Momentum Budget of a Squall Line with Trailing Stratiform Precipitation: Calculations with a High-Resolution Numerical Model

Abstract

In this paper, the authors investigate the momentum budget of a squall line with trailing stratiform precipitation by examining how the momentum balance varies with respect to the storm's internal structure. In particular, the authors determine differences between the momentum budgets of the convective and stratiform precipitation regions, which are physically distinct parts of the storm. The results from a high-resolution nonhydrostatic numerical simulation of the two-dimensional segment of the 10?11 June 1985 PRE-STORM squall line are used. The momentum equation is averaged over a 300-km-wide large-scale area for time periods of 1 h. On the 1-h timescale, the convective-scale temporal variations of horizontal and vertical velocities are nearly uncorrelated, and thus their contribution to the momentum flux is negligible. The remaining standing-eddy and mean-flow circulations account for the momentum flux on this timescale. The combination of the standing eddy and mean flow behave almost exactly like Moncrieff's idealization of two-dimensional steady-state squall line flow. Because the standing-eddy circulation and the pressure-gradient acceleration vary from one part of the storm to another, the interplay of forces leading to the large-scale momentum tendency also differs strongly from one subregion to another. The convective precipitation region dominates the momentum budget at low levels, where the standing-eddy flux convergence produces a forward acceleration that slightly outweighs the rearward pressure-gradient acceleration. At midlevels, both the convective and stratiform precipitation regions contribute to the net large-scale momentum tendency. The pressure-gradient forces in the convective and stratiform precipitation regions are both strong but oppositely directed; however, the rearward standing-eddy flux convergence in the convective precipitation region is also strong; thus, the net large-scale momentum tendency at midlevels is rearward. At upper levels, the momentum budget is completely dominated by the stratiform precipitation region, where a strong forward-directed pressure-gradient acceleration dominates the net large-scale momentum tendency. These differences between the momentum budgets of the convective and stratiform precipitation regions suggest that rather different large-scale momentum tendencies can arise as a function of storm structure; storms with strong convective precipitation regions and weak stratiform precipitation regions would produce momentum tendencies quite different from storms with well-developed stratiform precipitation regions.