On the role of parametric instability of internal gravity waves in atmospheric radar observations
Abstract
In the presence of a monochromatic finiteamplitude internal gravity wave, many properties of tropospheric and mesospheric radar observations can be explained consistently in terms of parametric instability. At small primary wave amplitudes, the fastest growing instability modes can be divided into two classes. In one class, the instability modes have scales comparable to that of the primary wave leading to a broadening of the wave number spectrum. The second class consists of smallscale instability modes with a continuous wave number spectrum so that the primary wave can be "seen" by radars even in the absence of conventional instability mechanisms such as shear or static instability. The smallscale instability modes generated by longperiod primary waves propagate almost vertically and form extended layers moving with the phase velocity of the primary wave. Thus, in accordance with observational results, the signal power of backscattered radar signals is strongest at near vertical antenna beam directions and shows a layered structure. The smallscale instability modes further satisfy Taylor's frozen turbulence field hypothesis so that the Doppler shift of scattered radar signals yields the space and time dependent fluid velocity of the primary wave. At sufficiently large primary wave amplitudes, there is in particular an isolated fast growing instability mode which has a frequency close to the mean VäisäläBrunt frequency and is probably due to shear instability. It may give rise to a cascade of successive instability modes that can explain organized substructures in layers of enhanced tropospheric UHF radar returns and the frequent occurrence of shortperiod evanescent waves in mesospheric VHF radar observations which cannot be attributed to KelvinHelmholtz instability of the mean shear flow.
 Publication:

Radio Science
 Pub Date:
 October 1990
 DOI:
 10.1029/RS025i005p00983
 Bibcode:
 1990RaSc...25..983K
 Keywords:

 Atmospheric Circulation;
 Atmospheric Sounding;
 Gravity Waves;
 Meteorological Radar;
 Atmospheric Turbulence;
 Radar Scattering;
 Wind Profiles;
 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics;
 Meteorology and Atmospheric Dynamics: Turbulence;
 Meteorology and Atmospheric Dynamics: Waves and tides;
 Radio Science: Remote sensing;
 Radio Science: Atmospheric propagation;
 Radio Science: Waves in plasma