Discrete Spectra and Continuous Spectrum of the Barotropic Quasi-Geostrophic Model-A Calculation of Meteorological Data

Atmospheric disturbances at 300 hpa are decomposed into normal modes, referred as discrete-spectrum disturbances which can propagate freely in the observed zonal mean flow, and non-modal transient disturbances, referred as continuous-spectrum disturbances which are continuously sheared and eventually absorbed by the zonal flow. It is shown that normal modes represent only a small fraction of the observed atmospheric disturbances, while continuous-spec-trum disturbances represent the majority of observed disturbances, even when the basic flow is unstable. Daily variabilities of the observed continuous-spectrum disturbances are presented. They are shown to follow the results of wave-packet theory. Calculations suggest that there are abundant sources to excite continuous-spectrum disturbances in the atmosphere.