A Three-Dimensional Numerical Model of an Isolated Thunderstorm: Part I. Comparative Experiments for Variable Ambient Wind Shear

The mature stage of an isolated convective storm in sheared surroundings is studied by means of an anelastic three-dimensional numerical model. Liquid precipitation and turbulence are included in parameterized form. Three comparative experiments are run with different vertical profiles of ambient wind: no ambient wind, uni-directional shear, and multi-directional shear dominated by strong low-level veering, the first shear profile being the west-east projection of the second. The cases are compared in regard to airflow, pressure, potential temperature and liquid water. The results were as follows: Both sheared storms exhibit a quasi-erect high-speed updraft, a deep cyclonic-anticylonic vortex couplet aloft, middle-level barrier flow around the updraft, and gradual splitting into cyclonic and anti-cyclonic cells moving to the right and left of the mean winds. The model storms show a slightly weaker growing stage with shear than without, but the mature stage is stronger and more persistent. Without shear, the main downdraft develops directly beneath the updraft, whereas with shear the main downdraft develops upshear of the updraft. Surface convergence between updraft inflow and downdraft outflow is much stronger with shear than without. The perturbed pressure field shows highs beneath the downdraft and at the updraft summit, and low pressure at intermediate levels. With shear, the low pressure shows two centers at the left and right flanks, inducing a divergent horizontal pressure gradient force field that may contribute to splitting. Thermal buoyancy and the perturbed vertical pressure gradient force oppose each other, in particular enabling parcels to accelerate upward against negative buoyancy at the bases of the sheared updrafts. With directional shear, the respective right and left flanks move slower and faster than the mean winds, and the right flank is stronger than the left. With shear, surface rainfall is lighter and less widespread than without shear, but maximum liquid water content aloft is greater and decreases much slower with time.