Wind tunnel experiments to evaluate the wind-induced bias on disdrometer measurements

Wind has a significant impact on precipitation measurement instruments, including disdrometers, by inducing aerodynamic disturbances around their bodies. These airflow features divert trajectories of falling hydrometeors, often reducing the amount of precipitation detected when compared to  windless conditions. Furthermore, the shape of disdrometers, which is non-radially symmetric, makes the wind-induced bias dependent on wind direction. Traditionally, field experiments have been used to develop corrections for the wind-induced bias. However, Computational Fluid Dynamics (CFD) simulations offer a more versatile approach for studying wind-induced bias on different instrument designs under varying climatic conditions. In this work a wind tunnel experimental campaign was conducted to show the interaction between wind and disdrometers and to validate a suitable CFD model by providing detailed data on drop trajectories. Full-scale tests were conducted in the high-speed test section of the Wind Tunnel facility available at Politecnico di Milano. The chamber (4m wide, 3.8m high and 6m long) is characterized by a nearly laminar flow and a narrow boundary layer. The disdrometers were fixed to the ground on a rotating plate to facilitate alignment with the flow direction. Furthermore, a specially designed drop generator – attached to a moving gantry – was used to release water drops into the wind flow, allowing precise control of drop diameter, release height and timing. Finally, a high-speed camera, operating at 1000 fps, recorded the trajectories of the drops approaching the sensing areas of the disdrometers. Images were processed to identify each drop, calculate their velocity and track their movement through the camera field of view. The study focused on two disdrometer models, the Thies CLIMA LPM and the OTT Parsivel², which use an optical method to measure drop size and velocity. The experiments were conducted for wind speeds of 10 m/s, drop diameters ranging from 1.0 to 1.2 mm, and three wind directions (0°, 45°, and 90°). Results showed that wind significantly alters drop trajectories, often diverting them away from the sensing area or causing them to collide with the instrument body. A numerical model - already used in e.g., Chinchella et al., (2024) – was validated by simulating the experimental conditions and comparing the results against observations. Validation shows that the numerical approach is suitable for developing adjustment curves to correct disdrometer measurements under windy conditions. This work further highlights the importance of addressing wind effects in precipitation measurements, by applying correction curves (see e.g., Chinchella et al., 2024) to enhance the accuracy of rainfall measurements obtained from disdrometers like the Thies CLIMA LPM or the OTT Parsivel².

ACKNOWLEDGMENTS

The wind tunnel campaign on disdrometers was carried out within the framework of the Italian national projects PRIN2022MYTKP4 “Fostering innovation in precipitation measurements: from drop size to hydrological and climatic scales”.

References:

Chinchella E., Cauteruccio, A., & Lanza, L. G. (2024). Quantifying the wind-induced bias of rainfall measurements for the Thies CLIMA optical disdrometer. Water Resources Research, 60(10), e2024WR037366. https://doi.org/10.1029/2024WR037366