Numerical evaluation of the wind-induced bias for the OTT Parsivel2 optical disdrometer

Non-Catching Gauges (NCGs) are increasingly employed to study precipitation microphysics and often serve as ground-based references for validating radar and satellite measurements. Their growing popularity is also due to their minimal maintenance requirements, that counterbalance their higher costs. However, wind-induced biases significantly affect NCG measurements by potentially diverting hydrometeors away from the instrument sensing area. This bias, already a concern for traditional catching gauges, is even more pronounced for NCGs due to their complex shapes, usually not radially symmetric (see e.g. Chinchella et al. 2024). This study investigates the wind-induced bias of measurements taken by the OTT Parsivel² disdrometer using Computational Fluid Dynamics (CFD) simulations coupled with a Lagrangian particle tracking model implemented in the OpenFOAM software. CFD simulations provide the wind velocity field around the instrument body for different combinations of wind speed and direction by numerically solving the Unsteady Reynolds-Averaged Navier-Stokes equations, using a k-ω SST turbulence model and a local time-stepping approach. Results show that wind parallel to the laser beam causes maximum disturbance on the instrument sensing area, while wind perpendicular to the laser beam minimizes the disturbance. Hydrometeor trajectories are modelled starting from the simulated velocity fields, by releasing drops ranging from 0.25 mm to 8 mm in diameter within the computational domain. The trajectories are tracked until the drops either reach the gauge, exit the domain, or fall below the sensing area. From these simulations, the Catch Ratio (CR) is calculated, representing the ratio of the number of droplets reaching the instrument sensing area in the presence of wind and their number in undisturbed conditions. For wind parallel to the laser beam, limited overcatch is shown at low wind speed (1–2.5 m/s), while severe undercatch occurs at high wind speed. For wind perpendicular to the laser beam, the bias is limited, with minor overcatch observed at high wind speed. By fitting the CR, adjustments to the measurements can be applied, provided the wind speed and direction are known at the installation site. Since the CRs strongly depend on the hydrometeors diameter, wind also affects the measured Drop Size Distribution (DSD), with small drops often missed entirely in certain wind conditions. Similar results are shown when integrating the CRs over the full range of drop sizes, obtaining the Collection Efficiency, that represent the ratio of the precipitation volume sensed by the instrument to the actual precipitation volume. The effect of free stream turbulence is also being tested by superimposing turbulent vortexes over the free stream flow at the inlet of the simulation domain. In conclusion, wind significantly affects the measurement of precipitation microphysical and integral properties, including the derived DSD and rainfall volume, when using the OTT Parsivel² disdrometer. These biases can be mitigated by applying adjustments as a function of wind speed and direction, thereby improving the reliability of NCG measurements in windy conditions.

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