SEASONAL-VARIATION OF GRAVITY-WAVE ACTIVITY IN THE LOWER ATMOSPHERE OBSERVED WITH THE MU-RADAR

In this paper we analyzed the seasonal variation of gravity wave activity in the troposphere and lower stratosphere (5-25 km) as revealed by monthly observations with the MU radar between December 1985 and December 1989. In the lower stratosphere, wind velocity variance due to gravity waves with periods of 5 min-21 hours showed a clear annual variation with a maximum in winter and a minimum in summer, agreeing fairly well with the seasonal variation of the jet stream intensity. This suggests that the excitation of gravity waves is closely related to the behavior of the background mean winds. The specific kinetic energy per unit volume of short-period gravity waves (ground-based period of 5 min-2 hours) was enhanced in winter and spring, its peak in the profile being located near the peak of the jet stream (similar to 12 km). On the other hand, the kinetic energy of the long-period (2-21 hours) component showed clear annual variation with a winter maximum and a summer minimum in the middle troposphere and lower stratosphere. The kinetic energy of the long-period component was smaller in the lower stratosphere than in the troposphere, which did not seem to penetrate the lower stratosphere. In the short-period component the ratio of the horizontal to the vertical wind variances near the jet peak height was observed to be large in winter and small in summer, probably due to the seasonal variation of the Brunt-Vaisala frequency. The wind variance ratio for the long-period component in the upper troposphere and lower stratosphere was greatly reduced in winter, possibly due to the Doppler shifting of gravity waves. The vertical flux of zonal momentum for the 5 min- to 21-hour period component showed negative values in the upper troposphere and lower stratosphere, implying that the upward propagating gravity waves in the lower stratosphere mostly traveled westward relative to the background wind. We did not find any significant seasonal variations in the vertical flux of meridional momentum.