Gravity wave spectra and direction statistics for the mesosphere as observed by MF radars in the Canadian Prairies (49°N–52°N) and at Tromsø (69°N)

Abstract

Observations of winds and gravity waves by MF radars in the Canadian Prairies (49°N–52°N) and Tromso (69°N) are used to provide frequency spectra and spectral variances of horizontal motions, and information on the predominant azimuthal directions of propagation for the waves. Data for several years (1992–1995) and two height layers (61–73 km, 76–88 km) in the mesosphere are considered. The variations of the monthly frequency spectra show strong universality of slope throughout the year at all locations, with slopes near −5/3 at upper heights in winter months, but increasingly smaller (absolute) slopes at higher frequencies (periods τ < 2 h) in summer months. At the lower heights, and at all frequencies, smaller slopes are observed. The power spectral densities, and spectral variances from band-pass filters (period-bands 10–100 min, 1.5–6h) show distinctive changes with latitude and locations, consistent with changes of background-mean zonal winds and gravity wave sources; at all sites semi-annual oscillations (SAO) dominate the 10–100 min band, and annual oscillations the 1.5–6h band. This is a Doppler-shifting phenomena. The orientations of the ellipses or ovals (which are fitted to the wind perturbations) also show significant monthly changes with location, consistent with changing directions for gravity wave propagation. Generally Saskatoon (52°N) is dominated by meridionally propagating waves, while at Robsart (49°N) and Tromso (69°N) zonal propagation dominates. The largest spectral variances also occur at these two latter locations.