The evolution of a sub-cloud layer structure associated with a severe storm-a case study

Using meteorological tower data from the NSSL (National Severe Storms Laboratory) in Oklahoma, evolutionary characteristics of a turbulent boundary layer are examined during the pre-storm and storm periods on May 20, 1977. Analyses include the vertical profiles of mean and turbulent quantities of wind, temperature, mixing ratio, momentum, and sensible and latent heat fluxes for the pre-storm and storm periods. The results indicate that a substantial modification occurs in the character of the lower boundary layer in each of these periods.

In the pre-storm (no precipitation) the boundary layer is characterized by a weakly stratified mixed layer; the energy source is found located in the surface layer, and the dominant term in the turbulent budget equation is shear generation. During the storm period, the mixed layer yields to a stable boundary layer, due to a net cooling effect associated with the storm-scale motions at lower levels. The structure of the boundary layer is mainly determined by the storm-scale fluxes originating from the cloud layer.