Potential Vorticity Anomalies Associated with Squall Lines

Abstract

This study involves observations and model simulations of potential vorticity anomalies in the wake of midlatitude squall lines. Using data from the Oklahoma?Kansas PRE-STORM experiment, we analyze potential vorticity fields near two squall lines?one with and one without a trailing stratiform region. From this observational analysis we suggest that squall lines with trailing stratiform regions can leave large, positive, midtropospheric potential vorticity anomalies in their wake. To further interpret these observations we consider a two-dimensional version of semigeostrophic theory formulated in isentropic and geostrophic coordinates, which results in a simple potential pseudodensity (inverse potential vorticity) equation. Using apparent heat source fields that model those computed diagnostically from PRE-STORM data, we find that theory does indeed predict large, midtropospheric potential vorticity anomalies for model squall lines with a trailing stratiform region but not for model squall lines that lack this feature. An interpretation of this result comes directly from the potential vorticity equation, which states that the material derivative of the potential vorticity depends on the derivative, along the vorticity vector, of the apparent heat source. In a squall line with a trailing stratiform region, large values of this derivative are found in the midtroposphere, above the lower-tropospheric evaporative cooling and below the upper-tropospheric stratiform condensational heating. This large derivative of the heating, coupled with the longer influence time associated with the width of the stratiform region, allows the potential vorticity signature of the stratiform region to dominate over the signature of the convective line. Thus, the midtropospheric mesoscale vortices often generated in the wake of squall lines are due in large part to the unique apparent heat source/sink pattern associated with the trailing stratiform region.