On the role of parametric instability of internal gravity waves in atmospheric radar observations

In the presence of a monochromatic finite‐amplitude 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 small‐scale 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 small‐scale instability modes generated by long‐period 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 small‐scale 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 Väisälä‐Brunt 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 short‐period evanescent waves in mesospheric VHF radar observations which cannot be attributed to Kelvin‐Helmholtz instability of the mean shear flow.