Observations of Turbulence in Stratified Flow

Abstract

Various theoretical properties of the structure function are evaluated. Additional functions are constructed to describe the overall influence of stratification, the anisotropy and intermittency of the turbulence, and the asymmetry of the main drafts. These functions and the usual spectral decomposition are computed from aircraft-measured turbulence data collected in nocturnal boundary layers and in turbulence over mountainous terrain. Certain features of the turbulence are found to depend more on stability than on the external situation. Three general types of turbulence are found: 1) intermittent turbulence driven by shear-driven overturning; 2) continuous turbulence where strong drafts in the presence of shear are characterized by sharp boundaries or microfronts, particularly on their upstream sides; and 3) weaker continuous turbulence. The costructure fields are generally consistent with vertical gradient transfer. However, the bora turbulence on larger scales is dominated by horizontal motions perpendicular to the mean shear.