A NUMERICAL INVESTIGATION OF SQUALL LINES .2. THE MECHANICS OF EVOLUTION

The physical processes involved in the evolution of the model squall l ine presented in Part I of this study are examined. It is found that b oth the thermal and dynamic effects are important in the development o f the midlevel meso-gamma-scale low pressure zone located just to the rear of the convective core. Based on this observation, a positive fee dback mechanism is proposed to explain the abrupt transformation of th e more or less upright convection line into a quasi-steady meso-beta-s cale convective system possessing an extensive trailing stratiform reg ion. In order to set the stage for this transformation to take place, the convective updraft is required to possess an initial upshear tilt. Qualitative arguments are given to show that this initial tilt might be the result of a moderate wind shear at low levels and none or rever se shear in the middle to high levels. Results from model sensitivity experiments are presented to support the theory. In addition to the dy namic effects of the low-level wind shear and the strength of the stor m-induced cold pool, model results show that other storm-induced featu res and environmental factors such as the middle-level wind shear also play an important role in determining the evolution of squall systems .