On the Temporal Characteristics of Radar Coherent Structures in Snow and Rain

摘要

Previous work showed that the magnitudes of the radar-backscattered amplitudes have statistically significant periodic components of frequencies (f) in excess of those arising from the Doppler velocity fluctuations of incoherent scatter. Analyses in both rain and snow in the earlier work revealed what is interpreted as pervasive coherent scatter. This coherency is thought to come from precipitation structures acting like gratings in resonance with the radar wavelength that, when they move with a velocity component transverse to the beam, induce the observedf. The purpose of this article is to characterize briefly the temporal structure off and, thereby indirectly, the temporal character of the structures producing the radar coherent backscatter. Itis found that these structures last considerably longer than the decorrelation times of a few to 10 milliseconds, characteristic of Doppler velocity fluctuations associated with incoherent scatter. For the data analyzed, though, most last no more than asignificant fraction of 1 s. Hence, for the observed transverse velocity of 2 ms~(-1), the dimensions of the gratings producing the radar coherent backscatter are only on the order of tens of centimeters to a few meters. Therefore, the typically large sampling volumes of most radars will contain many of these grids at any given time. Consequently, during 1 s of observations, one can envision the coherent scatter as coming from many individual grids twinkling on and off, much like the transient spectralreflections off ice crystals falling in sunlight.