The Nonlinear Dynamics of Cyclone Waves

Abstract

The development of atmospheric cyclones is studied from the viewpoint of the instability of large-scale wave perturbations superimposed on a zonal current. The stability properties of the observed mean January flow are investigated and the linear results are extended to include the effects of nonlinear processes on the growth of a cyclone-scale wave. An hemispheric model is employed in this investigation and solutions are obtained by spectral techniques. It is found that the observed atmospheric zonal current is highly unstable in a hydrodynamic sense. The instability is of a baroclinic character with barotropic stabilizing effects. The nonlinear computations show that the growth of the most unstable waves is brought to a halt when the perturbation kinetic energy reaches a level consistent with atmospheric observations. The barotropic energy exchanges are found to play a major role in this process by feeding a large amount of kinetic energy into the zonal flow when the baroclinic energy conversions reach a maximum. The damping effect of the nonlinear processes on the growth of the unstable wave is found to be slightly reduced when the horizontal resolution of the model is increased in either zonal or latitudinal directions.