A Numerical Model of Deep Moist Convection: Part II. A Prototype Experiment and Variations Upon It

Abstract

This study involves the dynamics of a deep convective cloud in conditionally unstable surroundings with moderate mid-tropospheric shear. A two-dimensional, anelastic numerical model is used to simulate convection of the squall-line type. A prototype experiment is run with liquid water drag, liquid precipitation, and effects of pressure perturbations upon the buoyancy included. The roles of these forces are inferred by running variations upon the prototype with one force suppressed in each case. In another variation, a sharp upper jet replaces a flat upper wind maximum. It is found that. The prototype storm exhibits a quasi-steady mature stage characterized by nearly time-independent streamline patterns for both air parcels and precipitation particles in and near the cloud core. Potentially warm low-level air feeds the updraft from downshear, while potentially cool middle-level air feeds the downdraft from upshear. During maturity, thermal buoyancy is the dominant vertical force, but is strongly opposed by the vertical perturbed pressure gradient force. The buoyancy due to pressure perturbations is appreciable, with a maximum value about one-fourth that for the thermal buoyancy. Liquid water drag is intermediate in importance between the two buoyancy components. The vertical and horizontal net accelerations are comparable to each other and to the pressure buoyancy. Dynamic entrainment of potentially cool air into the sides of the cloud eventually contributes to dissipation as the downdraft spreads laterally and isolates the updraft. Liquid water drag limits updraft intensity but is not necessary for downdraft formation, which is due instead to evaporative cooling. Fallout of precipitation is essential to storm dissipation; without fallout, liquid water accumulation at low levels is insufficient for significant downdraft development, and the cloud core evolves to a steady state. Negative pressure buoyancy in the upper portion of the cloud slightly limits the intensity of the developing updraft, but positive pressure buoyancy at and near the foot of the updraft reinforces its intensity during maturity. A sharp upper-level jet in place of a broad upper-level wind maximum delays and slightly prolongs the mature stage, but does not lead to a more intense updraft.