Listening to Raindrops

Jeffrey A. Nystuen

Applied Physics Laboratory
University of Washington
Seattle, Washington

Abstract: Six different types of automatic rain gauges, including tipping bucket, weighing, capacitance, optical, disdrometer and acoustical sensors, were deployed for 17 months (Sep 93-Jan 95) at the NOAA Atlantic Oceanographic and Meteorological Laboratory in Miami, Florida. Different rainfall conditions encountered during the experiment included wintertime stratiform frontal rainfall, intense springtime convective systems with extremely high rainfall rates (over 100 mm/hr), summertime convective storms, mesoscale convective systems in the rainy season (Sep-Oct) and one tropical storm (TS Gordon). Overall, all of the rain gauges performed well, with intercorrelations of order 0.9 or better using one-minute rainfall rates and biases of less than 10%, however each showed limitations under different rainfall situations. In particular, under extremely heavy rainfall rates (over 100 mm/hr), the disdrometer and tipping bucket rain gauges biased low while the optical rain gauge biased high. Under light rainfall rates (under 2 mm/hr), the capacitance and tipping bucket rain gauges showed significant instrument noise using the one minute sampling interval. The optical gauge was sensitive to the relative proportion of small to large raindrops within the rain. The raindrop distribution parameter N0, the coefficient of the exponential fit to the drop size distribution, could be used to predict the optical gauge bias. When N0 is large (relatively more small drops), the optical gauge biases high, when N0 is small (relatively more large drops), the optical gauge biases low. The acoustic rain measurement showed significant variability when compared to the other gauges. The acoustic measurement is very sensitive to the presence of very large raindrops (over 3.5 mm diameter) as these raindrops are extraordinarily loud underwater and prevent the smaller drop size populations from being heard and accurately counted when they are present. While the range of wind speeds encountered during the experiment was limited, wind did affect the performance of several of the gauges. At higher wind speeds (over 5 m/s), the disdrometer and acoustic rain gauges biased low and the instrument noise of the capacitance gauge increased significantly.

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