Average statistical characteristics of long gravity waves observed with the middle and upper atmosphere radar in the mesosphere

Average statistical characteristics of internal gravity waves (IGWs) with h orizontal wavelengths longer than 60 km are obtained at the altitudes 65-80 km from the observations with the middle and upper atmosphere radar at Shi garaki, Japan (35 degrees N, 136 degrees E) in 1986-1995. The numbers of si gnals selected with narrow frequency band fillers attributed to IGW having periods 0.1-6 hours vary between 870 and 2300 for different seasons and alt itudes. Long-period IGWs have larger amplitudes and larger vertical wavelen gths than short-period IGWs. For waves with periods 0.1-6 hours the mean va lues of the horizontal and vertical wavelengths are 100 - 300 km and 6 - 10 km, respectively. IGW amplitudes and momentum fluxes have maxima in winter and summer and minima in spring and autumn. Distributions of the azimuths of IGW horizontal wave vectors and momentum fluxes are nearly isotropic wit h maxima in zonal directions in summer. The westward momentum flux is large r than the eastward Aux in winter and is smaller than that in summer. The d ifferences between eastward and westward wave momentum fluxes are larger fo r long-period IGWs. The net zonal IGW momentum flux is mainly directed oppo site to the mean wind in the mesosphere.