CLOUD-DROPLET-SIZE DISTRIBUTION FROM LIDAR MULTIPLE-SCATTERING MEASUREMENTS

A method for calculating droplet-size distribution in atmospheric clou ds is presented, based on measurement of laser backscattering and mult iple scattering from water clouds. The lidar uses a Nd:YAG laser that emits short pulses at a moderate repetition rate. The backscattering, which is composed mainly of single scattering, is measured with a dete ctor pointing along the laser beam. The multiple scattering, which is mainly double scattering, is measured with a second detector, pointing at a specified angle to the laser beam. The domain of scattering angl es that contribute to the double-scattering signal increases monotonic ally as the pulse penetrates the cloud. The water droplets within the probed volume are assumed to have a constant size distribution. Hence, from the double-scattering-measured signal as a function of penetrati on depth within the cloud, the double-scattering phase function of the scattering volume is derived. Inverting the phase function results in a cloud-droplet-size distribution in the form of a log-normal functio n.