Numerical Simulation of a Midlatitude Squall Line in Two Dimensions

Abstract

A two-dimensional, anelastic cloud model was used in attempts to numerically replicate the observed structure of a midlatitude squall line. Initial conditions were adapted from observations of the 22 May 1976 Oklahoma line. Model simulations were made with and without considering the ice phase of water. These model storms have, within the constraints of the model's geometry, replicated the basic multicellular character and general line-normal airflow typical of these lines. In addition, the structure and intensity of the subcloud cold air pool and the propagation speeds developed by the storms appear to be reasonable. Further, it was found that the initial conditions chosen resulted in model storms which were not only long-lasting but also essentially repetitive, indicating that a state of quasi-equilibrium had been attained. The storms did not decay because the environmental conditions ahead of the storms were favorable and essentially unchanged during the course of the simulations. The inclusion of ice phase processes resulted in the production of more realistic appearing features in the trailing portion of the storm as well as more widespread precipitation. These were for the most part due to the enhanced rearward transport of precipitation particles from the convective cells which resulted from including ice, particularly low density snow. The underlying structures of these model storms were investigated by averaging model fields across time, smoothing out the transient components. These analyses indicated that the addition of ice had its greatest impact on the scale of the storm's circulation features.