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Last modified
7/28/2009 2:40:51 PM
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4/24/2008 2:55:43 PM
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Weather Modification
Title
Snow Accumulation Algorithm for the WSR-80D Radar: Final Report
Date
7/1/1998
Weather Modification - Doc Type
Report
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<br />5. SNOWFALL MEASUREMENTS <br /> <br />5.1 The National Network <br /> <br />Because of the problems noted and others discussed by Groisman and Legates (1994), the existing <br />national precipitation gage network was determined at the onset to be inadequate for the purposes of this <br />study. Most climatological gages are read daily and, therefore, do not have the needed time resolution. <br />Tipping bucket gages usually can resolve 0.01 inch of water, but most are unheated so they cannot <br />measure snowfall with any reasonable accuracy. Those that are heated can miss substantial portions of <br />actual snowfalls because of a thermal plume above the gage that deflects incoming snow particles. <br /> <br />Most existing recording gages in the national network are of the Fisher-Porter type with resolution of <br />only 0.10 inch water equivalent. This resolution is an order of magnitude less than the 0.01 inch (or less) <br />provided by a Belfort gage. In most regions of the U.S. which commonly experience snowfall, only a <br />small fraction of all snowfall hours have a melted water equivalent of 0.1 0 inch or more. Therefore, <br />Fisher-Porter gages have little utility for relating radar observations to hourly snowfall observations. <br /> <br />Even where gages with adequate mass resolution exist, they tend to be located near FOs, typically at <br />wide-open, windy airports. The undercatch of such gages can be serious and unknown in magnitude. <br />Snowfall measurements from such locations can add considerable variability to attempts to relate <br />snowfall accumulation to radar observations. <br /> <br />5.2 Wind Effects <br /> <br />As was stressed in Reclamation's proposal to develop the SAA, and discussed in Reclamation's first <br />annual report (Super and Holroyd, 1996), accurate S measurements are essential for "ground-truthing" <br />the SAA. Many articles and reports over the years discuss the need to protect precipitation gages from <br />wind-caused undercatch. The problem is most severe with snowfall. For example, Goodison (1978) <br />reported that gage undercatch by an unshielded Belfort Universal gage can exceed 50-percent with wind <br />speeds as light as 2.5 m s-l, and 75 percent with a 7 m sol wind. Furthermore, Goodison showed that even <br />a Belfort gage equipped with an Alter wind shield can exceed 50-percent undercatch with a 5 m sol wind <br />speed. Goodison and others have shown that the degree of under catch is even greater for some gage <br />types, including the Fisher-Porter, because of their shape. <br /> <br />Reclamation meteorologists with considerable experience in snow measurement carefully chose <br />protected gage sites within range of the WSR-88Ds near Denver, Cleveland and atop the Grand Mesa of <br />western Colorado, using the criteria of Brown and Peck (1962). Sites west-northwest of Minneapolis <br />were chosen by NWS personnel and were somewhat more exposed in the limited forest cover ofthat <br />area. Reclamation installed calibrated Belfort Universal gages in these four areas, all equipped with <br />Alter wind shields, and trained local personal to maintain them. All gages had standard 8-inch diameter <br />orifices except those on the Grand Mesa which had orifices with twice the standard area to minimize <br />snow capping. All gages used one rotation per 24 h gears. <br /> <br />Small clearings in widespread conifer forest, illustrated in figure 1, generally provide excellent snowfall <br />measurement sites. Clearings in thick deciduous forest, especially if low brush is common, can also <br />provide well-protected snowfall observing sites as were found near Cleveland. Such forest clearings, <br />together with gage wind shields, can almost eliminate gage undercatch caused by airflow around the gage <br />orifice. Gages installed and operated for this study were placed in forest clearings wherever possible. Of <br />course, most of the U.S. does not have widespread forest, and alternatives needed to be found for <br /> <br />11 <br />
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