An Instrument for the Measurement of Precipitation Rate by Near-Infrared Extinction

Abstract

The design, construction and performance of a near-infrared atmospheric precipitation sensing device is described. An infrared light emitting diode was used to generate a sensing beam of 0.92 ?m wavelength. The collimated sensing beam traversed an atmospheric path from the transmitter-receiver unit to a remotely positioned corner retroreflector. A phase sensitive detector was used for demodulation. A reference signal was obtained from the sensing beam using a beam splitter. Refractive optics were used for collimation and signal collection. In field tests, calibration curves for extinction versus the rate of rainfall and the rate of snowfall were obtained for path lengths of 100 and 50 m. Experimental results for the rainfall calibration curve produced smaller extinctions than predicted theoretically, with a maximum difference of 3 dB km?1 occurring at a rain rate of 80 mm h?1. Results for rainfall and snowfall experiments coincided with the findings of other authors to within approximately ±20%.