Zenith-angle dependence of VHF specular reflection echoes in the lower atmosphere

We studied the characteristics of specular echoes reflected from stratified layers in the troposphere and lower stratosphere. In particular, we observed echoes at antenna-beam zenith angles, theta, from 0 degrees to 28 degrees in steps of 2 degrees. When the radar measurements were averaged over about 30 min, the zenith angle dependence of the echo power normalized by the vertical power, (S) over tilde(theta), was generally the same for sufficiently intense reflection echoes. That is, the echo power was largest in the vertical direction, decreased to about -10 dB at 6 degrees, and then gradually decreased to a constant level between -15 and -25 dB at theta greater than or equal to 20 degrees. This constant level is interpreted as the isotropic turbulence scattering level <(S)over tilde(i)>. The width of (S) over tilde(theta) was significantly broader than expected for specular reflection from a perfectly horizontal layer. In order to explain this broadening, we developed two numerical models that describe statistically the slope of a reflection layer that has been distorted by vertical gravity-wave motions. With realistic gravity-wave spectra, the shape of (S) over tilde(theta) for theta = 0-6 degrees was successfully explained. However, from 8 degrees to 18 degrees the observed (S) over tilde(theta) was enhanced by as much as 7 dB over the model. From the observations we showed that all of the reflection echoes, including the enhanced echoes at theta = 8-18 degrees, are probably due to the same process. Then we showed that the discrepancy with the model may be the result of our neglect of the horizontal component of gravity-wave motions, which was done in order to constrain the number of calculations. (C) 1997 Elsevier Science Ltd.