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

R M Worthington,L Thomas

doi:10.1007/s005850050500

Abstract

VHF atmospheric radar is used to measure the wind velocity and radar echo power related to long-period 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 identified 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 differences are found for conditions of simultaneous high wind shear and high aspect sensitivity. It is suggested that the effect might arise from tilted specular reflectors or anisotropic turbulent scatterers, a result of, for example, Kelvin-Helmholtz instabilities generated by the strong wind shears. This radar power-difference effect could offer information about the onset of saturation in long-period waves, and the formation of thin layers of turbulence.

Highlights

Long-period wind perturbations, often identi®ed as gravity waves, are commonly detected in the lower stratosphere; their characteristics have been measured by both VHF radar (e.g. Cornish and Larsen, 1989; Thomas et al, 1992; Cho, 1995; Yamanaka et al, 1996) and balloons (e.g. Thompson, 1978)
This study examines their inuence on the echo power from VHF radar transmissions and, in particular, the imbalances between the echo powers of a symmetric pair of radar beams
The echo power of the pulses returned from a region of atmosphere generally depends on the zenith angle at which the radar beam is pointing, an eect known asaspect sensitivity'; the power is usually highest using a vertically pointing radar beam, and falls o as the zenith angle of the beam increases

Summary

Introduction

Long-period wind perturbations, often identi®ed as gravity waves, are commonly detected in the lower stratosphere; their characteristics have been measured by both VHF radar (e.g. Cornish and Larsen, 1989; Thomas et al, 1992; Cho, 1995; Yamanaka et al, 1996) and balloons (e.g. Thompson, 1978). The tropopause is near 11±12 km echo power between 0 and 12 beams is approximately 7 dB for the lower case and 25 dB for the upper; i.e. the wind shear seems to be associated with large power dierences only when aspect sensitivity is strong.

Results

Conclusion