VHF radar observations of cat's-eye-like structures at mesopheric heights

Abstract

Observations of noctilucent clouds indicate the frequent occurrence of wave-like motions and Kelvin–Helmholtz billows at the high latitude summer mesospause^1,2 (∼83 km), but to date no other technique appears to have produced direct evidence of their pres-ence at these heights³. High-powered mesosphere–stratosphere–troposphere very high frequency Doppler radars with good height and time resolution (∼150∼300 m and ∼10 s respectively) capable of making such observations have recently become available, and the backscattering regions in the mesosphere detected by such radars often take the form of thin layers that appear to be pro-duced by dynamical instabilities in the region of maximum wind shear⁴. Such wind conditions should produce Kelvin–Helmholtz instabilities, and here we present two examples of mesospheric radar observations that suggest the characteristic 'cat's-eye'-like turbulent structures often associated with them in other parts of the atmosphere⁵, the ocean⁶ and in laboratory experiments⁷.