Statistical analysis of gravity waves observed with the middle and upper atmosphere radar in the middle atmosphere .1. Method and general characteristics

A statistical procedure is developed to obtain the parameters of spectral harmonics of internal gravity waves (IGW) with horizontal wavelengths larger than similar to 60 km using measurements of wind velocity in the middle atmosphere from the middle and upper atmosphere (MU) radar. The method is applied to find sets of IGW parameters for the winter and summer seasons of 1987-1988 for 6-km-thick layers centered at 70, 75, and 80 km at Shigaraki, Japan (34.9 degrees N, 139.4 degrees E). The numbers of selected IGW harmonics having periods from 5 min up to 6 hours vary between 360 and 2000 depending on the season and altitude. Mean vertical wave numbers are 0.6-0.9 km(-1) in winter and 0.5-1 km(-1) in summer, and horizontal wave numbers are (3-4)x10(-2) km(-1) and (2-4)x10(-2) km(-1), respectively. Predominant azimuths of wave vectors and momentum fluxes concentrate near 64 degrees in winter and 90 degrees in summer at all altitudes. In general, the difference between the azimuths of the wave vector and momentum flux is close to zero or 180 degrees. Turbulent and instrumental noise can contribute 20-30% of the observed intensity of the wind variations at altitudes of 70-85 km. The IGW momentum fluxes are 0.2-2.7 m(2)s(-2) in winter and 0.2-1.9 m(2)s(-2) in summer. The wave drag of the mean flow is 0.5-45 m(2)s(-1) in winter and 2-25 m(2)s(-1) in summer. In most cases the momentum flux and wave drag are directed oppositely to the mean wind.