Frequency spectra of vertical velocity from Flatland VHF radar data

Abstract

The vertical wind velocity over very flat terrain was observed every 153 s in the troposphere and lower stratosphere by the Flatland radar, near Champaign‐Urbana, Illinois. Several hundred frequency spectra were calculated from all accepted 6‐hour time series from March through May 1987. By stratifying the spectra in various ways we find the following: (1) The spectra were independent of altitude within the troposphere or lower stratosphere, but the spectra in the two regions differed in amplitude and frequency; (2) At a given altitude the spectra were independent of the wind shear dū/dz, the buoyancy frequency N, and the maximum wind speed below 16 km; (3) The change of spectral shape and amplitude with increasing background wind speed ū was much less than at stations near mountains. The variance of the spectra, equal to twice the vertical kinetic energy per unit mass, roughly doubled as ū increased by 10 m s−1; (4) The spectra were consistent with being due to a spectrum of gravity waves, as indicated by the sharp drop in spectral amplitude near N at small ū and by the fact that the observed change of shape with increasing ū was quite consistent with the change of shape of model Doppler‐shifted gravity wave spectra; (5) The results of comparison between the observed and model spectra are consistent with an intrinsic gravity wave spectrum that is invariant with ū, dū/dz, etc., contrary to expectations from gravity wave theory; (6) The results are insensitive to the azimuthal distribution of gravity wave energy, as long as the distribution is roughly symmetrical relative to the mean flow; (7) The resulting characteristic horizontal phase velocity c* of the intrinsic frequency spectrum was about 6 m s−1 in both the troposphere and the stratosphere. The corresponding characteristic vertical wavelengths were about 3300 and 1800 m, respectively, consistent with previous estimates.