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

A fairly simple stratospheric model of the three longest planetary waves 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 negative. In order to allow comparison with the observed behaviour of planetary waves the model is forced near the tropopause with observed wave amplitudes and the model waves propagate over the observed zonal-mean state. The model is able to mimic the behaviour of the observed waves fairly accurately, with the thermal damping, interference and wave breaking terms all being important to its success. Good results are obtained with a Value of local wave-breaking diffusion coefficient between 10(6) and 10(7)m(2)s(-1). It is shown that the deficiencies in Matsuno'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 absorbs 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 sometimes has large values within the vortex, as well as in the midlatitude surf zone, with the two regions of diffusion being separated by the vortex edge. The difficulty in estimating the diffusion coefficient required for a tracer is discussed, as well as the behaviour of the zonal-mean potential vorticity flux during a wave-breaking episode. The assumption of turbulent diffusion in reversed-gradient regions implies a positive diffusive contribution to the potential vorticity flux. It 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 did not depend strongly on a movement of the tropical zero-wind line.