Modeling and observations of the atmospheric response to squall lines

Abstract

In order to study the atm0spheric response to a moving squall line a two-dimensional version of the semi-geostrophic theory on isentropic coordinates is used. This model reduces to a. system of two equations which includes a predictive equation for the potential pseudo-density u* (or inverse potential vorticity), and an invertibility principle which can be solved for the Bernoulli function M*. Wind and mass fields can be diagnosed from the solution for M*. The invertibility principle must be solved by an iterative technique; in this study both a simple Gauss-Seidel (GS) relaxation and a Full Multigrid (FMG) method are used and the results using each method are compared. Both methods converge to identical solutions for M* Model results show that the squall line leaves in its wake a positive potential vorticity anomaly in the lower troposphere and a negative potential vorticity anomaly in the upper troposphere. Associated with these potential vorticity anomalies are geostrophic flows which have cyclonic shear in the lower troposphere and anticyclonic shear in the upper troposphere. The stability profile is also modified due to the squall line passage such that the lower troposphere is stabilized while the mid to upper troposphere is destabilized. An observational study is also presented to provide verification for the model results . Supplemental sounding data from the 1985 OK PRE-STORM field study were composited over three times centered around 27 June 1985 OOOOZ at which time the squall line had reached its maximum intensity. Cross sect ions of potential vorticity and the component of the geostrophic flow along the squall line were created at three locations. These show that the observed squall line has left in its wake a reg ion of relatively high potential vorticity) in the lower troposphere and a region or relatively low potential vorticity in the upper troposphere. Cross sections of the component of the observed wind along the squall line are also studied to investigate whether any agreement exists between the observed and geostrophic flows. These flows do show some agreement and indicate cyclonic shear in the lower troposphere and anticyclonic shear in the upper troposphere. The two-dimensional model incorporates very simple physics , specifically it does not capture the complicated moist physics associated with a squall line. However, comparisons of the model and observational results show that the model does predict many of the features associated with the observed squall line.