An Optical Disdrometer for Measuring Size and Velocity of Hydrometeors

TLDR

The prototype optical disdrometer can detect hydrometeors ranging from 0.3 to 30 mm in diameter with velocities up to 20 m s⁻¹. The study presents the characteristics of this prototype optical disdrometer. Measurements of rain and snow were obtained with the device and compared against a Joss–Waldvogel disdrometer, a Hellmann rain gauge, and a research spectrometer. The instrument is easy to handle, robust, low cost, reliably detects small drops, agrees well with reference instruments, shows <5 % size repeatability (1.4–8.0 mm) and 10–25 % velocity repeatability, and can distinguish rain, snow, drizzle, graupel, hail, and clear conditions.

Abstract

The characteristics of a prototype optical disdrometer are presented. Particles are detectable in the diameter range from 0.3 to 30 mm having velocities of up to 20 m s−1. Advantages of the new system are (i) it is easy to handle, robust, and low cost, allowing a cluster of instruments to investigate the spatial and temporal fine-scale structure of precipitation; (ii) it provides reliable detection of the range of small drops; and (iii) it allows the possibility of snow measurements. Results of rain measurements are compared with data from a Joss–Waldvogel disdrometer and a Hellmann rain gauge. Furthermore, some snow measurements are presented and compared with results of a research spectrometer. The overall agreement is good. The repeatability of particle size estimation was checked in the diameter range between 1.4 and 8.0 mm and yielded a standard deviation of less than 5%. For drop velocities the standard deviation varies between 25% (0.3-mm drops) and 10% (5-mm drops). The optical disdrometer can also serve as a present weather sensor, detecting and differentiating among rain, snow, drizzle, graupel, hail, and the absence of precipitation.