Effects of Vertical Shears and Midlevel Dry Air on Tropical Cyclone Developments

Abstract

set of idealized experiments using the Weather Research and Forecasting model (WRF) were designed to investigate the impacts of a midlevel dry air layer, vertical shear, and their combined effects on tropical cyclone (TC) development. Compared with previous studies that focused on the relative radial position of dry air with no mean flow, it is found that the combined effect of dry air and environmental vertical shear can greatly affect TC development. Moreover, this study indicates the importance of dry air and vertical shear orientations in determining the impact. The background vertical shear causes the tilting of an initially vertically aligned vortex. The shear forces a secondary circulation (FSC) with ascent (descent) in the downshear (upshear) flank. Hence, convection tends to be favored on the downshear side. The FSC reinforced by the convection may overcome the shear-induced drifting and ?restore? the vertical alignment. When dry air is located in the downshear-right quadrant of the initial vortex, the dry advection by cyclonic circulation brings the dry air to the downshear side and suppresses moist convection therein. Such a process disrupts the ?restoring? mechanism associated with the FSC and thus inhibits TC development. The sensitivity experiments show that, for a fixed dry air condition, a marked difference occurs in TC development between an easterly and a westerly shear background.