CORRELATIONS OF GRAVITY-WAVES AND TIDES IN THE MESOSPHERE OVER SASKATOON

Measurements in the Mesosphere and Lower Thermosphere (MLT), from the Saskatoon MF (medium frequency) radar (52 degrees N, 107 degrees W), a re used for the years 1992 and 1993 to study gravity waves (GW) and th eir variability. GW-hand time series (10-100 min; 2-6 h), their spectr a and their 12, 24, 48 h oscillations are used. Fifty days of data fro m three seasons are selected, which are themselves marked by strong di fferences in middle atmospheric winds and solar tides. Encouragingly c onsistent modulations of the GW variances at 12, 24, 48 h periods are noted during the summer months; for both short events (a few days) and for the entire 50 days (for the 2-6 h). The inferred GW propagation d irections (toward the NE) are internally consistent with calculations of GW propagation-directions using a new correlation method and:consis tent with independent measurements of gravity waves discussed in earli er articles from Saskatoon. In this season, all three oscillations are -comparable and moderately large in the hourly-mean winds. During the winter, when the 12 h tide is dominant, there is apparent modulation o f the GW variances for short events only, despite the size of the tida l wind oscillations (larger than summer). The inferred GW propagation directions are closest to eastward, although the inference is not stro ng, due to the weakness and variability-of the modulations. Surprising ly, in the autumn months, when the 12 h tide is at its annual maximum, the modulation is very weak. However, an event of a few days was iden tified when some consistency was identified. Spectra from a Lomb-Scarg le spectral analysis of the variances for nine years are also used to provide a climatology. There is dominance of peaks near 24, 12 and eve n 48 h during summer months with a much weaker tendency for peaks to o ccur in winter and autumn. The existence of peaks at 6 h, although pos sible when the propagation directions of the GW fluxes are isotropic, is actually a rare event. This also confirms earlier:results from Sask atoon, that indicate anisotropy of the GW directions is usually quite strong. The intermittent nature of the GW modulation at tidal periods suggests strongly that the wave sources are intermittent in strength a nd direction and that the background wind at lower-heights also contri butes variability. (C) 1998 Elsevier Science Ltd. All rights reserved.