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<br />for Algorithm development. Reclamation scientists did acquire the existing November <br />through February Denver WFO hourly snowfall data, as well as daily snowfall amounts from <br />all cooperative observer gages within range of the Denver radar. <br /> <br />Besides the problem of sparsity of Denver data during the dry winter of 1994-95, it was <br />discovered that the standard mode of operation that winter was not to use the Clutter Bypass <br />Map. Rather, because of frequent ground-based inversions and superrefraction, maximum <br />suppression was routinely applied to the entire radar area of coverage. Communication with <br />OSF specialists has revealed that suppression application might have commenced with radial <br />wind speeds (i.e., toward or away from the radar) as great as 10 knots. Suppression is <br />increased as the radial wind speed decreases (Chrisman et aI., 1994). Radial winds less than <br />10 knots are not uncommon over large portions of the lowest tilt radar beam (0.50 elevation <br />angle) during many Denver-area winter storm periods. Consequently, meteorological returns <br />were likely often suppressed even in regions without ground clutter. With the uncertainty <br />of how much suppression was applied at a given place and time and the scarcity of surface <br />snowfall observations it is difficult to see how much use can be made of 1994-95 winter data <br />from Denver for establishing a Ze-S relationship. However, some attempts were made. <br /> <br />All hourly precipitation data from the Denver wFO were examined for the three snowstorms <br />of the 1994-95 winter with associated Level II data. The only hourly values of record are <br />Trace, 0.01, 0.02 and 0.06 (1 value only) inches, so the available range is very limited. <br />Comparisons of these data withZe values directly above the WFO gage revealed large scatter. <br />The scatter was possibly caused in part by over-application of clutter suppression and <br />partially by the lack of range in the snowfall observations. <br /> <br />3 <br /> <br /> <br />Twenty-four-hour precipitation totals from all area cooperative observer gages were also <br />examined and compared to Ze values directly overhead. This comparison resulted in even <br />larger scatter than that observed with the Denver WFO hourly data. At that point, it was <br />decided that resources would be better spent working with the upcoming 1995-96 winter <br />observations than dealing further with the limited and uncertain measurements from the <br />prior winter. <br /> <br />3. Observations from the 1995-96 Winter <br /> <br />3.1 Snowfall Measurement Considerations <br /> <br />Accurate snowfall measurement is difficult because wind effects can cause significant to <br />severe gage undercatch as demonstrated by several authors over many decades. For example, <br />Goodison (1978) reported that gage undercatch by an unshielded Belfort Universal gage can <br />exceed 50 percent with wind speeds as low as 2.5 m s.t, and 75 percent with a 7-m S.l wind. <br />Furthermore, Goodison showed that even a Belfort gage equipped with an Alter wind shield <br />can exceed 50-percent undercatch with a 5 m S.l wind speed. Goodison and others have <br />shown that the degree of undercatch is even greater for some gage types, including the <br />Fisher-Porter, because of their shape. <br /> <br />Another problem is that many existing recording gages in the national network are of the <br />Fisher-Porter type with resolution of only 0.10-inch water equivalent. This resolution is an <br />order of magnitude less than the 0.01 inch (or less) provided by a Belfort gage. In most <br />regions of the U.S. which commonly experience snowfall, only a small fraction of all snowfall <br />hours have a melted water equivalent of 0.10 inch or more. The infrequent occurrence of <br />