A Relationship between Reflectivity and Snow Rate for a High-Altitude S-Band Radar

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) = alpha S-beta, where S is a liquid-equivalent snow rate and alpha and beta 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 National Weather Service radar, this work investigates the Z(e)-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. The Z(e)-S data pairs were fitted with beta = 2. The finding of this study for Cheyenne, alpha = 110 mm(4) h(2) m(-3), is bounded by previous determinations made at other high-altitude National Weather Service sites. Also investigated was the temperature dependence of alpha. A positive alpha-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.