The formation of layered structures by parametric instability of finite-amplitude gravity waves

Abstract

VHF Doppler radars occasionally indicate the presence of almost monochromatic long-period gravity waves in the mesosphere. The radar echo intensity occurs in the form of nearly horizontally extended layers which are assumed to be generated by enhanced turbulence although the Richardson numbers are significantly larger than unity, excluding both shear and static instabilities as primarymechanisms for transition to turbulence. In good agreement with these observations, the theory of parametric instability of finite-amplitude monochromatic gravity waves predicts fast growing anisotropic small-scale disturbances whose size is limited by molecular dissipation. The disturbances form turbulent layers moving with the phase velocity of the primary wave. They are “frozen” in the primary wave flow (Taylor's hypothesis) and thus confirm the general assumption that the fluid velocity can be obtained from the Doppler shift of backscattered radar echoes.