Potential vorticity anomalies associated with squall lines

Abstract

From observational analysis we suggest that squall lines with trailing stratiform regions can leave large, positive, midtropospheric potential vorticity anomalies in their wake. We find that theory does indeed predict large, midtropospheric potential vorticity anomalies for model squall lines with a trailing stratiform region. The potential vorticity equation 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. -from Authors