Double Fence Intercomparison Reference (DFIR) vs. Bush Gauge for “true” snowfall measurement

- Bush Gauge measures 20-50% more snowfall than DFIR for wind speeds of 6-7 m/s.
- DFIR measurements need corrections for wind-induced undercatch of snowfall.
- Past WMO study underestimated DFIR catch, impacting national gauge assessment.

SummaryThe Bush Gauge and the DFIR have been used as the references for precipitation gauge intercomparison experiments. This study analyzes and compares the long-term (1991-2010) data collected by these gauges at the Valdai experimental station in Russia. The results show that the Bush Gauge systematically catches more (snow and mixed) precipitation than the DFIR. Wind speed during precipitation is the most important factor related to gauge undercatch. The Bush Gauge measures 20-50% more snow over a 12 h period than the DFIR for wind speeds of 6-7 m/s. Therefore, correction of the DFIR for wind-induced undercatch is necessary in order to best represent true precipitation. This study derives new correction equations for the DFIR measurements of snow and mixed precipitation. In comparison to previous analysis, the new equations suggest lower (by 3-6%) snow undercatch by the DFIR relative to the Bush Gauge, which means better DFIR performance than what we documented before in the past WMO intercomparison. This result will affect the evaluation of national standard precipitation gauges against the DFIR. Our effort is underway to determine the impact of this discrepancy through field data collection in Canada and additional data analysis at selected WMO test sites.