A Numerical Study of Gravity Wave Saturation: Nonlinear and Multiple Wave Effects

Abstract

In this study we examine some of the effects of wave-wave interactions and convective adjustment on the propagation of gravity waves in the middle atmosphere. For both a nearly monochromatic wave and a super-position of waves, nonlinear wave-wave interactions, while reducing primary wave amplitudes somewhat, are found to be unable to prevent the formation of convectively unstable layers. In contrast, convective adjustment of the wave field causes significant amplitude reductions, resulting in amplitudes for a spectrum of wave motions that achieve only a fraction of their monochromatic saturation values. Neither process is found to cause a major disruption of the primary wave field. Both wave-wave interactions and convective adjustment are found to excite harmonies of the primary wave motions. Excitation by convective adjustment appears to dominate for a monochromatic wave, whereas both processes become important for a spectrum of wave motions. In each case, the characteristics of the excited wave motions (i.e., phase tilt, intrinsic frequency, and direction of propagation) are found to be largely consistent with those of the primary waves. These results are seen to be in qualitative agreement with atmospheric observations.