The Frise heated tipping bucket is the precipitation accumulation gauge used with the Automated Surface Observing System (ASOS). The sensor measures liquid accumulation, but is not specifically designed to accurately measure freezing or frozen precipitation. The accurate measurement of liquid equivalent accumulations in all types of liquid, solid, and mixed precipitation is an important part of weather observations. 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 CRN cooperative effort with NCDC, and to assure that ASOS provides representative measurements of precipitation in all conditions, the NWS has undertaken a program to compare AWPAG measurements in the gauge and shield configuration as installed on ASOS to an AWPAG with a Tretyakov shield inside a DFIR. Obviously the measurement will be lower, but following an approach developed by the WMO, the measurement in the standard ASOS configuration can be corrected to be in close agreement with the measurement inside the DFIR. The approach will be 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 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 2005 - 2006, at a test site in Johnstown, Pennsylvania. This paper presents the data analyses and test results.