Integral Scales for the Nocturnal Boundary Layer. Part II: Heat Budget, Transport and Energy Implications

Roland B. Stull Department of Meteorology, University of Wisconsin, Madison, WI 53706

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Abstract

In Part I, external forcings such as pressure gradient, terrain roughness and imposed cooling were used to forecast the thickness and strength of an exponentially-shaped (ES) nocturnal boundary layer (NBL) temperature profile. In Part II, it is suggested that the evolution of the ES temperature profile can be explained by simple models for background radiative, surface-induced radiative, and turbulence contributions to the total cooling. One partitioning model sets the ratio of turbulent to surface-induced radiative components to be a constant (∼3.35). The exponentially-shaped heat-flux profile implied by that ratio agrees favorably with the Minnesota field experiment profile of Caughey et al. Differences between an ES and a mixed-layer (ML) model for the NBL am presented using potential energy (PE) arguments, where a thinner ML yields the same PE change as a thicker ES. Differences are also apparent using eddy diffusivity (K) theory, where the bulging K-profile for a ML is dissimilar to the linear K-profile found for an ES. The implications of using velocity scales from Part I with the PE calculations done here are that over 90% of the turbulence kinetic energy is dissipated by viscosity, as opposed to smaller percentages suggested by others.

Abstract

In Part I, external forcings such as pressure gradient, terrain roughness and imposed cooling were used to forecast the thickness and strength of an exponentially-shaped (ES) nocturnal boundary layer (NBL) temperature profile. In Part II, it is suggested that the evolution of the ES temperature profile can be explained by simple models for background radiative, surface-induced radiative, and turbulence contributions to the total cooling. One partitioning model sets the ratio of turbulent to surface-induced radiative components to be a constant (∼3.35). The exponentially-shaped heat-flux profile implied by that ratio agrees favorably with the Minnesota field experiment profile of Caughey et al. Differences between an ES and a mixed-layer (ML) model for the NBL am presented using potential energy (PE) arguments, where a thinner ML yields the same PE change as a thicker ES. Differences are also apparent using eddy diffusivity (K) theory, where the bulging K-profile for a ML is dissimilar to the linear K-profile found for an ES. The implications of using velocity scales from Part I with the PE calculations done here are that over 90% of the turbulence kinetic energy is dissipated by viscosity, as opposed to smaller percentages suggested by others.

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