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Path-Averaged Measurements of Rain Rate and Raindrop Size Distribution Using a Fast-Response Optical Sensor

Ting-I. WangNOAA/ERL/Wave Propagation Laboratory, Boulder, CO 80302

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K. B. EarnshawNOAA/ERL/Wave Propagation Laboratory, Boulder, CO 80302

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R. S. LawrenceNOAA/ERL/Wave Propagation Laboratory, Boulder, CO 80302

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Abstract

Path-averaged terminal velocity distribution of raindrops is determined from the temporal covariance function of signals from two vertically spaced linear optical detectors that respond to raindrop-induced amplitude scintillations of a projected laser beam. The known monotonic relationship between drop size and terminal velocity permits the measured velocity distribution to be converted to path-averaged drop-size distribution and, in turn, to rain rate. The large capture area of the measurements over a 200 m path allows drop-size distribution to be measured in short time intervals. We present measurements of path-averaged rain rate and raindrop size distribution made at 42 s intervals. The terminal velocity distribution during a storm that contained a mixture of rain and hail clearly shows the two-component nature of the precipitation.

Abstract

Path-averaged terminal velocity distribution of raindrops is determined from the temporal covariance function of signals from two vertically spaced linear optical detectors that respond to raindrop-induced amplitude scintillations of a projected laser beam. The known monotonic relationship between drop size and terminal velocity permits the measured velocity distribution to be converted to path-averaged drop-size distribution and, in turn, to rain rate. The large capture area of the measurements over a 200 m path allows drop-size distribution to be measured in short time intervals. We present measurements of path-averaged rain rate and raindrop size distribution made at 42 s intervals. The terminal velocity distribution during a storm that contained a mixture of rain and hail clearly shows the two-component nature of the precipitation.

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