Study of the Solid Precipitation Wind Induced Under-Catch Using Snowfall Intensity

The aerodynamic response of solid precipitation gauges when exposed to the wind is responsible for a significant reduction of their collection performance as a function of the wind speed. This is due to the deformation of the snowflakes trajectories upstream of the gauge. Hence, the use of windshields to reduce the impact of wind on solid precipitation measurements is common. For example, a Geonor T-200B vibrating wire gauge located in a Single Alter shield is part of the reference system in the ongoing WMO Solid Precipitation InterComparison Experiment (SPICE). This study investigates the collection efficiency (CE) of shielded and unshielded gauges using field observations performed at the Marshall field site (CO, USA) from 2013 to 2015. The influence of the environmental temperature (T) and the measured snowfall intensity (SI) on the wind-induced undercatch is analysed by means of data classification at different time resolutions. A non-gradient optimization algorithm is used to achieve optimal CE binning according to the parameter under analysis. The proposed methodology allows the assessment of a new set of horizontal wind speed vs. collection efficiency curves and the residual uncertainty of precipitation measurements after correction is evaluated. The advantage of classifying the precipitation measurements basing on the observed snowfall intensity instead of the environmental temperature is demonstrated by lower uncertainty values. The proposed methodology also avoids approximations related to the use of T observations to make assumptions on the type of precipitation. In this case the horizontal wind speed is the only environmental variable needed to corrected snow measurements from the wind-induced undercatch.