in Tropical Storm Jangmi. The dropsonde analysis shows that the peak tangential wind occurs persistently within the boundary layer and strongly suggests that significant supergradient winds are present there. An examination of ELDORA data in Tropical Storm Jangmi indicates that multiple rotating updrafts are present near the eye underneath extremely cold cloud tops of ≤ -65°C. In particular, there is a 12 km-wide, upright, updraft of 9 m s^-1 with co-located strong low-level (z < 2 km) convergence of 2 x 10^-3 s^-1 and intense relative vorticity values 4 x 10^-3 s^-1. From these results, it is surmised that extremely cold cloud tops in a rotating storm are an indicator of rotating deep convection below. Analysis of the infrared satellite imagery of 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 inner-core spin-up occurs within the boundary layer and rotating deep convective cells are predominant during tropical cyclone intensification.