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, refer red as discrete-spectrum disturbances which can propagate freely in the obs erved zonal mean flow, and non-modal transient disturbances, referred as co ntinuous-spectrum disturbances which are continuously sheared and eventuall y absorbed by the zonal flow. It is shown that normal modes represent only a small fraction of the observed atmospheric disturbances, while continuous -spectrum disturbances represent the majority of observed disturbances, eve n when the basic flow is unstable. Daily variabilities of the observed continuous-spectrum disturbances are pr esented. They are shown to follow the results of wave-packet theory. Calcul ations suggest that there are abundant sources to excite continuous-spectru m disturbances in the atmosphere.