THE BEHAVIOUR OF PLANETARY WAVES IN AN ATMOSPHERIC MODEL BASED ON SPHERICAL HARMONICS.
MCGILL UNIV MONTREAL (QUEBEC) ARCTIC METEOROLOGY RESEARCH GROUP
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The grid point method commonly used in numerical calculations presents serious problems in experiments that require a global coverage of the meteorological variables. The shape of the earth and the form taken by the meteorological equations in a system where longitude and latitude are the basic coordinates, suggest the use of spherical harmonics for the horizontal specification of the variables. This method eliminates grid points and all the truncation errors due to the finite difference approximations. It also permits the retention of all the terms in the meteorological equations including those that would normally exhibit an anamalous behaviour near the poles. A model based on five levels and 15 coefficients was integrated for 200 days starting from an atmosphere at rest. The integration was then continued for another 20 days with 45 coefficients. Cross-sections show a jet stream in each hemisphere and low level easterlies along the equatorial belt. The amplitudes, the phase speeds and the structure of the planetary waves in the model compare favourably with their atmospheric equivalents. The results of this integration indicate that spherical harmonics could be used profitably in general circulation models and for the preparation of extended range forecasts. Author