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, which is 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 Particle Size Velocity (PARSIVEL) optical disdrometer by using a large dataset corresponding to light and moderate rainfall and collected from two 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 N(t) and the median-volume diameter D(0)-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 Z(h), and the differential reflectivity Z(dr)) is quantified as well for different weather radar frequencies. The relative sampling uncertainty associated with estimates of N(t) is below 13% for time steps longer than 60 s. For D(0), it is below 8% for D(0) values smaller than 1 mm. The associated sampling uncertainty for estimates of R is on the order of 15% at a temporal resolution of 60 s. For Z(h), the sampling uncertainty is below 9% for Z(h) values below 35 dBZ at a temporal resolution of 60 s. For Z(dr) 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.