Studies of gravity-wave filtering and tidal interaction in the middle atmosphere

Abstract

Based on the spectral character of the observed gravity wave field, a parameterization scheme for gravity wave propagation and effects in the lower and middle atmosphere has been proposed. This effort begins with spectral expressions for gravity wave energy and momentum fluxes and prescribes the manner in which such idealized spectra respond to variations in the gravity wave environment. Means of specifying spectral amplitudes in response to specific wave sources at lower levels and the spread of wave influences horizontally as the spectrum propagates vertically through the lower and middle atmosphere are suggested. The parameterization scheme relies on a separable and invariant spectral form that is consistent with observations throughout the atmosphere. By choosing representative gravity wave parameters, the scheme yields predictions of mean fluxes of momentum for winter and summer CIRA 1986 model atmosphere profiles that agree well with mean flux observation in the lower and middle atmosphere. In the troposphere and lower stratosphere, both winter and summer profiles imply a negative zonal mean flux of $\sim$0.2$m\sp2s\sp{-2},$ consistent with airborne and radar measurements and the needs of large-scale GCM models. In the mesosphere, mean fluxes tend to oppose the zonal mean wind with flux maxima above the wind maxima and vertical profiles in general agreement with radar estimates at these heights. To assess the importance of gravity wave-tidal interactions in the middle atmosphere, the parameterization scheme is further applied to a static mean wind plus a typical tidal wind structure. The results suggest that the gravity wave-tidal interaction is likely to be strong where tidal amplitudes become large, yielding a significant modulation in the anisotropy of the gravity wave field due to differential filtering and saturation processes. Consistent with observations, this filtering leads to wave momentum fluxes that are approximately anti-correlated with the tidal wind field and which may far exceed mean values in magnitude.