Analysis of a Numerically Simulated Squall Line Along A Front

Guangming Zhou, Robert Wilhelmson, and Mohan Ramamurthy
Department of Atmospheric Sciences and National Center for Supercomputing Applications (NCSA) University of Illinois at Urbana-Champaign

A squall line that occurred on June 10-11, 1985, during the Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central (OK PRE-STORM) experiment, has been simulated numerically using the non-hydrostatic COMMAS model (Collaborative Model for Multiscale Atmospheric Simulation) with 4 km horizontal resolution. The model domain covers an area of 780 km by 760 km in horizontal and 20 km in vertical. The three dimensional initial conditions for the cloud model were obtained from a regional model and are thus non-homogeneous. The cloud model was initiated at 22 Z, June 10, 1985 and run for eight hours. A well defined squall line developed during the first two hours of integration. The central portion of the line moved from around 315^o to south-east at the average line speed of 14-16 m s^-1, comparable with observations.

The simulated squall line possesses many features in common with the observed squall line, including a stratiform precipitation region, a bright band in stratiform region, a rear inflow jet, a surface low-high-low pressure pattern, and positive relative vorticity along the front side of the line with negative vorticity behind and positive vorticity further behind at low levels. We found that this vorticity pattern is primarily maintained by the tilting and stretching terms with the tilting term being much more important than stretching term over the convective region due to the gradient of vertical velocity over that region. The vorticity pattern of this numerical simulated squall line agrees qualitatively with the results from observational study by Biggerstaff and Houze (1991). The reasons they give for this pattern will be expanded upon with clarifications based on the model results. Mid-level closed circulation patterns also formed in association with convective cells individually or when clustered together. The size of these circulations at mid-levels ranges from about 30 km to about 90 km.

More detailed results will be presented.

Corresponding author addresses:

Guangming Zhou
105 S. Gregory Ave.
Urbana, IL 61801
Tel: (217) 333-6296
Fax: (217) 244-4393
E-mail: zhou@uiatma.atmos.uiuc.edu

Mohan K. Ramamurthy
105 S. Gregory Ave.
Urbana, IL 61801
Tel: (217) 333-8650
Fax: (217) 244-4393
E-mail: mohan@uiatma.atmos.uiuc.edu

Robert B. Wilhelmson
NCSA, 5249 Beckman Institute
405 N. Mathews,
Urbana, IL 61801
Tel: (217) 244-6833
Fax: (217) 244-2909
E-mail: bw@ncsa.uiuc.edu