Experimental evidence of the wind-induced bias of precipitation gauges using particle image velocimetry and particle tracking in the wind tunnel

Wind tunnel (WT) experiments were conducted to reproduce the wind-induced deviation of rain drop trajectories when approaching the collector of catching-type gauges. Three typical outer shapes of the instrument body (cylindrical, inverted-conical and "chimney" shapes) were tested using full-scale models. The airflow pattern upstream of and above the collector was measured with a Particle Image Velocimetry technique, after injecting a passive tracer in the flow. Tests were performed by releasing water drops in the WT flow to mimic the raindrops fall. The drop trajectories near the gauge collector were tracked and quantitatively measured using a high-speed camera. The deviation of the trajectories, induced by the bluff-body aerodynamics of the gauge, is interpreted here by comparing the measured airflow pattern around the three instruments to highlight the effect of their different outer shapes. The "chimney" shape, that is typical of some weighing type gauges, demonstrated lower performance with respect to the cylindrical and inverted-conical shape, showing the most relevant effect in deviating the trajectories of the approaching drops when immersed in a wind field.