Evidence of internal gravity waves in the lower thermosphere at midlatitudes

Abstract

Wind measurements were made by the CNR Meteor Radar Station at Budrio (45°N, 12°E) in the April–September 1978 period in order to emphasize the role of internal gravity (IG) waves in the lower thermosphere ((80÷110) km). After briefly describing the basic properties of these waves, we determine some of their characteristics using both the conventional spectral-analysis methods and alternatively, for short-time data series, the maximum-entropy method (MEM). The Fourier analysis of these data indicates that the gravity waves with periods of 2<T<8 h have amplitudes increasing with height (up to (30÷40) ms−1 at 100 km, in June), vertical wave-lengths of (8÷40) km and horizontal wave-lengths exceeding the corresponding ones by a factor of 50 or more. In May and September, by using the MEM method with a sampling of (10°20) min, gravity waves with periods of about 100 min and 2 h, respectively, have been tentatively identified. In August, the rates of energy loss per unit mass for internal gravity waves range from 0.015 to 0.045 W kg−1 in the (80°100) km region and the momentum deposition by these waves is 2.4 m s−1 h−1 at 90 km. Under some specific conditions, the observed accelerations of the prevailing wind ((3÷4) m s−1 d−1) due to momentum deposition over a period of 4 consecutive days (24–27 August) may approach the above-calculated maximum value.