Precipitation Bragg scatter in radar observations at zenith

As precipitation sediments and interacts with turbulence, spatial structures appear as the familiar ‘streamers' of precipitation sweeping across the road during a thunderstorm or like those so obvious in snow which is back-lite. Some of these structures occur on scales which resonate with the radar wavelength. Consequently, they produce coherent back-scatter (precipitation Bragg scatter, pBs). Recently, and in contrast to incoherent scattering, it was found that the power normalized cross-correlation functions of back-scattered complex amplitudes in neighboring range bins (ρ₁₂) averaged over time exist. Moreover, they are identical to the fractional contributions made by radar coherent back-scatter in the radial direction to the total back-scattered power in rain and snow. This coherent power can significantly affect some radar techniques for measuring precipitation intensity because it depends upon the square of the particle concentrations rather than the linear dependence in the case of incoherent back-scatter. However, all of these observations were made by radars looking tangentially to the ground. Yet for many purposes, including the global measurements of precipitation from space, radar observations in precipitation are at zenith or nadir to the surface of the Earth. A natural question, then, is, Can coherent back-scattered power be found in observations in precipitation at zenith and nadir as well? Here, radar observations collected at zenith in a convective shower and snow are analyzed. It is found that ρ₁₂ and, hence, pBs exists in zenith observations as well. Moreover, the intensity of the Bragg scatter appears to be independent of the size of sample volume. Reasons for these findings and some implications are presented.