Wind-induced precipitation undercatch of the Hellmann gauges

The Hellmann gauges have been widely used as the official precipitation mea surement instruments in 30 countries. From 1986 to 1993, the accuracy and p erformance of the Hellmann gauges were evaluated during the WMO Solid Preci pitation Measurement Intercomparison at 4 stations in I;inland, Russia, Ger many, and Croatia. The double fence intercomparison reference (DFIR) was th e reference standard used at all the Intercomparison stations. The data for the Hellmann gauges were compiled from measurements made at the 4 WMO inte rcomparison 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 investiga ted. For snow and mixed precipitation, wind speed was found to be the most important factor determining the gauge catch and air temperature had a seco ndary effect. The relations of gauge catch ratio versus wind speed and temp erature on a daily time scale were derived and presented for snow and mixed precipitation. Independent tests of the relations have been conducted at t he WMO intercomparison stations and reasonable agreement between the correc ted precipitation and the DFIR observation has been obtained. These relatio ns are therefore recommended to be used for test correction of gauge measur ed 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.