Convective Bursts, Downdraft Cooling, and Boundary Layer Recovery in a Tropical Storm

Tropical Storm Edouard (2002) experienced strong vertical wind shear as it moved slowly westward east of Florida. An intense convective outbreak, measured by a dense core of lightning flashes, was centered at the 45 km radius just left of downshear. It lasted only two hours, after which all electrified convection ceased within 75 km of the center. The evolution of the storm was tracked using three flights by Air Force reconnaissance, the first one during the convection and the others after it had ceased. These flights followed similar paths from right to left of the ambient shear vector at elevations of 300-600 m, and thus allowed a direct measure of the evolution of the boundary layer. Although the azimuthally-averaged wind changed little, a strong local wind maximum developed left of shear that reached hurricane force during the convective outbreak. Thereafter, the tangential wind axisymmetrized. The convection stopped in response to broad downdraft cooling downshear and left of shear, shown by very low values of boundary layer temperature, moisture, and equivalent potential temperature. Upshear right values were not altered. Over the following 12 hours, the boundary layer recovered, eventually leading to a new downshear left convective outbreak. Implications of these results will be discussed.