Dual-Polarization Radar to Identify Drizzle, with Applications to Aircraft Icing Avoidance

Abstract

Freezing drizzle has been identified as a primary aircraft in-flight icing hazard. New evidence shows that it can be a greater hazard than freezing rain, even though the latter can be significant. Freezing drizzle may form by droplet coalescence without an intermediate melting process, so that it also can be more difficult to detect than freezing rain. Theoretical calculations of microwave scattering and verification by the initial field studies presented here demonstrate that freezing drizzle should be detectable and distinguishable from other hydrometeor types in short-wavelength, dual-polarization radar measurements of elliptical and linear depolarization ratios. A practical procedure is suggested for identifying and monitoring this aviation hazard with the radar and concurrent atmospheric temperature measurements. The WSR-88D radar (NEXRAD) has the potential to add dual-linear polarization to operationally apply this method, within certain constraints