Evaluating the catching performance of aerodynamic rain gauges by means of field comparisons and CFD modelling

Accurate precipitation measurement is a fundamental requirement in a broad range of applications including flood risk management and hydrological studies. At present, the most widely used method of measuring precipitation is the ‘rain gauge’, which is often also considered to be the most accurate. In the context of hydrological modelling, measurements from rain gauges are interpolated to produce an areal representation, which forms an important input to drive hydrological models. The results of these models may be applied in a variety of contexts, such as evaluating the hydrological impacts of climate change. In each stage of such a process another layer of uncertainty is introduced. The initial measurement errors are propagated through this chain, compounding the overall uncertainty. This study looks at the fundamental source of error, the precipitation measurement itself, and specifically addresses the systematic ‘wind-induced’ error. The shape of a precipitation gauge significantly affects its collection efficiency (CE), with respect to a reference measurement. This is due to the airflow around the gauge, which causes a deflection in the trajectories of the raindrops or snowflakes near the gauge orifice. Computational Fluid-Dynamic (CFD) simulations are used to evaluate the time averaged airflows realized around the EML ARG100, EML SBS500 and EML Kalyx-RG rain gauges, when impacted by wind. Terms of comparison are provided by the results obtained for standard precipitation gauge shapes manufactured by Casella and OTT which, respectively, have a uniform and a tapered cylindrical shape. The simulations were executed for five different wind speeds; 2, 5, 7, 10 and 18 ms-1.