Spectral Structure of Five-Year Time Series of Horizontal Wind Speed at the Boulder Atmospheric Observatory

We investigate the spectral structures of 5-yr, 1-min time series of horizontal wind speeds at 100 and 10 m heights at the Boulder Atmospheric Observatory tower located in the eastern slope of the Rocky Mountains, U.S.A. In the full-scale spectra, the diurnal spectral peak, which is usually insignificant at a coastal or offshore site, is the most significant at both heights. The spectrum is enhanced on the low frequency side of the diurnal peak during winter, but on the high frequency side during summer, which indicates frequent synoptic weather events during winter supplanted by mesoscale events during summer. In terms of the spectral density in the spectral gap of Van der Hoven [1957], separating boundary layer turbulence from the synoptic-scale fluctuations, at a frequency between 10^-4 and 10^-3 Hz, we rank the daily time series at 100 m height, and sample the summer top and winter bottom 10 percentile cases. The winter cases of the reduced spectral density in the gap region present the f^-3 spectrum (f is frequency) and negatively skewed velocity increment distributions, which are the signatures of enstrophy (the integral of squared vorticity) cascade of turbulent two-dimensional (2D) flows. In contrast, the summer cases of the enhanced spectral density present the f^-5/3  spectrum and positively skewed velocity increment distributions, which are the signatures of upscale energy cascade of 2D flows. In these mesoscale events that fill up the gap, the turbulence intensity - wind speed relationship is very sensitive to the choice of the averaging period.