Abstract

The rapid intensification (RI) of hurricanes is stuied in two different scenarios: the vertically alignted hurricane under weak shear and vertically tilted hurricane under moderate shear. In the first scenario, hurricane Wilam (2005) was chosen because of its record low pressure and small size.  The RI of Wilma is examined by analyzing the relationship between an upper-level warm core, convective bursts (CBs), sea surface temperature (SST), and surface pressure falls from 72-h cloud-permitting predictions with the finest grid size of 1 km. Results show that both the upper-level inertial stability increases and static stability decreases sharply 2–3 h prior to RI, and that the formation of an upper-level warm core, from the subsidence of stratospheric air associated with the detrainment of CBs, coincides with the onset of RI. It is found that the development of CBs precedes RI, but most subsidence warming radiates away by 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°C) region. Furthermore, considerable CB activity can still take place in the outer eyewall as the storm weakens during its eyewall replacement. A sensitivity simulation, in which SSTs are reduced by 1°C, shows pronounced reductions in the upper-level warm-core intensity and CB activity. It is concluded that significant CB activity in the inner-core regions is an important ingredient in generating the upper-level warm core that is hydrostatically more efficient for the RI of TCs, given all of the other favorable environmental conditions. In the second scenario, hurricane Earl (2012) was chosen because of its unexpected RI under moderate wind shear. Examination of the high-resolution forecast data reveals that the tilt was large at the RI onset and decreased quickly once RI commenced, suggesting that vertical alignment is the result instead of the trigger for RI. The RI onset is associated with the development of upper-level warming in the eye, which results from upper-level storm-relative flow advecting the warm air caused by subsidence warming in the upshear-left region toward the low-level storm center. This activity does not occur until persistent convective bursts (CB) are concentrated in the downshear-left quadrant. The temperature budget calculation indicates that horizontal advection plays an important role in the development of upper-level warming in the early RI stage. The upper-level warming associated with the asymmetric intensification process occurs by means of the cooperative interaction of the convective-scale subsidence, resulting from CBs in favored regions and the shear-induced mesoscale subsidence. When CBs are concentrated in the downshear-left and upshear-left quadrants, the subsidence warming is maximized upshear and then advected toward the low-level storm center by the storm-relative flow at the upper level. Subsequently, the surface pressure falls and RI occurs.