Intermittent turbulence Events observed with a sonic Anemometer and Minisodar during CASES99

A sonic anemometer mounted on a tower at a height of 7 m and a collocated minisodar were used to investigate intermittent turbulence events at the Beaumont KS site of the Atmosphere Boundary Layer Experiments (ABLE) facility during the Cooperative Atmosphere Surface Exchange Study 1999 (CASES99) in October, 1999. Time series of fluctuating winds and temperatures were subjected to a filtering process to remove low frequency contributions such as waves and diurnal variations and heat fluxes were calculated from the resultant high-pass time series; intermittent turbulence events were then defined to occur when turbulent heat fluxes exceeded a threshold value. On nights with relatively light winds, intermittent turbulence was found to occur randomly both in time of occurrence and in duration. When minisodar estimates of temperature structure parameter for multiple heights between 10 and 200 m were processed in a similar manner, similar patterns of occurrence were found, particularly for the lowest few range gates; however there were also a few time periods during which there was elevated sodar signal but little turbulent heat flux. The height to which the intermittent turbulence defined from sonic anemometer data penetrated (as deduced from minisodar data) was found to vary significantly; possible dependencies on the strength of the surface turbulence and NBL depth are presented. Analysis of the minisodar data for the possibility of initiation of intermittent turbulence by elevated disturbances is presented.