A Numerical Model of Deep Moist Convection: Part I. Comparative Experiments for Variable Ambient Moisture and Wind Shear

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  • 1 Dept. of Meteorology, University of Wisconsin, Madison 53706
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Abstract

This study concerns the influence of ambient conditions upon the behavior of deep moist convection in the atmosphere. By means of a two-dimensional numerical model, an anelastic system of hydrodynamic and thermodynamic equations is integrated in order to simulate convection of the squall-line type. Liquid precipitation and the effect of pressure perturbations upon the buoyancy are included.

The joint influence of low-level relative humidity and mid-tropospheric wind shear upon the intensity and persistence of the convection at maturity is investigated. Nine comparative experiments are performed using three values of each parameter, including cases without shear. The various cases are compared with regard to airflow and rainfall patterns. Basic features of the perturbation fields for flow, temperature and pressure are interrelated.

It is found that the greater the moisture supply and the weaker the shear, the more intense is the convection in terms of both peak updraft velocity and rainfall rate. Strength and persistence do not necessarily correspond. Specifically:

  1. With a very large moisture supply and weak shear, an intense but relatively short-lived updraft results. The main rainfall and downdraft occur downshear of the updraft, isolating it from its source.
  2. A long-lived or quasi-steady updraft of moderate to strong intensity is supported by moderate shear, or even strong shear if moisture is sufficiently abundant. Rainfall and downdraft develop upshear of the updraft, tending to perpetuate it.
  3. With insufficient moisture, strong shear cannot support an intense or long-lived storm. Rainfall and downdraft are upshear of the updraft, but the lowest-lying air flows under and past it, limiting its buoyancy.

Low-level evaporative cooling produces a surface meso-high under the mature storm core. Mid-level warming due to compensatory subsidence contributes to surface meso-lows on either side, the downshear meso-low being further enhanced by warm outflow into the anvil.

Abstract

This study concerns the influence of ambient conditions upon the behavior of deep moist convection in the atmosphere. By means of a two-dimensional numerical model, an anelastic system of hydrodynamic and thermodynamic equations is integrated in order to simulate convection of the squall-line type. Liquid precipitation and the effect of pressure perturbations upon the buoyancy are included.

The joint influence of low-level relative humidity and mid-tropospheric wind shear upon the intensity and persistence of the convection at maturity is investigated. Nine comparative experiments are performed using three values of each parameter, including cases without shear. The various cases are compared with regard to airflow and rainfall patterns. Basic features of the perturbation fields for flow, temperature and pressure are interrelated.

It is found that the greater the moisture supply and the weaker the shear, the more intense is the convection in terms of both peak updraft velocity and rainfall rate. Strength and persistence do not necessarily correspond. Specifically:

  1. With a very large moisture supply and weak shear, an intense but relatively short-lived updraft results. The main rainfall and downdraft occur downshear of the updraft, isolating it from its source.
  2. A long-lived or quasi-steady updraft of moderate to strong intensity is supported by moderate shear, or even strong shear if moisture is sufficiently abundant. Rainfall and downdraft develop upshear of the updraft, tending to perpetuate it.
  3. With insufficient moisture, strong shear cannot support an intense or long-lived storm. Rainfall and downdraft are upshear of the updraft, but the lowest-lying air flows under and past it, limiting its buoyancy.

Low-level evaporative cooling produces a surface meso-high under the mature storm core. Mid-level warming due to compensatory subsidence contributes to surface meso-lows on either side, the downshear meso-low being further enhanced by warm outflow into the anvil.

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