A Comparison Between Axisymmetric and Slab-Symmetric Cumulus Cloud Models

Abstract

Axisymmetric and slab-symmetric cumulus cloud models with Kessler's parameterizations for microphysical processes are developed. By using a staggered grid arrangement and applying a modified upstream difference scheme, erroneous behavior in the center of a simulated cloud, which would result with the use of the ordinary upstream difference scheme, is eliminated. A comparison between the present two models of different geometries confirms in general the conclusions reached in previous studies: the updraft in an aixsymmetric model grows more vigorously than in a slab-symmetric model. However, the ratio of the maximum updraft in the slab-symmetric model to that in the axisymmetric model is 0.53 in this study, notably larger than Murray's 0.12. An analysis of the pressure gradient force associated with could motions reveals that the vertical pressure gradient force due to perturbed pressure is: 1) of the same order of magnitudes as that of the thermal buoyancy force in the core region of the cloud; is 2) acting in the opposite direction of the net force due to excess heat, moisture, and the weight of liquid water; and 3) is larger in absolute magnitude in the slab-symmetric model than in the axisymmetric one. Also included are differences in the evolution of the modeled clouds in relation to different intensities of initial buoyant elements used in initiating convection in a conditionally unstable atmosphere and in relation to differences in the size of integration domains.