A POSSIBLE ORIGIN OF LINEAR DEPOLARIZATION OBSERVED AT VERTICAL INCIDENCE IN RAIN

Recent observations by two different nadir-pointing airborne radars wi th some polarization capabilities have detected surprisingly large lin ear depolarization ratios at times in convective tropical rain. This d epolarization can be explained if the rain is considered to be a mixtu re of a group of apparent spheres and another group of drops that are distorted in the horizontal plane perpendicular to the direction of pr opagation of the incident wave. If confirmed in future observations, t his suggests that at times the larger raindrops are oscillating, in pa rt, because of collisions with smaller drops. Since many of the interp retations of radar polarization measurements in rain by ground-based r adars presume that the raindrop shapes correspond to those of the well -known ''equilibrium'' drops, the present observations may require adj ustments to some radar polarization algorithms for estimating rainfall rate, for example, if the shape perturbations observed at nadir also apply to measurements along other axes as well.