Wind-induced Precipitation Undercatch of the Hellmann Gauges

The Hellmann gauges have been widely used as the official precipitation measurement instruments in 30 countries. From 1986 to 1993, the accuracy and performance of the Hellmann gauges were evaluated during the WMO Solid Precipitation Measurement Intercomparison at 4 stations in Finland, Russia, Germany, and Croatia. The double fence intercomparison reference (DFIR) was the reference standard used at all the Intercomparison stations. The data for the Hellmann gauges were compiled from measurements made at the 4 WMO intercomparison sites. These data represent a variety of climates, terrains and exposures. The effects of meteorological factors, such as wind speed, type of precipitation and temperature, on gauge catch efficiency were investigated. For snow and mixed precipitation, wind speed was found to be the most important factor determining the gauge catch and air temperature had a secondary effect. The relations of gauge catch ratio versus wind speed and temperature on a daily time scale were derived and presented for snow and mixed precipitation. Independent tests of the relations have been conducted at the WMO intercomparison stations and reasonable agreement between the corrected precipitation and the DFIR observation has been obtained. These relations are therefore recommended to be used for test correction of gauge measured data. It is expected that implementation of these correction procedures to the current and archived records will significantly improve the accuracy and homogeneity of precipitation data.