STRUCTURE AND EVOLUTION OF THE 22-FEBRUARY-1993 TOGA-COARE SQUALL LINE - ORGANIZATION MECHANISMS INFERRED FROM NUMERICAL-SIMULATION

Mechanisms responsible for meso- and convective-scale organization wit hin a large tropical squall line that occurred on 22 February 1993 dur ing the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Resp onse Experiment are investigated using a three-dimensional numerical c loud model. The squall line occurred in an environment typical of fast -moving tropical squall lines, characterized by moderate convective av ailable potential energy and moderate-to-strong vertical shear beneath a low-level jet with weak reverse vertical shear above.A well-simulat ed aspect of the observed squall line is the evolution of a portion of its leading convective zone from a quasi-linear to a three-dimensiona l bow-shaped structure over a 2-h period. This transition is accompani ed by the development of both a prominent mesoscale vortex along the n orthern edge of the 40-60-km long bow-shaped feature and elongated ban ds of weaker reflectivity situated rearward and oriented transverse to the leading edge, within enhanced front-to-rear system relative midle vel flow, near the southern end of the bow. The vertical wind shear th at arises from the convectively induced mesoscale how within the squal l-line system is found to be a critical factor influencing 1) the deve lopment of the vortex and 2) through its associated vertical pressure gradients, the pronounced along-line variability of the convective upd raft and precipitation structure. The environmental wind profile is al so critical to system organization since the orientation of its vertic al shear (in layers both above and below the environmental jet height) relative to the local orientation of the incipient storm-induced subc loud cold pool directly influences the onset of the convectively induc ed mesoscale flow.