Long-period unstable gravity-waves and associated VHF radar echoes

VHF atmospheric radar is used to measure the wind velocity and radar echo power related to longperiod wind perturbations, including gravity waves, which are observed commonly in the lower stratosphere and tropopause region, and sometimes in the troposphere. These wind structures have been identi®ed previously as either inertia-gravity waves, often associated with jet streams, or mountain waves. At heights of peak wind shear, imbalances are found between the echo powers of a symmetric pair of radar beams, which are expected to be equal. The largest of these power di€erences are found for conditions of simultaneous high wind shear and high aspect sensitivity. It is suggested that the e€ect might arise from tilted specular re ̄ectors or anisotropic turbulent scatterers, a result of, for example, Kelvin-Helmholtz instabilities generated by the strong wind shears. This radar power-di€erence e€ect could o€er information about the onset of saturation in long-period waves, and the formation of thin layers of turbulence.