Assessment of the Thies optical disdrometer performance

Optical disdrometers have the potential to be low maintenance instruments that can measure drop-size distributions and drop-size distribution moments such as rainfall rate, radar reflectivity, optical extinction, and others. As with any other measurement device, their output is affected by different sources of uncertainty. To better understand these uncertainties, we compared rainfall accumulations that were measured by three dual tipping bucket stations, three Vaisala WXT510 compact weather stations, and four Thies optical disdrometers. We detected considerable bias between disdrometers and tipping buckets, as well as among disdrometers, that we attempted to minimize through calibrating diameter measurements. Although the calibration successfully decreased bias among disdrometers, it increased bias between disdrometer rainfall accumulations and tipping bucket accumulations, indicating that there are other relevant sources of error in the estimation of rainfall accumulation from disdrometer measurements. We developed a simulation capable of mimicking the disdrometerʹs operation during the calibration procedure and studied the effects of using three arbitrary laser beam patterns into the measurement of the spheresʹ diameters. The simulation successfully reproduced some of the behavior observed in calibration data, showing the dependence of the bias and the spread of sphere diameter measurements on the studied beam patterns. Furthermore, we designed a Monte-Carlo-based simulation to propagate the uncertainties of the measurement of spheresʹ diameters into the estimation of rainfall accumulations. We used the calculated uncertainties to correct rainfall accumulations which improved the agreement among disdrometers, although apparently overcorrected the accumulation for one of the instruments. Moreover we found the average correction to be related to both average and standard deviation of the diameter error and insensitive to the diameter error skewness. Overall we observed considerable bias between instruments with different principles of measurements, which could not be solely explained by uncertainty in the evaluation of the diameters, or by wind effects, indicating that difficulties in the determination of the optical disdrometerʹs sensing area could be the cause.