On the rapid intensification of Hurricane Wilma (2005). Part IV: Inner-core dynamics during the steady radius of maximum wind stage

Abstract

Recent studies show that some hurricanes may undergo rapid intensification (RI) without contracting the radius of maximum wind (RMW). A cloud-resolving mesoscale model prediction of Hurricane Wilma (2005) is used herein to examine what controls the RMW contraction and how a hurricane could undergo RI without contraction. Results show that the processes controlling the RMW contraction are different within and above the planetary boundary layer (PBL). In the PBL, radial inflow contributes to contraction, whereas frictional dissipation acts as an inhibiting factor. Above the PBL, radial outflow and vertical motion are the two main factors governing the RMW contraction, with the former inhibiting it. A budget analysis of absolute angular momentum (AAM) shows that the radial AAM flux convergence is the major process accounting for the spin-up of the maximum rotation in the PBL as the RMW contracts, while the vertical flux divergence of AAM and the friction oppose the spin-up. During the RI stage with no RMW contraction, the local AAM tendencies in the eyewall are however smaller in magnitude and narrower in width than those during the contracting RI stage. In addition, the AAM following the time-dependent RMW decreases with time in the PBL and remains nearly constant aloft during the contracting stage, whereas it increases during the non-contracting stage. These results reveal different constraints for the RMW contraction and RI, and help explain why a hurricane vortex can still intensify after the RMW ceases contraction.