A Statistical Analysis of Steady Eyewall Sizes Associated with Rapidly Intensifying Hurricanes

Despite considerable progress in tropical cyclone (TC) research, our ability to predict TC intensity change is still limited. It is even more challenging to predict the timing and location of rapid intensification (RI) of TCs, because of lacking understanding of the dynamical mechanisms by which RI occurs. In recent years, novel observational techniques accompanied by high-resolution numerical simulations have allowed the inner-core structures of rapidly intensifying TCs to be studied in great detail. Some recent modeling studies have shown rapid contraction of the radius of maximum wind (RMW)/eyewall prior to the onset of rapid intensification (RI), followed by a steady state in the RMW (S-RMW) during RI of hurricanes. A statistical analysis of rapidly intensifying hurricanes without RMW contraction is performed to examine how frequently, and at what intensity and radius S-RMWs occur using the Extended Best-Track dataset during the 25 years of 1990-2014. Results show that about 69% (53%) of 139 (164) 24-h (12-h) RI events associated with 55 hurricanes exhibit S-RMWs. More importantly, S-RMWs tend to appear more frequently in more intense storms or when the RMWs of intensifying hurricanes become small. Absolute angular momentum (AAM) budgets, using a 72-h high-resolution prediction of Hurricane Wilma (2005), are calculated, following the evolution of the RMW, to examine how Wilma's RI could take place after its RMW becomes a steady state. Results indicate that both radial and vertical AAM flux divergence increase rapidly during Wilma's RI stage, but with the net local tendency remaining positive at the same order of magnitude prior to and during the RI stage. Of importance is that both the rapid RMW contraction and the locally positive AAM tendency resulting from the surplus of radial over vertical AAM flux divergence contribute equally to Wilma's early RI development, but only the latter plays a role in determining its RI when its RMW becomes a steady state.