One of the sessions at the 12th Conference on Applied Climatology dealt with the development of the U.S. Climate Reference Network (CRN). An important concern identified in the session was the undercatch of frozen precipitation in a windy environment that results from the comparatively high drag coefficient of snow. Another concern was the need for real-time measurement of snowfall. In response to minimizing precipitation undercatch in CRN and providing real-time measurements, an experiment was initiated to compare temporal accumulations of liquid-equivalent frozen precipitation from identical heated weighing-recording precipitation gauges in six different types of windshields beginning in January 2001 at the NCAR Marshall Field site south of Boulder, CO.

The Geonor vibrating-wire precipitation gauge was used along with a specially designed heater and control. We found that controlled heating applied to the exterior collection cylinder wall prevented snow accumulation on the interior and subsequent "dumping" when melting occurred. The six different windshields comprised a DFIR (double-fence intercomparison reference), a 2/3 diameter DFIR, a Wyoming shield, a half-scale Wyoming, a double-Alter, and a single Alter. Comparisons were performed among the six gauges and were also compared with the NWS 8" unheated manual gauge located in a single Alter shield. Total snowfall from 13 events was approximately 54 in.

The data show that the largest total catch was measured in the DFIR shield, a result consistent with a WMO comparison among various windshields, one of which was a DFIR. Liquid-equivalent accumulation over all events from the gauge in the 2/3 diameter DFIR was 96% of that in the DFIR. Similar ratios for the gauges in the Wyoming, half-scale Wyoming, double-Alter, and single Alter were, respectively, 94%, 89%, 82%, and 75%. The ratio for the NWS 8-inch manual gauge was 81%. These ratios show that multiple fences reduce undercatch more than single fences. Similar ratios, but computed over 15-minute intervals, show considerable scatter when plotted against wind speed. In addition, the ratios from the gauges in the two Wyoming and single Alter windshields decrease significantly with increasing wind speed.