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HMZS/WD/1988/07

Convection Development in a Sheared Wind Field

Marjan Divjak

Abstract

Effects of the ambient wind profile on the atmospheric convection development are studied using a three-dimensional numerical model. Temperature and humidity fields used, and microphysical processes included are directed specifically toward the studying of mid-latitude summertime continental thunderstorms.

Three experiments are run with different vertical profiles of ambient wind: no shear, positive speed shear but no directional shear, and positive speed shear with low level veering. In all cases, the same vertical profiles of temperature and humidity are used. Convection is triggered by the initial temperature and momentum perturbation in a form of a shallow radial symmetric warm updraft. The cases are investigated in regard to airflow, pressure, temperature, and precipitation patterns. The following is found.

1. The growing stage is weaker with shear than without, and to trigger the convection of nearly the same intensity of the mature stage, stronger initial perturbations are needed for cases with shear than without it. The mature stage in the sheared environment is more persistent. Without shear, the main downdraft develops directly beneath the updraft, whereas with shear the main downdraft develops downwinds of the updraft.

2. An internal gravity wave is triggered at upper levels when the cloud top approaches the tropopause.

3. Sheared storms exhibit a nearly erect updraft, a vortex doublet aloft, middle level barrier flow around the updraft, and gradual splitting into two cells.

4. During the growing stage, the perturbed pressure field exhibits a meso-low under the cloud, and a meso-high near its top. With shear, the meso-low is displaced downwinds of the meso-high. During the mature stage, highs are found beneath the downdraft and at the updraft summit, and low at intermediate levels.

5. Thermal buoyancy and the perturbed vertical pressure gradient force oppose each other.

6. With perturbations used, secondary convection develops in the neighbourhood of the downdraft only in the veered environment. New cell growth occurs at the right flank.

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