The Relationship between Storm Motion, Vertical Wind Shear, and Convective Asymmetries in Tropical Cyclones

Abstract

The influence of the direction of storm motion on the azimuthal distribution of electrified convection in 35 Atlantic basin tropical cyclones from 1985 to 1999 was examined using data from the National Lightning Detection Network. In the inner 100 km, flashes most often occurred in the front half of storms, with a preference for the right-front quadrant. In the outer rainbands (r = 100?300 km), flashes occurred predominantly to the right of motion, although the maximum remained in the right-front quadrant. The results are shown to be consistent with previous studies of asymmetries in rainfall, radar reflectivity, and vertical motion with respect to tropical cyclone motion. The motion effect has been attributed to the influence of asymmetric friction in the tropical cyclone boundary layer. The authors previously found a strong signature in the azimuthal distribution of lightning with respect to vertical wind shear. Because both effects show clearly, vertical wind shear and storm motion must themselves be systematically related. It was found that more than three-quarters of 12-hourly periods contained a storm motion vector that was left of (i.e., counterclockwise from) the shear vector. These results support the importance of a downshear shift in the upper anticyclone, which produces motion left of shear for all directions of shear. The results are further broken down by direction of shear, and it is shown that the beta effect also plays a significant role in the relationship between motion and vertical wind shear. These results also suggest that substantial downshear tilt of the cyclonic part of the tropical cyclone vortex is uncommon, because that alone produces motion right of shear. The relative importance of asymmetric friction and vertical wind shear on the azimuthal asymmetry of convection was determined by examining circumstances in which the two effects would place maximum lightning in different quadrants. Without exception, the influence of vertical wind shear dominated the distribution. Although asymmetric friction creates vertical motion asymmetries at the top of the boundary layer, these apparently do not produce deep convection if vertical wind shear?induced circulations oppose them.