Airglow imaging of gravity waves 2. Critical layer filtering

Images of the variations of OH intensities have been recorded by a three-st ation network of scanning radiometers in southwestern Ontario during the pe riod from May until September 1998. The ensemble of gravity waves seen in a ll three instruments is analyzed and observed to have a strong dependence i n their propagation direction with the majority of waves seen at each of th e three sites having a horizontal wave vector in the northeast direction. I n order to understand these results a model of the transmission of gravity waves through the atmosphere was employed. The gravity waves observed by th e OH Scanning Radiometers (OHSCRs) are assumed to originate in the troposph ere with an isotropic spectrum, meaning that the average wave amplitudes ar e equal in all directions. The gravity waves propagate upward where they wi ll undergo a selection process owing to the effect of the "background winds ." Using the horizontal winds from the the Horizontal Wind Model (HWM-93), the transmission of gravity waves from a tropospheric source is modeled and shown to be consistent with the observations. The strong zonal winds accou nt for the summer eastward and winter westward propagation, but it is the t idal components of the meridional wind that account for the northward tende ncy of wave propagation (and the corresponding relative absence of southwar d propagating waves). In order to further validate the transmission, model winter measurements from the OHSCR data set are examined and shown to be co nsistent.