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An important application of radar reflectivity measurements is their interpretation as precipitation intensity. Empirical relationships exist for converting microwave backscatter retrieved from precipitation particles (represented by an equivalent reflectivity factor Z e) to precipitation intensity. The reflectivity-snow-rate relationship has the form Z e 5 αS β, where S is a liquid-equivalent snow rate and α and β are fitted coefficients. Substantial uncertainty exists in radar-derived values of snow rate because the reflectivity and intensity associated with snow tend to be smaller than those for rain and because of snow-particle drift between radar and surface detection. Uncertainty in radar-derived snow rate is especially evident at the few available high-altitude sites for which a relationship between reflectivity and snow rate has been developed. Using a new type of precipitation sensor and a NationalWeather Service radar, this work investigates the Ze-S relationship at a high-altitude site (Cheyenne, Wyoming). The S measurements were made 25 km northwest of the radar on the eastern flank of the Rocky Mountains; vertical separation between the radar range gate and the ground was less than 700 m. A meteorological feature of the snowstorms was northeasterly upslope flow of humid air at low levels. TheZe-S data pairs were fitted with β=2. The finding of this study for Cheyenne, α=110 mm 4 h 2 m -3, is bounded by previous determinations made at other high-altitude NationalWeather Service sites. Also investigatedwas the temperature dependence of α.Apositive α-T relationship is evident and is hypothesized to result from ice crystals produced by heterogeneous ice nucleation, at cloud top, followed by diffusional crystal growth during sedimentation. © 2012 American Meteorological Society.




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