INTERPRETATIONS OF OBSERVED CLIMATOLOGICAL PATTERNS IN STRATOSPHERIC GRAVITY-WAVE VARIANCE

Observational analyses of gravity waves in the stratosphere have revea led various climatological patterns in gravity wave activity. Seasonal , geographical, and vertical variations have all been observed. In thi s work, a linear model of gravity wave propagation is applied to inves tigate the underlying causes of some of the observed patterns. A colle ction of monochromatic gravity waves that represent a broad spectrum o f wavenumbers and frequencies is input at 6-km altitude in the model. Propagation of the waves through realistic background atmospheric wind and stability fields is treated with linear ray theory and a simple s aturation condition to limit amplitudes to stable values. The wave spe ctrum at the 6-km source height is specified to be constant at all lat itudes, longitudes, and times, so the variability that appears at high er altitudes is due entirely to background atmosphere variations. Befo re the model results are compared to the observations, the spectrum of waves is filtered in a way that mimics the limitations of each of the observation techniques. The filtering is described in terms of vertic al wavelength and is referred to as the ''observational filter.'' In a vertically varying background wind, gravity waves are Doppler-shifted in intrinsic frequency and refracted to different vertical wavelength s as they propagate vertically through the atmosphere. The observation al filter and the wave refraction effects can thus couple in interesti ng ways that have not been explicitly considered in previous work. The model shows that this coupling can give rise to geographical, seasona l, and vertical variations in gravity wave observations without any va riations in the spectrum or amplitude of gravity wave sources in the t roposphere. Thus careful consideration of both the background wind pro file and observational filter can greatly affect the interpretation of the observed climatological patterns in gravity wave activity.