Diagnostic and Semiprognostic Studies of Cumulus Effects in the Presence of Mesoscale Circulations

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  • 1 Department of Atmospheric Sciences, University of California, Los Angeles, Los Angeles, California
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Abstract

Heat and moisture budgets of two mesoscale convective systems of a characteristic horizontal scale of approximately 450 km are studied using the Atmospheric Variability Experiment-Severe Environmental Storms and Mesoscale Experiment mesoscale data with a cutoff wavelength of 150 km. The scale dependence of budget residuals is examined by further varying horizontal resolutions of data with low-pass spatial filters. The results show that the horizontal distributions of the vertically integrated heat source 〈Q1〉 and moisture sink 〈Q2〉 are dependent on horizontal resolutions of data. For the mesoscale data, the horizontal distributions of 〈Q1〉 and 〈Q2〉 show pronounced “dipole” patterns that are closely related to the horizontal fluxes of heat and moisture due to mesoscale circulations. For the larger-scale data, the dipole pattern disappears and the horizontal distributions of 〈Q1〉 and 〈Q2〉 show positive values in the area of convection. However, the vertical profiles of the observed heat source and moisture sink averaged over a large domain (600 km × 600 km) are not significantly dependent on horizontal resolutions of data.

The observed budget residuals are compared with the cumulus-induced heating and drying obtained semi-prognostically using the Arakawa-Schubert parameterization with the mesoscale and larger-scale data. The major features of the budget residuals are reproduced by the cumulus-induced heating and drying. When the larger-scale data are used, additional contributions from the condensation and evaporation due to mesoscale stratiform clouds and precipitation are needed to explain the budget residuals. When the mesoscale data are used, the representation of hydrometeors generated and transported from the region of cumulus convection to the region of stratiform clouds is required to accurately reproduce the budget residuals. The Arakawa-Schubert quasi-equilibrium assumption becomes more accurate for the data-resolving mesoscale circulations.

Abstract

Heat and moisture budgets of two mesoscale convective systems of a characteristic horizontal scale of approximately 450 km are studied using the Atmospheric Variability Experiment-Severe Environmental Storms and Mesoscale Experiment mesoscale data with a cutoff wavelength of 150 km. The scale dependence of budget residuals is examined by further varying horizontal resolutions of data with low-pass spatial filters. The results show that the horizontal distributions of the vertically integrated heat source 〈Q1〉 and moisture sink 〈Q2〉 are dependent on horizontal resolutions of data. For the mesoscale data, the horizontal distributions of 〈Q1〉 and 〈Q2〉 show pronounced “dipole” patterns that are closely related to the horizontal fluxes of heat and moisture due to mesoscale circulations. For the larger-scale data, the dipole pattern disappears and the horizontal distributions of 〈Q1〉 and 〈Q2〉 show positive values in the area of convection. However, the vertical profiles of the observed heat source and moisture sink averaged over a large domain (600 km × 600 km) are not significantly dependent on horizontal resolutions of data.

The observed budget residuals are compared with the cumulus-induced heating and drying obtained semi-prognostically using the Arakawa-Schubert parameterization with the mesoscale and larger-scale data. The major features of the budget residuals are reproduced by the cumulus-induced heating and drying. When the larger-scale data are used, additional contributions from the condensation and evaporation due to mesoscale stratiform clouds and precipitation are needed to explain the budget residuals. When the mesoscale data are used, the representation of hydrometeors generated and transported from the region of cumulus convection to the region of stratiform clouds is required to accurately reproduce the budget residuals. The Arakawa-Schubert quasi-equilibrium assumption becomes more accurate for the data-resolving mesoscale circulations.

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