ASPECT SENSITIVITY MEASUREMENTS OF VHF BACKSCATTER MADE WITH THE CHUNG-LI RADAR - PLAUSIBLE MECHANISMS

A new technique, using beam broadening effects, has been developed to measure the aspect sensitivity of atmospheric clear air VHF radar echoes. It uses the relatively broad antenna beam of the vertical-pointing antenna of the new Chung-Li stratospheric-tropospheric radar (25°N, 120°E). The aspect sensitivity measurement using this method is straightforward and free from convolution effects introduced by the finite width of the antenna beam pattern. The observed results agree very well with other measurements. The authors propose a turbulent layer model to explain the aspect sensitivity of the echoes. In this model, anisotropic turbulence is confined to a very thin (few meters) region at the boundary of a turbulent layer. This region is responsible for the aspect sensitivity of the echoes obtained from the vertical direction. The isotropic echoes obtained from the oblique beam arise from the isotropic turbulence embedded in the center of the layer, with 30–300 m in vertical extent. We show in an appendix that the magnitude of the partial reflection coefficient is much more sensitive to the shape, length scale, and smoothness, than to the slope of the refractive index profile. Therefore the functional shape of the refractive index profile is very important for estimating the reflection coefficient. Large errors can be made when assuming, for simplicity, nonphysical profiles. For partial reflecting mechanisms to be important, steplike discontinuities, confined within length scales of the order of a meter, would be required.

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