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

Abstract

urricane Irene (1999) rapidly intensified from 65 to 95 kt (~33.4 to 48.9 m s?1) in 18 h. During the rapid intensification (RI) period, the northeastward storm motion increased from 10 to 18 m s?1, the ambient southwesterly vertical wind shear increased from 6?7 to 10?13 m s ?1, and the downshear tilt of the inner core vortex increased. The azimuthal wavenumber-1 asymmetric convection that developed was consistent with a superposition of shear-induced and storm motion?induced forcing for vertical motion downshear and ahead of the center. Although the diabatic heating remained strongly asymmetric, it was of sufficient intensity to dramatically increase the azimuthally averaged heating. 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 asymmetric forcing from the large vertical wind shear and rapid storm motion were responsible for RI. An unanswered question is what determines whether the heating will develop within the radius of maximum winds. Extraordinarily deep cells developed in the inner core toward the end of the RI period. Rather than causing RI, these cells appeared to be an outcome of the same processes noted above.