STRUCTURE AND EVOLUTION OF THE 22 FEBRUARY 1993 TOGA COARE SQUALL LINE - NUMERICAL SIMULATIONS

In this study a numerical cloud model is used to simulate the three-di mensional evolution of an oceanic tropical squall line observed during the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Respons e Experiment and investigate the impact of small-scale physical proces ses including surface fluxes and ice microphysics on its structure and evolution. The observed squall line was oriented perpendicular to a m oderately strong low-level jet. Salient features that are replicated b y the model include an upshear-tilted leading convective region with m ultiple updraft maxima during its linear stage and the development of a 30-km scale midlevel vortex and associated transition of the line to a pronounced bow-shaped structure. In this modeling approach, only su rface fluxes and stresses that differ from those of the undisturbed en vironment are included. This precludes an unrealistically large modifi cation to the idealized quasi-steady base state and thus allows us to more easily isolate effects of internally generated surface fluxes and stresses on squall line evolution. Neither surface fluxes and stresse s nor ice microphysics are necessary to simulate the salient features of the squall line. Their inclusion, however, results in differences i n the timing of squall line evolution and greater realism of certain s tructural characteristics. Significant differences in the convectively induced cold pool strength occur between the early stages of simulati ons that included ice microphysics and a simulation that contained onl y warm-rain microphysical processes. The more realistic strength and d epth of the cold pool in the simulations that contained ice processes is consistent with an updraft tilt that more closely resembles observa tions. The squall-line-induced surface fluxes also influence the stren gth but, more dramatically, the areal extent of the surface cold pool. For the majority of the 6-h simulation, this influence on the cold po ol strength is felt only within several hundred meters of the surface. Significant impact oi squall-line-induced surface fluxes on the evolv ing deep convectional the leading edge of the cold pool is restricted to the later stages (t greater than or equal to 4 h) of simulations an d is most substantial in regions where the ground-relative winds are s trong and the convectively induced cold pool is initially weak and sha llow.