Spectral Analysis and Diagnosis of Nonlinear Interactions of Large-Scale Moving Waves at 200 mb in the GLAS General Circulation Model

Abstract

Wavenumber-frequency spectral analysis of a 90-day winter (15 January - 15 April) wind field simulated by a climate experiment of the GLAS (Goddard laboratory for Atmospheric Sciences) atmospheric circulation model is made using the space-time Fourier analysis which is modified with Tukey's numerical spectral analysis. Computations are also made to examine the nonlinear interactions of model wave disturbances in the wavenumber-frequency domain linear interactions. Results are compared with observation, especially Kao and Lee's (1977) study. It is found that equatorial easterlies do not show up in this climate experiment at 200 mb. The zonal kinetic energy and momentum transport spectral of the model are generally in good agreement with observation. However, some distinct features of the model's spectra are revealed. The wavenumber spectra of kinetic energy show that the eastward moving waves of low wavenumbers have larger meridional motion compared with observation. Furthermore, the eastward moving waves show a band of large of spectral value in the medium-frequency regime. The frequency spectra in the high-frequency regime decrease faster than observation as frequency increases. The scheme proposed by Kao and Lee for the contribution to kinetic energy spectra by nonlinear interactions in middle latitudes is not applicable over the whole model globe because of the disappearance of equational easterlies. The contribution to momentum flux spectra by nonlinear interactions in Northern Hemisphere middle latitudes is similar to that of kinetic energy spectra. The primary nonlinear interactions of kinetic energy and momentum flux are contributed by those between mean zonal flow and long and medium waves with low and medium frequencies. The stationary waves do not play a significant role in the nonlinear interactions as found in observation.