Surface-based modification of convectively-generated mesovortices and its implications for tropical cyclogenesis

Numerical simulations using the Penn State/NCAR mesoscale model MM5 have been performed on a long-lived midlevel mesovortex over the continental United States that was instrumental in initiating and organizing multiple cycles of deep convection. These simulations showed how the cycles of convective redevelopment amplify the mid- to upper-level warm core of the vortex, causing the cyclonic vortcity to strengthen in the midlevels and penetrate down into the lower troposphere, briefly reaching the surface. However, low-level cooling caused by convective and mesoscale downdrafts and adiabatic ascent offsets the impact of the mid- to upper-level warming, limiting the extent to which the cyclonic vorticity penetrates to the surface.

In this study the impact of weakening the surface-based cold pool on the structure of the simulated mesovortex is investigated. Methods for weakening the cold pool in the simulations include: eliminating the convective (subgrid-scale) and mesoscale (resolvable-scale) downdrafts and replacing the lower land boundary with a water surface of varying temperatures. The resultant structures of the mesovortex for these different conditions are compared to explain how a mesovortex, with cyclonic vorticity initially confined to the midlevels, can grow downward to the surface, marking a key step in the tropical cyclogenesis process.