A Three-Dimensional Numerical Model of an Isolated Deep Convective Cloud: Preliminary Results

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  • 1 Department of Meteorology, University of Wisconsin, Madison 53706
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Abstract

The development of an isolated convective storm in a sheared environment is studied with an anelastic three-dimensional numerical model. Each grid cell has horizontal dimensions of Δxy=3.2 km and a vertical dimension of Δs=0.7 km. Although it is ultimately planned to use at least a 31×31×20 grid with turbulence and liquid precipitation included, the present study uses an 11×11×8 trial grid with both of these processes suppressed, simulating only early cloud growth.

Comparative experiments are run for three vertical profiles of ambient wind: no ambient wind, positive speed shear but no directional shear, and positive speed shear with veering. The cases are compared in regard to airflow, pressure and thermal patterns. It is found that:

  1. A vortex doublet develops at middle levels when ambient shear is present. A contributing factor may be tilting of horizontal vorticity into the vertical by differential vortex-tube lifting.
  2. Shear introduces asymmetry, with upshear dominance of perturbation outflow and horizontal gradients of physical properties. Updraft development is weaker with shear than without it; this may he partly due to displacement of the maximum thermal buoyancy upshear of the updraft core at middle levels, making upward beat transport less efficient
  3. The perturbed pressure field exhibits a meso-low under the cloud, and a mesa-high near its top. With shear, the meso-low is displaced downshear of the meso-high.
  4. Thermal buoyancy and the vertical perturbed pressure gradient force are the dominant vertical forces, but strongly oppose each other. At the cloud base, negative thermal buoyancy is overcome by upward- directed vertical pressure gradient and pressure buoyancy forces.
  5. With directional shear, the middle-level horizontal pressure gradient force is directed to the right of the direction of cloud motion, suggesting a potential propagation mechanism.

Abstract

The development of an isolated convective storm in a sheared environment is studied with an anelastic three-dimensional numerical model. Each grid cell has horizontal dimensions of Δxy=3.2 km and a vertical dimension of Δs=0.7 km. Although it is ultimately planned to use at least a 31×31×20 grid with turbulence and liquid precipitation included, the present study uses an 11×11×8 trial grid with both of these processes suppressed, simulating only early cloud growth.

Comparative experiments are run for three vertical profiles of ambient wind: no ambient wind, positive speed shear but no directional shear, and positive speed shear with veering. The cases are compared in regard to airflow, pressure and thermal patterns. It is found that:

  1. A vortex doublet develops at middle levels when ambient shear is present. A contributing factor may be tilting of horizontal vorticity into the vertical by differential vortex-tube lifting.
  2. Shear introduces asymmetry, with upshear dominance of perturbation outflow and horizontal gradients of physical properties. Updraft development is weaker with shear than without it; this may he partly due to displacement of the maximum thermal buoyancy upshear of the updraft core at middle levels, making upward beat transport less efficient
  3. The perturbed pressure field exhibits a meso-low under the cloud, and a mesa-high near its top. With shear, the meso-low is displaced downshear of the meso-high.
  4. Thermal buoyancy and the vertical perturbed pressure gradient force are the dominant vertical forces, but strongly oppose each other. At the cloud base, negative thermal buoyancy is overcome by upward- directed vertical pressure gradient and pressure buoyancy forces.
  5. With directional shear, the middle-level horizontal pressure gradient force is directed to the right of the direction of cloud motion, suggesting a potential propagation mechanism.
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