Distribution of Helicity, CAPE, and Shear in Tropical Cyclones

Abstract

The previous study of helicity, CAPE, and shear in Hurricane Bonnie (1998) was extended to all eight tropical cyclones sampled by NASA during the Convection and Moisture Experiments (CAMEX). Storms were categorized as having large or small ambient vertical wind shear, with 10 m s?1 as the dividing line. In strongly sheared storms, the downshear mean helicity exceeded the upshear mean by a factor of 4. As in the previous study, the helicity differences resulted directly from the tropical cyclone response to ambient shear, with enhanced in-up-out flow and veering of the wind with height present downshear. CAPE in strongly sheared storms was 60% larger downshear. Mean inflow near the surface and the depth of the inflow layer each were 4 times larger downshear. At more than 30% of observation points outside the 100-km radius in the downshear right quadrant, midlatitude empirical parameters indicated a strong likelihood of supercells. No such points existed upshear in highly sheared storms. Much smaller upshear?downshear differences and little likelihood of severe cells occurred in storms with ambient wind shear below 10 m s?1. In addition to these azimuthal asymmetries, highly sheared storms produced 30% larger area-averaged CAPE and double the area-averaged helicity versus relatively unsheared storms. The vortex-scale increase in these quantities lessens the negative impact of large vertical wind shear.