Mesoscale Atmospheric Processes Seminar: Da-Lin Zhang

University of Maryland, College Park

Previous studies have focused mostly on the roles of environmental factors in the rapid intensification (RI) of tropical cyclones (TCs) due to the lack of high-resolution data. In this study, the RI of TCs is examined by analyzing the relationship between an upper-level warm core, convective bursts (CBs), sea surface temperatures (SSTs) and surface pressure falls from 72-h cloud-permitting predictions of Hurricane Wilma (2005) with the finest grid size of 1 km. Results show that an upper-level warm core forms in the same layer as the upper outflow, in coincidence with the onset of RI. The warm core results from the subsidence of stratospheric air associated with the detrainment of CBs. The upper divergent outflow appears to play an important role in protecting the warm core from ventilation by environmental flows. Results also show the development of considerable CBs preceding RI, but most subsidence warming radiates away by gravity waves and storm-relative flows. In contrast, much 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. Sensitivity simulations reveal that the upper-level warm core and CB activity depend critically on warm SSTs. We conclude 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 to the RI of TCs, given all the other favorable environmental conditions.