VHF RADAR OBSERVATIONS OF MESOSCALE MOTIONS IN THE TROPOSPHERE - EVIDENCE FOR GRAVITY-WAVE DOPPLER SHIFTING

A VHF radar at Adelaide (35°S, 138°E) has been used to study small‐scale horizontal velocity fluctuations in the troposphere. The measurements have been analyzed in the frequency domain and show that the motions are often “polarized,” such that they show a preference to be aligned along the direction of the mean background wind. This azimuthal anisotropy is especially apparent at high observed frequencies and becomes more pronounced as the wind speed increases. The cause of the polarization is analyzed in terms of both the gravity wave and vortical mode explanations of the mesoscale wind fluctuations. It is shown that the observed behavior is very consistent with the predicted effects of Doppler shifting on a spectrum of atmospheric gravity waves by non zero background winds. The results, however, are inconsistent with the predicted behavior of a spectrum of vortical modes advected past a ground‐based observer. It is concluded that internal gravity wave motions dominate the energy of the tropospheric mesoscale wind field in these observations. These findings have a number of implications for studies which use ground‐based techniques to study gravity wave wind fluctuations in the frequency domain because Doppler shifting of wave energy can produce significant biases, especially if narrow frequency bands are used. This problem and possible solutions are discussed. Copyright 1990 by the American Geophysical Union.