Dual-polarization Radar Analysis of Orographic Wintertime Clouds with Freezing Drizzle

The presence of supercooled drizzle-sized drops (hereafter, freezing drizzle) in winter clouds poses a unique in-flight icing hazard to certain classes of aircraft. Detecting the presence of supercooled large drop (SLD) icing conditions is critical in order to provide relevant information to the aviation community to help aircraft avoid these hazardous conditions. In particular, the information needs to be reliable so that it can be used to prevent susceptible aircraft from flying into regions of SLD icing and/or to guide aircraft to exit SLD icing conditions while in flight.

Ikeda et al. (2010) proposed an algorithm to identify freezing drizzle based upon recurrent features observed in operational NEXRAD data common to cases with known freezing drizzle. This algorithm employs several statistical parameters related to the radar reflectivity, in order to assess how uniform the echo pattern is, utilizing operational NEXRAD data available at the time of publication. With the expansion of dual-polarization radars across the NEXRAD system, however, the opportunity is ripe to update this algorithm for freezing drizzle detection. In particular, one challenge with this algorithm’s assumption of uniformity arises in cases where conditions are mixed-phase, yet freezing drizzle, and therefore SLD icing conditions, are still present. This paper aims to examine NEXRAD dual-polarization radar data from cases of orographic wintertime clouds in which the cloud was predominantly composed of freezing drizzle (as determined by in situ measurements), yet a few dendrites also were present making the radar return less uniform. The goal of this study is to determine if the dual-polarization data can provide additional guidance on the threat of SLD icing conditions.

This research is in response to requirements and funding by the Federal Aviation Administration (FAA). The views expressed are those of the authors and do not necessarily represent the official policy or position of the FAA.