The Effects Of Convective Cells On Hurricane’s Intensity In Highly-sheared Wind Environments

The concept of helicity has proven useful for the prediction of supercells in middle and high latitudes, which it is rarely used in the deep convection of tropical cyclones (TC) currently. The distribution and magnitudes of storm relative environmental helicity (SREH) and convective available potential energy (CAPE) under an intense vertical-wind-sheared (VWS) environment are examined using a MM5simulation of Hurricane Bonnie (1998) with a high resolution. The model-simulated fields are compared to those observed by Molinari and Vollaro (2008), showing that the distribution and magnitudes of the SREH and CAPE are contract with ambient VWS, especially in the downshear part of Bonnie. When Bonnie is in the highly-sheared environment, its intensity is still increasing. Compared to observation results, model results show the SREH, CAPE and wind more continuous in space and the relationship with SREH and ambient VWS. We also analyzed the time series of the intensity of Bonnie and ambient VWS over time, and select three typical times to discuss the distribution of SREH and CAPE. When the ambient VWS is greater, the asymmetry of the horizontal structure is more obvious.Then, the3D distribution of convective bursts (CBs) in relation to the CAPE,SREH, and intense ambient VWS and the generation of vertical relative vorticity in spiral rainbands are studied. Results show the generation of intense cyclonic vorticity in CBs in the downshear-left quadrant, accounting for the maintenance of rapid rotation of the hurricane vortex. Vorticity budget calculations indicate that intense cyclonic (anticyclonic) vorticity tends to be generated inside (outside) the updraft core of the eyewall by tilting the horizontal vorticity associated with the intense VWS, whereas the opposite is true for vortex stretching. The net result indicates the dominant generation of cyclonic vorticity by stretching of the hurricane vortex in the planetary boundary layer due to the presence of strong convergence. Further vorticity budget calculations reveal the important role of tilting by deep convection in the outer portion of spiral rainbands in generating vortices at their early stages and the subsequent stretching of the vortices as they propagate cyclonically into the inner core region. We conclude that while intense VWS tends to suppress hurricane development, the presence of large CAPE would allow deep convection to tilt the VWS-associated horizontal vorticity to vertical cyclonic vorticity that is then stretched and cyclonically inward-advected, leading to the rapid rotation of the hurricane eyewall.