MESOSCALE CONVECTIVE INFLUENCES ON THE RAPID INTENSIFICATION OF HURRICANE IRENE (1999)

During the evening of 18 Oct 1999, Hurricane Irene tracked northeastward offshore the southeast coast of North Carolina and passed over the Gulfstream. During the period from 0205 and 0756 UTC, the central pressure of Hurricane Irene decreased 15 HPa (973 HPa to 958 HPa) in less than 6 h. The sharp drop in pressure coincided with a nearly doubling of reconnaissance flight level winds and ground-based Doppler radar velocities. The eye diameter also decreased sharply from 14.8 km to 5.6 km.

The rate of pressure change of 2.5 mb/hr easily classifies Irene's short term strengthening as a period of rapid deepening (Holliday and Thompson, 1979) and very nearly explosive deepening (Dunnavan, 1981). It is believed that the cause of such rapid intensification was due to strong vortex stretching in the eyewall by a penetrative burst of of deep convection which infrared satellite imagery and ground-based radar data indicated as cloud tops colder than -91C and echo tops above 19 km, respectively.