Short period fluctuations of the horizontal wind measured in the upper middle atmosphere and possible relationships to internal gravity waves

Abstract

Horizontal wind fluctuations between heights of 61 and 105 km as obtained from the M.F. radar measurements at Saskatoon (52N, 107W) are analyzed with respect to their direction and intensity in two period ranges (about 10–60 min and 1–6 h). Frequency distributions of horizontal perturbation directions and monthly mean r.m.s. deviations of the horizontal wind are presented for both period ranges. Strong annual, semiannual and interannual variations of the wind variability dependent on height are evident. Wind perturbation directions exhibit pronounced anisotropies, especially a clear N-S maximum in the short period range which markedly varies with season. The observations are discussed in the framework of gravity wave theory. The tendency to a preferably north-southward orientation of wind fluctuation is attributed to directional filtering of transient gravity waves with phase velocities deviating from zero in the prevailing zonal wind below the height range of observation (below 60–70 km). Seasonal changes of gravity wave source strength and distribution are inferred from the Saskatoon direction statistics. Previously recognised mean meridional wind reversals with height appear to be consistent with the N-S polarization of the gravity wave field above the mesopause. The shorter and longer period fluctuation intensities show significant differences of their seasonal variation in the mesosphere. Eddy dissipation rates estimated from horizontal wind fluctuations also exhibit regular changes with height and season. They show a clear semiannual oscillation at heights below about 95 km and a dominant annual variation above with maximum values in the winter.