On the Interaction between the Subcloud and Cloud Layers in Tropical Regions

Yoshimitsu Ogura Laboratory for Atmospheric Research, University of Illinois, Urbana 61801

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Han-Ru Cho Laboratory for Atmospheric Research, University of Illinois, Urbana 61801

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Abstract

A model for the subcloud layer in tropical regions is developed to investigate the effect of the mass exchange between the subcloud and the cloud layers on the heat and moisture budgets in the subcloud layer. This mass exchange occurs in association with the updraft inside ascending cumulus clouds and the induced downdraft outside these clouds.

The model is first applied in discussing the moisture balance in the subcloud layer in weak traveling easterly waves, using Reed and Recker’s composite data. In the trough-axis region the upward transport of moisture due to ascending cumulus clouds and the downward transport at the cloud base level due to the induced downdraft is found to be much larger in magnitude than the large-scale horizontal convergence of moisture in the subcloud layer or the evaporation from the ocean surface. Steady-state solutions of the model are obtained which describe the relations between the large-scale convergence, the vertical convective mass flux, the energy flux at the ocean surface, and the thermodynamic properties of the subcloud layer. It is found that the vertical mass flux due to cumulus clouds at the cloud base (Mc) thus obtained agrees fairly well with that determined earlier by Cho and Ogura from consideration of the beat and moisture budgets in the layer above the cloud base. It is also found that Mc increases as the rate of evaporation from the ocean surface increases when all other conditions are identical.

Abstract

A model for the subcloud layer in tropical regions is developed to investigate the effect of the mass exchange between the subcloud and the cloud layers on the heat and moisture budgets in the subcloud layer. This mass exchange occurs in association with the updraft inside ascending cumulus clouds and the induced downdraft outside these clouds.

The model is first applied in discussing the moisture balance in the subcloud layer in weak traveling easterly waves, using Reed and Recker’s composite data. In the trough-axis region the upward transport of moisture due to ascending cumulus clouds and the downward transport at the cloud base level due to the induced downdraft is found to be much larger in magnitude than the large-scale horizontal convergence of moisture in the subcloud layer or the evaporation from the ocean surface. Steady-state solutions of the model are obtained which describe the relations between the large-scale convergence, the vertical convective mass flux, the energy flux at the ocean surface, and the thermodynamic properties of the subcloud layer. It is found that the vertical mass flux due to cumulus clouds at the cloud base (Mc) thus obtained agrees fairly well with that determined earlier by Cho and Ogura from consideration of the beat and moisture budgets in the layer above the cloud base. It is also found that Mc increases as the rate of evaporation from the ocean surface increases when all other conditions are identical.

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