A statistical climatology of mesospheric gravity wave activity over Urbana

Five years (1991-1995) of quasi-continuous medium frequency (MF) radar observations collected at the Urbana Atmospheric Observatory are presented. These observations are analyzed from the viewpoint that the mesospheric wind perturbations with temporal scales between the buoyancy and inertial frequencies are dominated by a spectrum of propagating gravity waves. An alternative approach, time-domain-interferometry (TDI), to estimating the vector winds using the measured radial velocity and angle-of-arrival of the scattered signal is investigated and extended to the estimation of wind variances and momentum flux. Comparisons between the TDI and the more traditional spaced antenna full correlation analysis (SA-FCA) technique highlight instrumental (receiver saturation, antenna beam tilt) and processing (filtering, rejection criteria) biases inherent in the two techniques. Regardless of the differences between techniques, they both estimate wave variances with scale heights that imply a partially to severely saturated gravity wave field, which surprisingly appears to become less saturated with height. These variances are seen to have a repeatable seasonal structure, semiannual below 87 km and nearly constant above, across the five years of observations. A simple model based on linear saturation theory is found to be inadequate in reproducing the observed wave variances. However, it is also shown that critical layer filtering by a height dependent wind field could be important in controlling the seasonal characteristics of the variance of the wave field.Ope