Seasonal and diurnal changes in wind variability from Flatland VHF profiler observations

Climatological results are presented on the hourly variance of the wind observed in the midtroposphere (3 to 9 km MSL). This quantity roughly indicates the energy in the atmospheric wind field for variations with periods roughly less than 1 hour. Observations are from the Flatland VHF research wind profiler, located near Champaign/Urbana, Illinois, well away from significant orographic features. The period of record covers two years, September 1990 through August 1992. The values of the variance of the winds along vertical and oblique (15 degrees from zenith in the cardinal directions) beams are presented versus height, season, time-of-day, and beam pointing direction. It is found that the hourly variance values have approximately lognormal frequency distribution. The mean hourly variance is significantly larger for the oblique wind observations than for the vertical. Mean wind variances also tend to be larger in the east/west steering plane than in the north/south plane. The mean variance generally increases with height, but faster than would be expected if it were due solely to the decrease in atmospheric density, implying the presence of local source/sinks of wind energy. The rate of change with height is noticeably different for the vertical and oblique beams, being much less for the vertical beam, in some seasons even decreasing with height. With respect to season, the mean hourly variance is smallest in the summer and largest in the winter. With respect to diurnal changes, the variance is maximum during the afternoon for spring, summer, and autumn, with the maximum up to a factor of two larger than the minimum. In winter, the diurnal change is much smaller, with little indication of an afternoon maximum.