LIGHT-SCATTERING BY NONSPHERICAL RAINDROPS - IMPLICATIONS FOR LIDAR REMOTE-SENSING OF RAINRATES

The single scattering properties of nonspherical raindrops have been c alculated by means of the geometric optics approximation to ascertain the usefulness of lidar remote sensing of rainrates. Based on the theo retical hydrodynamical studies of Chuang and Beard (J. Atmos. Sci., 19 90, 47, 1374-1389), a Chebysheff-series of shape coefficients has been selected to account for the size dependent particle nonsphericity. Th e single scattering calculations for randomly oriented raindrops with particle radii ranging from 0.5 to 4.5 mm exhibit a very pronounced de pendence of the phase matrix on particle shape. However, most of these changes are not monotonic with increasing size, which complicates cor relations between rainrates and the radiative properties of the raindr ops. A comparison of ray tracing results by Chebysheff-type particles and axis-ratio equivalent spheroids shows significant differences for particles with radii larger than 1 mm. Backscattering intensity as wel l as linear and circular depolarization ratios for horizontally orient ed raindrops show a non-monotonic increase with particle size. The siz e distribution averaged backscattering properties are poorly correlate d with rainrates. We conclude that lidar remote sensing of rainrates d oes not seem to be a promising attempt. However, this conclusion may b e subject to changes if raindrop oscillations, which have not been con sidered in this study, affect the size distribution averaged backscatt ering properties. (C) 1998 Elsevier Science Ltd. All rights reserved.