MOMENTUM BUDGET OF A SQUALL LINE WITH TRAILING STRATIFORM PRECIPITATION - CALCULATIONS WITH A HIGH-RESOLUTION NUMERICAL-MODEL

In this paper, the authors investigate the momentum budget of a squall line with trailing stratiform precipitation by examining how the mome ntum balance varies with respect to the storm's internal structure. In particular, the authors determine differences between the momentum bu dgets of the convective and stratiform precipitation regions, which ar e physically distinct parts of the storm. The results from a high-reso lution nonhydrostatic numerical simulation of the two-dimensional segm ent of the 10-11 June 1985 PRE-STORM squall line are used. The momentu m equation is averaged over a 300-km-wide large-scale area for time pe riods of 1 h. On the I-h timescale, the convective-scale temporal vari ations of horizontal and vertical velocities are nearly uncorrelated, and thus their contribution to the momentum flux is negligible. The re maining standing-eddy and mean-flow circulations account for the momen tum flux on this timescale. The combination of the standing eddy and m ean flow behave almost exactly like Moncrieff's idealization of two-di mensional steady-state squall line how. Because the standing-eddy circ ulation and the pressure-gradient acceleration vary from one part of t he storm to another, the interplay of forces leading to the large-scal e momentum tendency also differs strongly from one subregion to anothe r. The convective precipitation region dominates the momentum budget a t low levels, where the standing-eddy flux convergence produces a forw ard acceleration that slightly outweighs the rearward pressure-gradien t acceleration. At midlevels, both the convective and stratiform preci pitation regions contribute to the net large-scale momentum tendency. The pressure-gradient forces in the convective and stratiform precipit ation regions are both strong but oppositely directed; however, the re arward 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 complet ely dominated by the stratiform precipitation region, where a strong f orward-directed pressure-gradient acceleration dominates the net large -scale momentum tendency. These differences between the momentum budge ts of the convective and stratiform precipitation regions suggest that rather different large-scale momentum tendencies can arise as a funct ion of storm structure; storms with strong convective precipitation re gions and weak stratiform precipitation regions would produce momentum tendencies quire different from storms with well-developed stratiform precipitation regions.