Spatial and temporal occurrences of intermittent turbulence during CASES-99

Occurrences of intermittent turbulence in very stable conditions during the CASES-99 field study near Leon, Kansas were detected at several sites separated by horizontal distances from 1 km to 25 km using sonic anemometers, minisodars and a laser scintillometer. Periods with significant turbulent heat fluxes were separated by extended quiescent periods with little or no flux, and most of the flux during a night was realized in relatively small fractions (<20%) of the total time. There appeared to be no relationship between this intermittency fraction and the median z/L (z being height and L the Obukhov length) value for the night, although overall sensible heat flux values on very stable nights were significantly less than those on less stable nights. The intermittency fraction at 7 m was found to increase with mean wind speed at 20 m and, to a lesser extent, with wind shear between 20 m and 30 m. While correspondence of turbulent episodes at two sites separated by 1 km was common, it was less common at separations on the order of 20 km. There were time periods, however, during which enhanced turbulence levels were seen nearly simultaneously at large separation distances. Turbulence episodes were found to propagate upward or downward at different times with no readily defined large-scale controlling mechanism.