Abstract
In order to investigate the mechanism for the amplification of planetary waves, various analyses (structure of waves, energetics, relationships with the zonal mean flow and so on) are made for large-amplitude waves of wavenumbers 1 through 3 based on geopotential height and temperature data for 10 winter seasons.
The results are as follows: The wave disturbances of wavenumber 3 amplify after baroclinic waves develop in longitudes where the nonlinear term due to the baroclinic waves has in phase relation with the zonal wind component of wavenumber 3 in the mean zonal wind with weak lateral shear in 50°-55°N (the latitude band where the large-amplitude wave of wavenumber 3 is formed). It is also found that the wave disturbances of wavenumber 3 again amplify after baroclinic waves of wavenumber 6 grow at the decay stage of wavenumber 3. Thus, the large-amplitude wave of wavenumber 3 appears several times in one winter.
The development of wavenumber 2 occurs in many cases when a westward-moving barotropic mode with a long period approaches a stationary mode. For such a situation, the northward wind component of wavenumber 2 becomes in-phase with the temperature wave of wavenumber 2. Hence the zonal available potential energy is converted into the eddy available potential energy of wavenumber 2.
The mechanism for the amplification of wavenumber 1 seems to be similar to that of wavenumber 3 from view of energetics, but possibilities of other mechanisms are also suggested.
Discussions are also made for comparison between our observational results and theoretical studies.
