Evaluation of the WMO-SPICE Transfer Functions for Adjusting the Wind Bias in Solid Precipitation Measurements

A significant result of the World Meteorological Organization (WMO) Solid Precipitation Inter-Comparison Experiment (SPICE) was the development of transfer functions for adjusting the systematic bias due to wind in the automated gauge measurement of solid precipitation. Precipitation data collected at eight SPICE sites during the intensive intercomparison period (winter seasons of 2013/2014 and 2014/2015) was combined to produce a set of transfer functions for a variety of gauge and wind shield configurations that were intended to be widely applicable across many climate regimes. However, the evaluation of these transfer functions was limited to the data collected during the two winter seasons of the SPICE field campaign. To extend the evaluation, data for two additional winter seasons (2015/2016 and 2016/2017) from the same eight SPICE sites was collected, quality controlled, and post-processed. 30-minute and seasonal adjusted and unadjusted measurements from single-Alter shielded and unshielded Geonor T-200B3 and OTT Pluvio2 precipitation gauges were compared to the WMO Double Fence Automated Reference (DFAR) and the performance of the transfer function adjustment was assessed in terms of relative total catch (RTC), root mean square error (RMSE), and Pearson correlation (r). Further to previous evaluations, this analysis isolated the occurrences of snow and reported these adjustment metrics as well as those for total precipitation. The evaluation shows that the transfer function performance varies substantially by site. Adjusted RTC ranges from 54% to 123%, RMSE from 0.07 mm to 0.38 mm, and r from 0.28 to 0.94, depending on precipitation phase, site, and gauge configuration. Generally, windier sites such as Haukeliseter (Norway) and Bratt’s Lake (Canada) exhibit a net under-adjustment (17% to 46%), while the less windy sites such as Sodankylä (Finland) and Caribou Creek (Canada) exhibit a net over-adjustment (2% to 23%). It was also shown that the windier sites, Bratt’s Lake in particular, exhibit a relatively large number of snowfall events that are measured by the DFAR (> 0 mm) but not measured by the single-Alter shielded or unshielded gauges (i.e. reported as 0 mm), and therefore cannot be adjusted with the transfer functions.