Vertical Structure and Turbulence in the Very Stable Boundary Layer

Abstract

The structure of turbulence in a strongly stratified nocturnal boundary layer is studied using fast-response aircraft data collected under clear sky conditions with weak ambient flow. The principal source of turbulence is shear generation near the top of the surface inversion layer. This shear is induced by the development of surface flow which appears to be cold air drainage. The downward heat flux in the turbulent shear zone acts to warm the upper part of the inversion layer and is opposed by clear-air radiative cooling and advection of cold air. The horizontal structure of the turbulence is studied using conditional means and other properties of the joint frequency distribution and analysis of the eigenvectors of the correlation matrix. Often, the turbulence exhibits statistical properties consistent with shear instability on horizontal scales near 300?400 m. Although modified by stratification, the main motions are turbulent-like with sharp horizontal boundaries and lead to net downward heat transport. The overturning also appears to lead to some buoyancy instability on somewhat smaller scales. The smaller scale turbulence is more three-dimensional but some influence of stratification is evident on horizontal scales even smaller than 100 m.