The heated tipping bucket was the initial precipitation accumulation gauge used when the Automated Surface Observing System (ASOS) was deployed. The sensor measures liquid precipitation accumulation, but is not designed to accurately measure freezing and frozen precipitation. The accurate measurement of all types of precipitation is an important part of weather observations; therefore the National Weather Service (NWS) ASOS Product Improvement (PI) team has conducted compliance testing of the All-Weather Precipitation Accumulation Gauge (AWPAG) since 2001. Additionally, the NWS and National Climatic Data Center (NCDC) are cooperating in a field test of different precipitation gauges and shielding configurations. The ultimate goal is to standardize precipitation measurements in the United States. For this purpose, the NWS test sites at Sterling, Virginia, and Johnstown, Pennsylvania, have been included in NCDCs Climate Reference Network (CRN).

The AWPAG specification requires comparability with a standard NWS 8-inch non-recording precipitation gauge with a single metal Alter shield. However, wind can significantly reduce precipitation catch, particularly when the precipitation is in the form of snow. This has resulted in the World Meteorological Organization (WMO) developing an internationally recognized reference windshield (Goodison, B.E, Louie, P.Y.T, and Yang, D., 1998), the Double Fence Intercomparison Reference (DFIR) which will improve precipitation gauge catch efficiency. As part of the NWS cooperative effort with the NCDC CRN program, and to ensure that ASOS provides representative measurements of precipitation in all conditions (especially wind driven snow), the NWS has undertaken a program to compare measurements of a standard ASOS AWPAG/Tretyakov shield configuration to an AWPAG with a Tretyakov shield inside a DFIR. Obviously the precipitation measurement will be lower; but following an approach developed by the WMO, the precipitation measurement in the standard ASOS configuration can be corrected for improved comparability with the precipitation measurement inside the DFIR. The approach was to use wind speed, temperature, and knowledge of the precipitation type (information that is available from ASOS sensors) to derive the ratio of the two precipitation measurements. The equation so derived, referred to as the transfer function, can then be implemented on ASOS to provide more accurate real-time measurements of precipitation, even in windy snow conditions.

Testing was conducted during the winter of 2006 - 2007, at a test site in Johnstown, Pennsylvania. This paper presents the data analyses and test results.