The Effects of Vertical Wind Shear on the Distribution of Convection in Tropical Cyclones

Abstract

The influence of vertical wind shear on the azimuthal distribution of cloud-to-ground lightning in tropical cyclones was examined using flash locations from the National Lightning Detection Network. The study covers 35 Atlantic basin tropical cyclones from 1985-99 while they were over land and within 400 km of the coast over water. A strong correlation was found between the azimuthal distribution of flashes and the direction of the vertical wind shear in the environment. When the magnitude of the vertical shear exceeded 5 m s 1 , more than 90% of flashes occurred downshear in both the storm core (defined as the inner 100 km) and the outer band region (r 100-300 km). A slight preference for downshear left occurred in the storm core, and a strong preference for downshear right in the outer rainbands. The results were valid both over land and water, and for depression, storm, and hurricane stages. It is argued that in convectively active tropical cyclones, deep divergent circulations oppose the vertical wind shear and act to minimize the tilt. This allows the convection maximum to remain downshear rather than rotating with time.