A Study of Tornadic Thunderstorm Interactions with Thermal Boundaries

Abstract

It has been frequently observed that thunderstorms which interact with a warm front, or an old thunder-storm outflow boundary, are likely to increase in severity and become tornadic. The physical mechanisms responsible for this observed characteristic of severe storm evolution are not well understood. A physical model of subcloud wind profiles near thermal boundaries has been developed and a number of cases have been analyzed. Within a hot, moist and conditionally unstable air mass, warm thermal advection and surface friction cause the winds to veer and increase with height. Whereas within a cool, Moist air mass (such as a thunderstorm outflow region) cool thermal advection and friction combine to produce a wind profile that has maximum speeds near the surface and veers little with height. The spatial distribution of differing vertical wind profiles and moisture contents within the boundary layer may act in concert to maximize mesoscale moisture contents, convergence and cyclonic vorticity within a narrow mixing zone along the thermal boundary. These characteristics may explain, in part, why storms often reach maximum intensity within the environment attending thermal boundaries.