CLIMATOLOGY OF MESOSPHERIC GRAVITY-WAVE ACTIVITY OVER URBANA, ILLINOIS (40-DEGREES-N, 88-DEGREES-W)

A 5-year (1991-1995) climatology of mesospheric gravity wave activity observed with the Urbana medium frequency (MF) radar is presented. We focus on the monthly mean horizontal wind variances associated with gr avity waves with observed periods between 10 min and 8 hours. These re sults are compared with the seasonal characteristics from other gravit y wave observations. The total wave variance, <(u'(2))over bar> + <(v' (2))over bar>, displays a dominant semiannual variation (maxima summer /winter, minima spring/fall) below 87 km with little seasonal variatio n above 87 km. The zonal wave variance exceeds the meridional wave var iance, v'(2)/u'(2) < 1, except around regions of zonal wind reversal. In all seasons the observed variance scale height is greater than that of a nondissipating wave field. A simple zero mean wind model based o n linear saturation theory is found to be inadequate in reproducing th e seasonal characteristics of the observed wave variances. An alternat ive model based on critical layer filtering by a height dependent wind field can reproduce the observed semiannual oscillation in the wave V ariance given a specific source input.