A SPECTRAL APPROACH FOR STUDYING MIDDLE AND UPPER ATMOSPHERIC PHENOMENA

We report on the application of a newly developed spectral code to the study of the middle upper atmosphere. The spectral approach offers co nceptual and practical convenience for analyzing the generation and in teraction of different components of atmospheric activity through the decomposition of the dynamical fields into components with different z onal wave numbers (m). As examples and tests. we obtain solutions for the m = 0 (the mean circulation) and 1 (the diurnal tides) components separately (no mutual interactions), as well as the m = 1 component un der the influence of the mean circulation. By simulating gravity wave effects with Rayleigh friction and eddy diffusion peaking near 90 km a ltitude, the mean circulation thus generated can reproduce the observe d mesospheric temperature anomaly under solstice conditions. The compu ted diurnal tides are in good agreement with results obtained earlier by other authors. The large temperature gradient (associated with the m = 0 component) set up by the mesospheric temperature anomaly under s olstice conditions creates a condition favorable for the development o f baroclinic instabilities in the mesopause layer, especially near the summer pole. In our time dependent calculation, waves with approximat ely 4-day period are generated in the m = 1 component, superimposing w ith the 1-day period tides.