A REALISTIC 3-COMPONENT PLANETARY WAVE MODEL, WITH A WAVE-BREAKING PARAMETRIZATION

A fairly simple stratospheric model of the three longest planetary wav es is constructed, which includes interference terms between the three wave components, and a diffusive parametrization of wave breaking in regions where the local meridional potential vorticity gradient is neg ative. In order to allow comparison with the observed behaviour of pla netary waves the model is forced near the tropopause with observed wav e amplitudes and the model waves propagate over the observed zonal-mea n state. The model is able to mimic the behaviour of the observed wave s fairly accurately, with the thermal damping, interference and wave b reaking terms all being important to its success. Good results are obt ained with a Value of local wave-breaking diffusion coefficient betwee n 10(6) and 10(7)m(2)s(-1). It is shown that the deficiencies in Matsu no's (1970) model are probably due mainly to the neglect of wave-wave interactions and an over-simplified zonal-mean temperature structure. It is shown that Garcia's (1991) idea that wave breaking completely ab sorbs any build-up of zonal-mean wave activity is probably too severe, but that better results are obtained if the local nature of the wave- breaking is taken into account. The zonal-mean diffusion coefficient s ometimes has large values within the vortex, as well as in the midlati tude surf zone, with the two regions of diffusion being separated by t he vortex edge. The difficulty in estimating the diffusion coefficient required for a tracer is discussed, as well as the behaviour of the z onal-mean potential vorticity flux during a wave-breaking episode. The assumption of turbulent diffusion in reversed-gradient regions implie s a positive diffusive contribution to the potential vorticity flux. I t was also found that changes in the equatorial zonal wind result in a modulation of the extra-tropical potential vorticity flux similar to that found by Dunkerton and Baldwin (1991), but that the modulation di d not depend strongly on a movement of the tropical zero-wind line.