Heat, Moisture, and Momentum Budgets of Isolated Deep Midlatitude and Tropical Convective Clouds as Diagnosed from Three-Dimensional Model Output. Part II: Sensitivity to Ice Phase and Small Changes in Ambient Shear Strength and Low-Level Moisture Supply

Abstract

This project uses a three-dimensional anelastic cloud model with a simple ice phase parameterization to evaluate the feedback of isolated deep convective clouds over a horizontal scale comparable to one grid cell in typical mesoscale numerical weather prediction models. A more specific focus in this paper is the sensitivity of the feedback to modest changes in the initial vertical wind shear intensity and low-level moisture supply, as well as to the ice phase. Two parallel sets of comparative simulations are run for a quasi-steady severe Oklahoma supercell thunderstorm in strong vertical wind shear versus a weaker, less persistent, and narrower tropical Atlantic cumulonimbus with a slowly decaying and pulsating updraft in much weaker shear. The horizontal Reynolds averaging approach of Anthes is adopted to diagnose the budgets for heat, moisture, and horizontal momentum. Several similarities and differences between the midlatitude and tropical control experiments were delineated in Part I. The main findings of the sensitivity study are described below. The midlatitude storm evolves to maturity somewhat later (earlier) for stronger (weaker) shear, though with little effect on peak updraft speed or basic storm structure. Quantitatively, the convection is more sensitive to moisture supply changes, although basic structure is again preserved. With increased moisture the peak updraft speed increases by ?15% and the apparent heating and drying amplitudes by ?40%, and vice versa for the drier run. The vertical eddy fluxes are the main modulating factors. Without ice the peak updraft is ?10% weaker, though with no systematic effect on downdraft speed, the later stages show gradual weakening in contrast to the quasi-steady control case, and the apparent heating and drying amplitudes are ?25% lower due to decreased condensation and also (for heat) the absence of any latent heat release by glaciation. The tropical cumulonimbus is for the most part less sensitive to shear intensity than its midlatitude counterpart. The pulsations are weaker in stronger shear and vice versa, but varying the shear has no systematic effect on either downdraft intensity or updraft evolution, affecting the budgets to a modest degree chiefly through the vertical eddy transport profiles. Omitting ice also affects the tropical cumulonimbus less than the midlatitude supercell storm, only slightly affecting updraft speed and the various budgets, especially for momentum. However, the tropical cumulonimbus is much more sensitive to moisture supply than the midlatitude supercell. The updraft is almost 25% weaker in the dry run and ?45% stronger with slower decay and stronger pulsations in the moist run, which also produces a deeper cloud with less downshear tilt and a more extensive anvil. Apparent heating and drying amplitudes are roughly doubled in the moist run and halved in the dry run, modulated mainly by condensation and vertical eddy transport amplitudes. The momentum budget is also notably sensitive to moisture supply, especially in the moist variation, in which the upper-level horizontal pressure gradient force promotes the enhanced anvil blowoff and reduced cloud tilt.