Title: The "optimal state" for cold pools in shear and its relevance to mesoscale convective systems

Abstract:

Deep precipitating convection often produces layers of relatively cool air near the surface by evaporation and melting of hydrometeors. These "cold pools" play a role in the upscale development of many mesoscale organized convective systems. This study examines the dynamics of cold pools when they spread into environments with vertical wind shear, with most of the presentation focusing on the theoretical "optimal state" in which the sheared environmental flow is turned into a vertical oriented jet. Steady analytic solutions are presented from two perspectives: 1) the vorticity perspective, which was first presented in an article by Rotunno, Klemp, and Weisman; and 2) the flow-force-balance perspective, which is based on conservation of mass and momentum. The latter perspective reveals that the shear-layer depth must be three-quarters of the cold pool depth for the oncoming flow to turn exactly into the vertical. Next, two- and three-dimensional numerical simulations of cold pools in shear are examined, and it is shown that a statistically steady flow can be maintained with all the important elements of the theoretical solution. Most notably, the front-relative flow is negligible at all levels behind the surface gust front, and the net generation of vorticity by buoyancy within a control volume closely matches the horizontal flux of environmental vorticity on the right side of the control volume. The relevance of these results to mesoscale convective systems is then made by comparison with recent observational and numerical studies of these more complex systems.


Speaker's website: http://www.mmm.ucar.edu/people/bryan/
Host: Marty Singh