PROPAGATION AND BREAKING AT HIGH-ALTITUDES OF GRAVITY-WAVES EXCITED BY TROPOSPHERIC FORCING

An anelastic approximation is used with a time-variable coordinate tra nsformation to formulate a two-dimensional numerical model that descri bes the evolution of gravity waves. The model is solved using a semi-L agrangian method with monotone (nonoscillatory) interpolation of all a dvected fields. The time-variable transformation is used to generate d isturbances at the lower boundary that approximate the effect of a tra veling line of thunderstorms (a squall line) or of flow over a broad t opographic obstacle. The vertical propagation and breaking of the grav ity wave field (under conditions typical of summer solstice) is illust rated for each of these cases. It is shown that the wave field at high altitudes is dominated by a single horizontal wavelength, which is no t always related simply to the horizontal dimension of the source. The morphology of wave breaking depends on the horizontal wavelength; for sufficiently short waves, breaking involves roughly one half of the w avelength. In common with other studies, it is found that the breaking waves undergo ''self-acceleration,'' such that the zonal-mean intrins ic frequency remains approximately constant in spite of large changes in the background wind. It is also shown that many of the features obt ained in the calculations can be understood in terms of linear wave th eory. In particular, linear theory provides insights into the waveleng th of the waves that break at high altitudes, the onset and evolution of breaking, the horizontal extent of the breaking region and its posi tion relative to the forcing, and the minimum and maximum altitudes wh ere breaking occurs. Wave breaking ceases at the altitude where the ba ckground dissipation rate (which in our model is a proxy for molecular diffusion) becomes greater than the rate of dissipation due to wave b reaking. This altitude, in effect, the model turbopause, is shown to d epend on a relatively small number of parameters that characterize the waves and the background state.