A numerical investigation of a simple spectral atmospheric model



A two-level quasi-nondivergent model containing 12 spectral components on a rectangular beta-plane is used to simulate a number of atmospheric phenomena. The nonlinear model contains two components that describe the zonal flow at each level permitting zonal winds with two maxima and two minima. The eddy fields at the two levels contain four components selected in such a way that the resulting eddy fields have transports of both sensible heat and momentum. The model permits a full description of energy generations, conversions and dissipations, because the eddy components are selected with such wave numbers that interactions take place between, the eddies and the zonal fields. Diabatic heating, topographical effects and dissipation of kinetic energy are included in the model. A limitation of the model is that it contains only one wave number in the zonal direction. The model is used to illustrate nonlinear developments of baroclinic waves on various horizontal scales in a case of forcing on the zonal components alone. With a long channel it is possible to simulate the development of long stationary waves forced by topography and/or heating. For special definitions of the heating on both the zonal and the eddy modes one may simulate the formation and maintenance of blocking situations as a result of interactions between the zonal components and the eddies. The eddy components will normally go into periodic or almost periodic motion in the phase domain unless the model is forced by heating, topography and friction. These unforced motions and their periods are investigated. We also show that the type of atmospheric circulation may change significantly as a function of the position of the maximum heating in the south-north direction, illustrating a change from single to double jets and the resulting change in the intensity and position of the waves.

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