Ground-Based Measurements of Raindrop Fallspeeds

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  • 1 Department of Physics, The University of Auckland, Auckland, New Zealand
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Abstract

Typical results of field measurements of the fallspeeds of natural raindrops are presented and instrumental effects investigated by numerically simulating the measurement processes. Deviations between the measured fallspeeds and those expected on the basis of theory and windtunnel results are smaller than previous investigators have found, probably because of the improved instrumentation used. The random component of these deviations is consistent with estimated experimental error, but on average drops are found to fall about 5%–10% slower than expected, and this mean deviation increased with drop size; these data are consistent with the existence of local updrafts and downdrafts. The effects of the observed fallspeed fluctuations on estimates of other quantities at ground level (e.g., drop radius) are considered and, in some cases, shown to be significant.

Abstract

Typical results of field measurements of the fallspeeds of natural raindrops are presented and instrumental effects investigated by numerically simulating the measurement processes. Deviations between the measured fallspeeds and those expected on the basis of theory and windtunnel results are smaller than previous investigators have found, probably because of the improved instrumentation used. The random component of these deviations is consistent with estimated experimental error, but on average drops are found to fall about 5%–10% slower than expected, and this mean deviation increased with drop size; these data are consistent with the existence of local updrafts and downdrafts. The effects of the observed fallspeed fluctuations on estimates of other quantities at ground level (e.g., drop radius) are considered and, in some cases, shown to be significant.

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