Heat, Moisture, and Momentum Budgets of Isolated Deep Midlatitude and Tropical Convective Clouds as Diagnosed from Three-Dimensional Model Output. Part I: Control Experiments

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  • 1 Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison, Madison, Wisconsin
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Abstract

This project uses a three-dimensional anelastic cloud model with a simple ice phase parameterization to evaluate the feedback between isolated deep convective clouds and their near surroundings. The horizontal Reynolds averaging approach of Anthes is adopted to diagnose the vertical profiles of the individual budget terms for heat, moisture, and horizontal momentum, as well as the resultant effects of each budget as defined by apparent sources or sinks. The averaging area, 33.75 km on a side, is comparable to one grid cell for typical mesoscale numerical weather prediction models.

Two comparative simulations are run, one for a severe Oklahoma thunderstorm in strong vertical wind shear and the other for a tropical Atlantic cumulonimbus in much weaker shear. The midlatitude cloud evolves to a vigorous quasi-steady mature stage with several supercell characteristics including an erect large-diameter updraft, a strong and vertically extensive mesolow, and a well-developed highly asymmetric cold pool that spreads rapidly. In contrast, the tropical updraft is much narrower and slower with a shallow weak midlevel mesolow, leans markedly downshear, and evolves early into slow decay modulated by bubblelike pulsations, while the cold pool is weak and quasi-circular and spreads slowly.

There are several similarities between corresponding budgets in the two runs. Most notably: 1) The heat and moisture budgets are dominated by condensation, which is maximized in the midtroposphere. 2) The horizontal pressure gradient force dominates the momentum budget. 3) Vertical eddy transport (flux divergence) is highly important to each budget. Thermodynamically, it acts to mainly cool and dry the lower troposphere, while warming and moistening the upper troposphere, though with a lower crossover level for moisture than for heat. 4) The altitudes of the peak apparent heat sources are determined by the vertical eddy transport of heat. 5) Net evaporation has ∼40% as much amplitude as the condensation. 6) Horizontal eddy transport contributes little to the heat and moisture budgets. 7) Other terms derived by Anthes but ignored in existing cumulus parameterizations (the resultant of eddy storage, transport of mean fields by the eddy wind, and transport of eddy fields by the mean wind) oppose significant portions of the apparent sources or sinks in each budget, though not quite proportionally.

On the other hand, the tropical budgets differ from their midlatitude counterparts in several aspects. Most notably: (i) The amplitudes of the feedback terms are an order of magnitude smaller. (ii) The condensational heating and drying have sharper and narrower peaks, and glaciation of rain is much smaller in relation to the condensation. (iii) The evaporative cooling and moistening have a sharp midtropospheric peak associated with an altostratus shelf that is absent in the midlatitude case. (iv) Vertical eddy transport of heat is less important in relation to condensational warming, and is more oscillatory with height than in the midlatitude case. (v) Vertical eddy transport does not contribute to the forcing of the principal peak in the apparent moisture sink. (vi) Horizontal eddy transport of momentum, significant in the midlatitude storm, is negligible in the tropical cloud.

Abstract

This project uses a three-dimensional anelastic cloud model with a simple ice phase parameterization to evaluate the feedback between isolated deep convective clouds and their near surroundings. The horizontal Reynolds averaging approach of Anthes is adopted to diagnose the vertical profiles of the individual budget terms for heat, moisture, and horizontal momentum, as well as the resultant effects of each budget as defined by apparent sources or sinks. The averaging area, 33.75 km on a side, is comparable to one grid cell for typical mesoscale numerical weather prediction models.

Two comparative simulations are run, one for a severe Oklahoma thunderstorm in strong vertical wind shear and the other for a tropical Atlantic cumulonimbus in much weaker shear. The midlatitude cloud evolves to a vigorous quasi-steady mature stage with several supercell characteristics including an erect large-diameter updraft, a strong and vertically extensive mesolow, and a well-developed highly asymmetric cold pool that spreads rapidly. In contrast, the tropical updraft is much narrower and slower with a shallow weak midlevel mesolow, leans markedly downshear, and evolves early into slow decay modulated by bubblelike pulsations, while the cold pool is weak and quasi-circular and spreads slowly.

There are several similarities between corresponding budgets in the two runs. Most notably: 1) The heat and moisture budgets are dominated by condensation, which is maximized in the midtroposphere. 2) The horizontal pressure gradient force dominates the momentum budget. 3) Vertical eddy transport (flux divergence) is highly important to each budget. Thermodynamically, it acts to mainly cool and dry the lower troposphere, while warming and moistening the upper troposphere, though with a lower crossover level for moisture than for heat. 4) The altitudes of the peak apparent heat sources are determined by the vertical eddy transport of heat. 5) Net evaporation has ∼40% as much amplitude as the condensation. 6) Horizontal eddy transport contributes little to the heat and moisture budgets. 7) Other terms derived by Anthes but ignored in existing cumulus parameterizations (the resultant of eddy storage, transport of mean fields by the eddy wind, and transport of eddy fields by the mean wind) oppose significant portions of the apparent sources or sinks in each budget, though not quite proportionally.

On the other hand, the tropical budgets differ from their midlatitude counterparts in several aspects. Most notably: (i) The amplitudes of the feedback terms are an order of magnitude smaller. (ii) The condensational heating and drying have sharper and narrower peaks, and glaciation of rain is much smaller in relation to the condensation. (iii) The evaporative cooling and moistening have a sharp midtropospheric peak associated with an altostratus shelf that is absent in the midlatitude case. (iv) Vertical eddy transport of heat is less important in relation to condensational warming, and is more oscillatory with height than in the midlatitude case. (v) Vertical eddy transport does not contribute to the forcing of the principal peak in the apparent moisture sink. (vi) Horizontal eddy transport of momentum, significant in the midlatitude storm, is negligible in the tropical cloud.

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