Challenges in measuring the size and velocity of large raindrops: a comparison of selected methods

The characteristics of raindrops play a crucial role in various fields, including meteorology, hydrology, agriculture, horticulture, weather forecasting, and atmospheric physics. Among the various techniques and measurement devices for characterising precipitation drops, disdrometers are currently most commonly used and high-speed cameras are becoming more popular. A significant challenge can be the characterisation of large drops, associated with e.g. throughfall phenomenon or heavy convective rainstorms, which are susceptible to shape changes as a result of oscillations. Thus, the aim of the study was to evaluate the suitability of selected methods for measuring the size and velocity of large drops. The study was based on the formation of single drops with specified diameters (3.2, 4.3 and 5.3 mm) and different heights of release (1, 3 and 5 m). The performed analyses included a comparison of data obtained from two types of laser disdrometers (Thies Clima LPM and Parsivel2), single high-speed camera and a set of synchronised high-speed cameras with 3D PTV module (i.e. particle tracking velocimetry). The measurements were complemented by a characterisation of drop shape deformation that may have influenced the results, which was assessed using high-speed imaging technique. Study showed that high-speed cameras allowed a very accurate analysis of the parameters of large individual drops (i.e. size, velocity and shape descriptors) in contrast to the tested disdrometers which showed substantial variability in the results. The coefficient of variation for the measured parameters was up to 5.5 % for the cameras, and up to 13.6 % for the disdrometers. In this context, high-speed cameras provide an alternative for measuring processes that are subject to significant errors (e.g. related to the irregularity and variability of the drop shape) in disdrometer-based measurements, such as throughfall, and serve as a valuable tool for validating widely used instruments. What is more, the use of camera set combined with a PTV module provides a full 3D mapping of the trajectory of falling drop, allowing insight into the velocity components (the vertical component, Vy, and horizontal components, Vx and Vz) of individual drops, which could be important in the case of drops whose flight has been disturbed by wind.