Rapid Intensification of a Sheared, Fast-Moving Hurricane over the Gulf Stream

Hurricane Irene (1999) rapidly intensified from 65 knots to 95 knots in 18 hours. During the rapid intensification (RI) period, the ambient southwesterly vertical wind shear increased from 6-7 m/s to 10-13 m/s and the northeastward storm motion increased from 8 m/s to 16 m/s. The resulting azimuthal wavenumber-1 asymmetric convection was consistent with a superposition of shear-induced and storm motion-induced forcing for vertical motion downshear and ahead of the center. The storm passed over the northwestern edge of the Gulf Stream during this time. The much-larger wind speed and upper oceanic heat content to the right of motion enhanced moist enthalpy fluxes that fed into the convective region. Although the resulting diabatic heating was strongly asymmetric, it was of sufficient intensity to dramatically increase the azimuthally averaged heating during RI. This heating occurred almost entirely inside the radius of maximum winds, a region known to favor rapid warm core development and spinup of the vortex.

It is hypothesized that the vertical shear, storm motion, and the Gulf Stream all contributed to RI. The role of extremely deep convective cells that developed at the end of the RI period will also be discussed. An unanswered question is what determines whether the heating will develop within the radius of maximum winds.