Identification of aircraft icing conditions with a polarimetric WSR-88D

In mixed-phase clouds, supercooled liquid water (SLW) is commonly obscured in the radar signal by much larger ice particles. Polarimetric radar, however, has the ability to discriminate between rimed and unrimed ice particles, thereby providing a means of obtaining indirect evidence of the presence or absence of SLW. As such, vertical profiles of radar reflectivity Z, differential reflectivity ZDR, specific differential phase KDP, and correlation coefficient rhoHV might be used to discern between systems that exhibit conditions favorable for aircraft icing and those that do not. More specifically, when SLW is present, water mass is acquired by ice particles through accretion, resulting in particles that are on average more heavily rimed, slightly more spherical in shape, and have a much lower aggregation rate than would be the case if SLW were not present. Other differences at higher levels, such as height of cloud top and the presence of enhanced deposition at temperatures colder than -10°C, leading to large stellar and dendritic ice crystals that would quickly sweep out any available cloud water at lower levels, thereby decreasing icing potential, are also observable by polarimetric radar.

In this study, data from the polarimetric KOUN WSR-88D radar are being analyzed to investigate polarimetric signatures associated with aircraft icing conditions. A database of 23 potential icing events has been created. All pilot reports (PIREPS) within 150 km of the KOUN radar for these events were then obtained from a database maintained by the National Center for Atmospheric Research. From this search, approximately 120 PIREPS, with reported icing conditions ranging from trace to heavy, have been identified at times when coincident KOUN data are available. These events will be examined to provide a better understanding of possible polarimetric signatures associated with aircraft icing conditions.