Freezing drizzle detection with WSR-88D radars

Freezing drizzle represents a significant in-flight icing hazard and can cause considerable structural damage to aircraft. In an attempt to develop operational radar based algorithms to identify icing regions, measurements from WSR-88Ds are being examined. Icing threat areas are designated by computing the texture, spatial mean, and standard deviation of the radar reflectivity field. Detections of drizzle are facilitated by identifying the presence of reflectivity bright bands because a melting layer is established. The task is difficult when bright bands are absent because echo characteristics for freezing drizzle largely overlap those for “light snow” as reported by routine surface observations. However, the two precipitation types may be separated when environmental factors, such as the vertical profile of temperature and humidity, surface conditions, and cloud top temperatures, are considered. A summary of radar echo characteristics from a number of freezing drizzle cases in Colorado, Minnesota, and Ohio are presented and compared to light snow events. The importance of alternate information sources is demonstrated. Preliminarily results show that likelihood of freezing drizzle increases for cloud top temperatures greater than -10C and reflectivity less than 5 dBZ at the surface. At cloud top temperatures near -12C, both light snow and freezing drizzle are possible, but in such conditions standard deviations of reflectivity indicate more homogeneity for freezing drizzle.