On the structure of atmospheric waves in middle latitudes

A. WIIN NIELSEN

Abstract

The investigation described in the present paper is concerned with the structure of atmospheric waves in the middle latitudes. The typical wave in the troposphere has, according to observational studies, a difference between the thermal field and the geopotential field in such a way that the trough and the ridge in the temperature field are located to the west of the corresponding curves for the geopotential field. A theory using a linear version of the two-level, quasi-nondivergent model including heating and dissipation is developed to explain these observed facts. Such a model will contain the possibility of baroclinic instability for certain wavelengths for a sufficiently large vertical windshear. It is thus necessary to treat the stable and the unstable cases separately. The applied techniques include both a numerical integration to steady state in the stable cases and a detailed treatment of the perturbation equations in the unstable case. A calculation of the vertical velocity is included in order to show that these waves convert eddy available potential energy to eddy kinetic energy. The model equations are simplified in such a way that the independent variables are the variations both in the zonal direction and in time. The model permits a calculation of the meridional transport of sensible heat, but due to its simplicity the meridional transport of momentum is excluded. The calculated structure of the wave is in good agreement with the observed atmospheric waves in middle latitudes.

Keywords

Atmospheric waves; middle latitudes; thermal and geopotential fields

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