Gravity Wave Sources, Source Variability and Lower and Middle Atmosphere Effects

Abstract

Considerable progress has been made in understanding gravity wave sources and variability in recent years. This paper reviews the source mechanisms believed to be most significant at this time and the evidence for source variability and atmospheric effects. The major sources of gravity wave motions at high frequencies include topography, convective and frontal activity, and wind shear and exhibit large variability in response to varying mean winds and meteorology. Data suggest that these sources lead to clear statistical enhancements of wave activity at greater altitudes, with occasional dramatic increases in wave amplitudes. Other sources are likely to be more important in exciting low-frequency motions, including geostrophic adjustment and local momentum or heat sources due to internal or external forcing. Gravity wave effects include, among others, flow accelerations and turbulent diffusion. Mean accelerations force residual circulations with small amplitudes at lower levels and large departures from radiative equilibrium conditions at greater altitudes. Forcing variability is large and likely has effects that penetrate to considerable heights, but these are poorly understood at this time.