On the Trigger of Rapid Intensification of Hurricane Wilma (2005)

The trigger of rapid intensification (RI) of Hurricane Wilma (2005) with a record-breaking deepening rate of more than 4 hPa hr-1 in the minimum sea-level pressure (PMIN) is examined using a 72-h nested-grid, cloud permitting prediction with the Weather Research and Forecast model at the finest grid size of 1 km. Results show the formation of an upper-level warm core, coinciding with the onset of RI, from the descending air of stratospheric origin in the eye, which reaches the peak amplitude of more than 18°C at the time of peak intensity. This peak warming occurs at the same layer as the upper-level outflow. We hypothesize that the upper divergent outflow layer favors the generation of an upper-level warm core by protecting it from ventilation by environmental flows. Use of the hydrostatic equation reveals that the upper-level warming, i.e., above 11 km, can account for a 64-hPa fall in PMIN at the peak intensity, which is more than twice as large as the lower-level warming. Time series of the vertical warming profile and PMIN indicates that it is the development of intense upper-level warming in the eye that triggers the RI of Wilma. A further analysis reveals that the upper-level warming is caused by vigorous convective bursts with remarkably strong vertical motions (e.g., peaked at 48 m s-1 at 14-km altitude), occurring mostly in the vicinity of the radius of maximum wind where enthalpy is peaked. These convective bursts can penetrate into the lower stratosphere, and their associated mass detrainment induces cyclonic radial inflows above the upper outflow layer that are responsible for the subsidence warming below. Sensitivity simulations indicate the intensity and number of convective bursts depend critically on the magnitude of sea-surface temperature (SST). Namely, colder SST results in fewer and weaker convective bursts, weaker upper-level warm core and weaker surface pressure falls. In the presentation, we will demonstrate that the upper level warming is more effective in reducing surface pressure. Based on the results, we recommend that more attention be paid to detecting the upper level temperature changes and storm-relative flows in the eye, rather than just the traditional 850-200 hPa layer vertical wind shear, in order to more accurately predict the RI of tropical cyclones.