An Optical Array Instrument for Shape and Fall Velocity Measurements of Hydrometeors

Abstract

A ground-based optical array instrument for the measurement of shapes, sizes, and fall velocities of freely falling hydrometeors is presented. The instrument, the Hydrometeor Velocity and Shape Detector (HVSD), is designed to accurately measure hydrometeors greater than 1 mm in diameter that yield the main contribution to radar backscatter and rain rate in moderate to heavy precipitation. The optical system of the HVSD consists of two horizontal and parallel light beams with a small vertical offset, directed toward two arrays of photodiodes. Each hydrometeor falling through the measuring area is recorded twice with a slight time difference. The two corresponding images of each particle are matched automatically, based on shape and fall pattern characteristics. After two images are matched, the fall velocity of the original hydrometeor is calculated and its actual cross section is reconstructed. The HVSD was calibrated using simulated raindrops and ice particles in the laboratory. It has an inherent undersizing problem of small raindrops, for which an empirical correction is derived. Size measurements of submillimeter hydrometeors are generally of significant uncertainty. The quality of the matching algorithm was analyzed by comparing rainfall data recorded with the HVSD, a Joss?Waldvogel disdrometer, and a rain gauge. The results demonstrate the HVSD's capability to measure properties of single hydrometeors and integral precipitation parameters. Snowfall measurements can also be used to investigate characteristics of aggregates and rimed particles. Median fall velocities and natural fall velocity variability can be studied.