Investigations of the Wavelength Dependence of Radar Backscatter from Atmospheric Turbulence

Abstract

It has long been realized that turbulent mixing of vertical refractive-index gradients is responsible for most clear-air echoes seen by UHF radars. The assumptions that the turbulence is isotropic and homogeneous then let us predict a scale dependence, and therefore wavelength dependence, for the strength of these Bragg scattered echoes. This dependence, λ−1/3, is quite different from the wavelength dependence of Rayleigh scatter from hydrometeors, λ−4. Three sensitive collocated clear-air radars were used in coordinated experiments to test the predicted λ−1/3 dependence. The radars have well-separated wavelengths allowing us to probe atmospheric turbulence at three Bragg scales of 34, 11.5, and 1.5 cm, and recent modifications made to the radars enabled us to collect measurements closely matched in space and time. Results from measurements taken at many altitudes show that the λ−1/3 dependence is frequently observed. However, many measurements deviate from the prediction and the measured power law is more accurately described as a distribution centered about λ−1/3. Possible explanations for this include temporal variability and anisotropy of the turbulence as well as measurement limitations. Observations of clouds and precipitation at the three wavelengths are also presented. The wavelength dependence of these measurements is explained by combined returns from Rayleigh backscatter from the hydrometeors and Bragg scatter from the clear air.