Rain gauge aerodynamic design to minimise wind-induced bias in precipitation measurement

Abstract. The presence of wind introduces biases in precipitation measurements obtained from traditional rain gauges due to the interaction between the airflow and the outer shell of the instrument, which acts like a bluff body immersed in a wind stream. The airflow is diverted, creating an updraft and accelerating the flow velocity. This interacts with the fall trajectories of hydrometeors near the gauge rim, causing some to deviate out of the collector. The resulting wind-induced undercatch can be minimised by adopting a suitable design for the outer shell of the gauge. While the effectiveness of an inverted conical shape for the outer walls of the gauge has already been demonstrated in the literature, this work proves that the size and geometric profile of the collector's rim are crucial in reducing the effect of wind on precipitation measurements. Building on the construction requirements derived from operational constraints, various single and dual inclination rim profiles were tested using computational fluid dynamics simulation and Lagrangian particle tracking to identify the most effective design. The proposed rim profile is then thoroughly tested in numerical simulations performed over an extended range of wind speeds and precipitation intensities. The scalability of the rim profile to suit different sizes of the collector area is demonstrated, and adjustment curves are obtained to address residual bias. A comparison with existing high-performance gauges on the market revealed that the proposed design significantly outperforms existing solutions.