Intradiurnal wind variations in the midlatitude and high-latitude mesosphere and lower thermosphere

Abstract

MF radar observations of neutral zonal and meridional wind components in the mesosphere and lower thermosphere region (78–98 km) made at Adelaide (35°S, 138°E) and Davis (68°S, 78°E) from January 1994 to December 1999 are analyzed with an emphasis on intradiurnal wind variations common to both stations. Periodograms were used to investigate the spectral content of the wave motions from 6 hours to 20 days and reveal the presence of a rich spectrum of waves. Motions due to tides and intradiurnal oscillations (6–15 hour periods) are particularly evident. Cross-spectral analyses were used to assess the global coherence of the intradiurnal oscillations. They are found to be short-lived (∼2–4 days) and have wave amplitudes in the range of 10–20 ms−1. The cross-spectral analyses revealed discrete intervals during which intradiurnal oscillations showed a high degree of coherence between the two stations. The cross-spectral phase differences suggest that oscillations with periods between 8 and 14 hours are westward propagating with zonal wave number 1. These oscillations are identified as the atmospheric manifestation of gravitational normal modes or Lamb waves.