Characteristics of Atmospheric Turbulence in Near Neutral Stability

Abstract

The published data on the characteristics of atmospheric turbulence are critically reviewed and analyzed in statistical terms. Assumptions of stationarity and homogeneity are examined. It is shown that in the rower part of the boundary layer the rims values of the fluctuating velocity components show little or no variation with height. The numerous proposed models for turbulence spectra of the three components arc critically discussed in the light of Similarity theory and it is concluded that the von Karman <i>[Proc. natn. Acad. Sci. U. </i> S. <i>A., </i> 34 (1948), 530] model provides the best fit with the available full scale data for the longitudinal and the vertical components and the Kaimal <i>[Boundary Layer Met., </i> 4 (1973), 289] model is the most general one and is based on the least amount of empiricism for the lateral component. In view of the large range of the sizes of eddies a number of definitions of scale have been proposed which are summarized here. It is noted that these scales, in general, are functions of height, thermal stability and surface conditions and this dependence is discussed quantitatively. Further, it is shown that the length scales are very sensitive to the shape of the spectrum assumed. Various methods of determining the length scales are discussed. Finally, validity of Taylor's hypothesis in shear flows is studied semi-empirically. It is tentatively concluded that Taylor’s hypothesis should be generally valid for turbulence fluctuations, even in shear flows.