The influence of preexisting boundaries on supercell evolution

A three-dimensional nonhydrostatic cloud model is used to study the evolution of supercell thunderstorms, with emphasis on the low-level mesocyclone, interacting with preexisting boundaries. The impacts of low-level environmental sheer, storm motion relative to boundary orientation, and boundary strength are assessed. In the low-level shear experiments, significant low-level rotation is consistently observed earlier, tends to be stronger, and is longer lived in storms interacting with a boundary than in storms initiated in a homogeneous environment. Low-level rotation is weaker in storms crossing the boundary and moving into the colder ail: In contrast, all storms moving along or into the warm air ahead of the boundary develop significant low-level rotation. Increasing the temperature gradient and shear across the boundary has little impact on the low-level mesocyclone evolution. Storms interacting with a boundary characterized by only horizontal shear produce weaker mesocyclones than those created when a temperature gradient also exists across the boundary. It will be shown that the mechanisms generating the low-level mesocyclone appear to be different for storms interacting with boundaries than those initiated in a homogeneous environment. Consistent with previous studies, storms initiated in a homogeneous environment derive their low-level rotation from tilting of streamwise horizontal vorticity generated along the storm's forward flank region. In contrast, for storms interacting with a boundary, a significant fraction of the air composing the low-level mesocyclone originates at low levels from the cool air side of the boundary. These parcels contain significant streamwise vorticity, which is tilted and stretched by the storms updraft. Vertical vorticity along the preexisting boundary may also have contributed to mesocyclogenesis. The forward-flank region appears to play a minor role in generating low-level rotation when a preexisting boundary is present.