Turbulence statistics of a Kelvin-Helmholtz billow event observed in the night-time boundary layer during the Cooperative Atmosphere-Surface ExchangeStudy field program

An apparent shear flow instability occurred in the stably stratified night- time boundary layer on 6 October 1999 over the Cooperative Atmosphere-Surfa ce Exchange Study (CASES-99) site in southeast Kansas. This instability pro moted a train of billows which appeared to be in different stages of evolut ion. Data were collected by sonic anemometers and a high-frequency thermoco uple array distributed on a 60 m tower at the site, and a high resolution D oppler lidar (HRDL), situated close to the tower. Data from these instrumen ts were used to analyze the characteristics of the instability and the bill ow event. The instability occurred in a layer characterized by a minimum Ri chardson number Ri similar to 0.13, and where an inflection in the backgrou nd wind profile was also documented. The billows, which translated over the site for approximately 30 min, were approximately L similar to 320 m in le ngth and, after billow evolution they were contained in a layer depth H sim ilar to 30 m. Their maximum amplitude, determined by HRDL data, occurred at a height of 56 m. Billow overturns, responsible for mixing of heat and mom entum, and high-frequency intermittent turbulence produce kurtosis, values above the Gaussian value of 3, particularly in the lower part of the active layer. (C) 2001 Elsevier Science B.V. All rights reserved.