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Ann. Geophys., 22, 1917-1929, 2004
www.ann-geophys.net/22/1917/2004/
© European Geosciences Union 2004
Quasi-two-day wave in an unstable summer atmosphere - some numerical results on excitation and propagation
E. G. Merzlyakov1 and Ch. Jacobi2
1Institute for Experimental Meteorology, SPA Typhoon, Obninsk, 249038, Russia
2Institute for Meteorology, University of Leipzig, D-04103 Leipzig, Germany
Abstract. Based on numerical calculations we demonstrate that small changes in the
smooth climatological background atmosphere may lead to an unstable mean
zonal wind distribution in the summer middle atmosphere. We relate these
changes to small ones because locations and power of the main circulation
structures are conserved, except for the acceleration of the easterly jet in
the stratosphere/mesosphere. The instability forces oscillations propagating
westward with a period of about 2 days and zonal wave numbers s=3 and/or 4.
There are variations in the mean zonal wind distribution due to the
excitation and transient propagation of these waves, and the numerical results
correspond to features of these variations observed in experimental studies.
The growing waves tend to remove the source of excitation. This process is
effective enough to reduce the strong easterly jet and to remove the strong
negative gradient of the zonal mean potential vorticity in the region of the
instability. Therefore, when these parameters are calculated as mean values
over a long time interval, the obtained values are too small to provide the
instability. Strong 2-day waves, in turn, are unstable and can generate
secondary waves with longer periods and lower zonal wave numbers. This effect
is only significant for extremely strong 2-day waves. Another process is
found to be more effective to produce secondary waves. We demonstrated that
the 2-day wave with s=3 forced by nonlinear interaction between the 10-14
day planetary waves and the 2-day wave of zonal wave number 4 is unstable.
This wave instability generates secondary waves with amplitudes that are
large enough to be observed by ground-based radars, for example.
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