Examination of the Catch Efficiency of the Double Fence Intercomparison Reference (DFIR) for Snow

The occurrence and intensity of precipitation during winter storms are key factors that can cause major disruptions to society. To predict such events, it is critical to measure accurately the amount of precipitation accumulated on the ground. Systematic errors in snowfall measurements are often observed due to the gauge-shield geometry and the weather conditions. For example, it is well documented that the catch efficiency of a gauge-shield configuration decreases with increasing wind speed. This catch efficiency is obtained by comparing the precipitation rate of the gauge-shield configuration with the reference, which is often defined as an automatic gauge placed in a Double Fence Inter Comparison Reference (DFIR). A lot of scatter can be observed for a given wind speed, which is mainly caused by the type of precipitation falling. Furthermore, inconsistencies in the measurements are sometimes observed. The goal of this study is to better understand the uncertainties associated with the measured precipitation by an automatic gauge placed in the DFIR. To address this issue, a detailed analysis of the flow field in the vicinity of the DFIR is conducted using computational fluid dynamics software ANSYS FLUENT. Using a Lagrangian model developed specifically to track different types of snow, the trajectory of particles is obtained to compute the collection efficiency associated with different precipitation types for varying wind speed. The results are compared with the crystal types measured by a 2D video disdrometer placed in a DFIR. The preliminary analysis shows some variation in the catch efficiency based on the type of precipitation as well as the wind direction. Overall, this study contributes to a better understanding of the sources of uncertainties associated with the DFIR shield.