Radar coherent scatter from precipitation: Part I

Classical radar theory only considers incoherent backscatter from precipitation. Such backscatter is associated with Gaussian-like auto-correlations of the amplitudes (A) characterized by particle reshuffling times on the order of a few to several milliseconds. Can precipitation generate coherent scatter as well?

Up to now the accepted answer has been, no. Why? First, consider clear air. Signs of coherent scatter in clear air have always been interpreted in terms of the index of refraction fluctuations caused by the turbulent energy cascade. Some investigators have extended this approach to explain apparent radar coherent scatter in clouds and smoke by treating the particles as a continuum in which the inhomogeneities in the spatial concentration of the droplets are equivalent to fluctuations in the index of refraction occurring on the appropriate Komolgorov turbulent scales. Such an approach, however, can not account for coherent scatter in precipitation because hydrometeors are distributed sparsely in space (relative to radar wavelength) so that the continuum assumption does not hold.

However, in this first part of a two part presentation, we show theoretically that the answer is potentially a yes because a completely different mechanism exists for generating radar coherent scatter in clouds and precipitation. It is based on the direct scatter by the hydrometeors forming transient spatially and temporally coherent periodic structures in resonance with the radar wavelength. This approach requires neither the continuum assumption nor Komolgorov turbulent scaling.