Modeling Subcloud Layer Structure and Interaction with a Shallow Cumulus Layer

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  • 1 Department of Atmospheric Science, Colorado State University, Fort Collins 80523
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Abstract

This paper couples a mixed subecloud layer model developed by several authors with the cumulus flux parameterization proposed by Betts (1975). Cloud base mass flux and mass flux gradient are related to sub-cloud layer parameters and cumulus layer gradients using two parameters, α and β. The α symbol is the ratio of a model transition layer depth to the subcloud layer depth, and β is the ratio of a model cloud base static energy flux to the surface flux. For the simple case of a steady-state transition layer, the subecloud layer beat and moisture budgets are predicted.

Data from a field experiment over Venezuela are used to illustrate mean subcloud layer structure and to derive heat and moisture flux profiles and model parameters from a simple budget analysis. The data give (α,δ)=(0.11, 0.41) and correspondingly, (αv1βv based on virtual static energy fluxes and profiles. During the budget time period (centered on local noon over land) the subeloud layer war and dries with a corresponding rise of cloud base. The steady-state transition layer model predictions showed qualitative agreement with the fluxes derived from the budget. The extension of the convective mass flux model into the subeloud layer was shown to be feasible. It illustrates the rapid decrease of convective mass flux across the transition layer.

The paper concludes that the model is a satisfactory diagnostic tool for subcloud layer budgets and the cloud-subdoud layer interaction, and may have predictive value.

Abstract

This paper couples a mixed subecloud layer model developed by several authors with the cumulus flux parameterization proposed by Betts (1975). Cloud base mass flux and mass flux gradient are related to sub-cloud layer parameters and cumulus layer gradients using two parameters, α and β. The α symbol is the ratio of a model transition layer depth to the subcloud layer depth, and β is the ratio of a model cloud base static energy flux to the surface flux. For the simple case of a steady-state transition layer, the subecloud layer beat and moisture budgets are predicted.

Data from a field experiment over Venezuela are used to illustrate mean subcloud layer structure and to derive heat and moisture flux profiles and model parameters from a simple budget analysis. The data give (α,δ)=(0.11, 0.41) and correspondingly, (αv1βv based on virtual static energy fluxes and profiles. During the budget time period (centered on local noon over land) the subeloud layer war and dries with a corresponding rise of cloud base. The steady-state transition layer model predictions showed qualitative agreement with the fluxes derived from the budget. The extension of the convective mass flux model into the subeloud layer was shown to be feasible. It illustrates the rapid decrease of convective mass flux across the transition layer.

The paper concludes that the model is a satisfactory diagnostic tool for subcloud layer budgets and the cloud-subdoud layer interaction, and may have predictive value.

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