A Semi-Empirical Dynamical Model for the Trade-Wind Convective Layer

Man-Kin Max Laboratory for Atmospheric Research, University of Illinois, Urbana, 61801

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Abstract

It is shown that it is feasible to investigate theoretically the steady state local structure of the trade-wind convective layer with semi-empirical models by systematically synthesizing the various published results on the mass and energy budgets for the Atlantic region. A thermodynamic model was first developed to clarify how the thickness and the virtual potential temperature of the layer depend upon the various parameters. Realistic results were obtained. A dynamical model was later constructed to examine the role of the vertical fluxes of momentum in addition to the thermodynamic processes.

It was found that there are five key adjustable parameters, namely the Coriolis parameter, the imposed interior flow, the surface horizontal divergence, the radiational cooling, and the heating due to precipitation. The variations of the structure of the convective layer with these five parameters and their physical implications are discussed. In particular, the cross-isobar flow was found to have a relatively maximum value at an intermediate latitude, implying a preferred latitude of frictionally induced mass convergence. Furthermore, systematic changes in the relationship between the momentum flux at the surface and the corresponding ones at the top of the convective layer are identified. It was found that while information on this relationship may be used to parameterize the momentum flux at the inversion level in terms of the surface value when only one parameter is varied, there does not seem to be a simple parameterization in general.

Abstract

It is shown that it is feasible to investigate theoretically the steady state local structure of the trade-wind convective layer with semi-empirical models by systematically synthesizing the various published results on the mass and energy budgets for the Atlantic region. A thermodynamic model was first developed to clarify how the thickness and the virtual potential temperature of the layer depend upon the various parameters. Realistic results were obtained. A dynamical model was later constructed to examine the role of the vertical fluxes of momentum in addition to the thermodynamic processes.

It was found that there are five key adjustable parameters, namely the Coriolis parameter, the imposed interior flow, the surface horizontal divergence, the radiational cooling, and the heating due to precipitation. The variations of the structure of the convective layer with these five parameters and their physical implications are discussed. In particular, the cross-isobar flow was found to have a relatively maximum value at an intermediate latitude, implying a preferred latitude of frictionally induced mass convergence. Furthermore, systematic changes in the relationship between the momentum flux at the surface and the corresponding ones at the top of the convective layer are identified. It was found that while information on this relationship may be used to parameterize the momentum flux at the inversion level in terms of the surface value when only one parameter is varied, there does not seem to be a simple parameterization in general.

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