Experimental Quantification of the Sampling Uncertainty Associated with Measurements from PARSIVEL Disdrometers

The variability of the (rain)drop size distribution (DSD) in time and space is an intrinsic property of rainfall, of primary importance for various environmental fields such as remote sensing of precipitation for example. DSD observations are usually collected using disdrometers deployed at the ground level. Like any other measurement of a physical process, disdrometer measurements are affected by noise and sampling effects. This uncertainty must be quantified and taken into account in further analyses. This paper addresses this issue for the Parsivel optical disdrometer, using a large data set corresponding to light and moderate rainfall and collected from 2 collocated Parsivels deployed during 15 months in Lausanne, Switzerland. The relative sampling uncertainty associated with quantities characterizing the DSD, namely the total concentration of drops Nt and the median-volume diameter D0, is quantified for different temporal resolutions. Similarly, the relative sampling uncertainty associated with the estimates of the most commonly used weighted moments of the DSD, i.e., the rain rate R, the radar reflectivity at horizontal polarization Zh and the differential reflectivity Zdr, is quantified as well for different weather radar frequencies. The relative sampling uncertainty associated with estimates of Nt is below 13% for time steps longer than 60 s. For D0, it is below 8% for D0 values smaller than 1 mm. Concerning R estimates, the associated sampling uncertainty is in the order of 15% at a temporal resolution of 60 s. For Zh, the sampling uncertainty is below 9% for Zh values below 35 dBZ at a temporal resolution of 60 s. For Zdr values below 0.75 dB, the sampling uncertainty is below 36% for all temporal resolutions. These analyses provide relevant information for the accurate quantification of the variability of the DSD from disdrometer measurements.