ON THE RAPID INTENSIFICATION OF HURRICANE WILMA (2005)

Abstract

Previous studies have focused mostly on the roles of environmental factors in the rapid intensification (RI) of tropical cyclones (TCs) due to the lack of high-resolution data in the inner-core regions. In this study, we examine the RI issue by analyzing 72-h cloud-permitting model predictions of Hurricane Wilma (2005) with the Weather and Research Forecast (WRF) model at the finest grid sizes of 1-2 km. The 72-h predictions cover Hurricane Wilma¡¦s initial 18-h spin up, an 18-h RI and the subsequent 36-h weakening stage. The model prediction uses the initial and lateral boundary conditions, including a bogus vortex, that are identical to the Geophysical Fluid Dynamics Laboratory's then-operational data, except for the time-independent sea surface temperature (SST) field. The model predicts an RI rate of more than 4 hPa h-1 for an 18-h period, with the minimum central pressure of less than 889 hPa. It was found that an upper-level warm core forms in the same layer as the upper outflow, in coincidence with the onset of RI. The warm core results from the subsidence of stratospheric air associated with the detrainment of convective bursts (CBs). The upper divergent outflow appears to play an important role in protecting the warm core from ventilation by environmental flows. Results also show the development of more CBs preceding RI, but most subsidence warming radiates away by internal gravity waves and storm-relative flows. In contrast, many fewer CBs occur during RI, but more subsidence warming contributes to the balanced upper-level cyclonic circulation in the warm core (as intense as 20,,aC) region. Furthermore, considerable CB activity can still take place in the outer eyewall as the storm weakens during its eyewall replacement. Sensitivity simulations reveal that the upper-level warm core and CB activity depend critically on warm SST. We conclude that significant CB activity in the inner-core regions is an important ingredient in generating an upper-level warm core that is hydrostatically more efficient to the RI of TCs, given all the other favorable environmental conditions.

The formation of a divergent upper-level outflow that prevents the warm core from ventilation is examined through asymmetric contraction processes associated with new rainbands forming inside the eyewall. The relative vorticity, generated in the downshear region and then advected cyclonically downstream, can induce convergence in the boundary layer. With the aid of high moisture content, the convergence can trigger deep convection and contribute to the formation of the new rainbands. Finally, the importance of a small eye size is demonstrated using three widely accepted approximations: angular momentum conservation, solid body rotation and gradient wind balance. Results show that the storm intensifies much faster for a given contraction speed if the eye size is small.