Comparative observations of short‐period gravity waves (10–100 min) in the mesosphere in 1989 by Saskatoon MF radar (52°N), Canada and the MU radar (35°N), Japan

Abstract

We analyzed the wind variances of gravity waves during 10–100‐min periods observed in the mesosphere at 67–80 km altitude with the MU radar at Kyoto (35°N, 136°E) and the MF radar at Saskatoon (52°N, 107°W) in 1989. In order to compare the results quantitatively we applied similar data analysis procedures to the data sets which were obtained with different observation techniques. We first calculated frequency spectra of wind velocity, then estimated variances, , , and by integrating the spectra. The frequency spectra of the horizontal wind u′ and υ′ generally showed a spectral slope of about −5/3 at both locations. The spectral slope of w′ at Saskatoon ranged from −2/3 to −1 in winter, and −1/3 to −1/2 in other seasons, while it became nearly zero in May and September at both locations. The slope for the MU radar observations was less steep and sometimes became +0.15–0.3 in the frequency region lower than 50–70 day−1 (periods more than 20–30 min). The variance, , , and , showed semiannual variation with a large maximum in summer, a secondary peak in winter, and minima in September and April. The annual average of the ratios of , , and between the MU and Saskatoon MF radars was 1.6, 1.5, and 1.6. This result is interpreted as suggesting that the variance of gravity waves with periods of 10–100 min at 35°N is stronger than that at 52°N, even if the possible biases are taken into account. The variances due to gravity waves are more conspicuously enhanced in summer at Kyoto than at Saskatoon.