Aircraft Icing Detection Using S-band Polarization Radar Measurements

The combination of supercooled liquid water content (SLWC) > 0.2 g/m3, cloud temperature approximately -10 C, and median volume diameter (MVD) > 30 micron causes significant aircraft icing. The amount of SLWC is highly variable and depends on moisture content and temperature profile. In regions where temperatures are below 0o C, super-cooled liquid water (SLW) is generated through transport of liquid droplets from warmer temperatures (updrafts), or the condensation of vapor into cloud droplets. These super-cooled droplets are subject to contact freezing, or riming, when they come into contact with a solid body such as hail stones or aircraft. Three parameters, namely liquid water content, MVD and temperature, are used for characterizing the severity of icing conditions. The presence of SLW is difficult to detect, yet constitutes a significant threat to aviation safety. The ability to detect and track SLW remotely from the ground would result in greatly increased air safety near airports.

A dual-wavelength (Ka- and X-band) radar technique can be used for detecting the amount of liquid water and droplet size in cloud. However, the technique and equipment are complicated and expensive to deploy and operate. Therefore, alternate methods of remote SLW detection are under investigation. Recent studies have shown the utility of polarimetric radars to distinguish hydrometeor particle types. In this paper, we investigate the detection of SLW using ground based S-band polarization radar measurements. The NCAR S-Pol radar (a 10 cm wavelength dual polarimetric ground based radar) was operated during the Mesoscale Alpine Program (MAP) in coordination with several aircraft equipped with microphysical instrumentation (including 2-D probes, and the Rosemount icing probe). Based on flight reports and icing detection instruments, we have identified several time periods that the planes experienced icing conditions within regions of S-Pol observations. The radar observations, as well as the particle identification output, are compared to the 2-D probe measurements in regions the aircraft observed super-cooled drops. In this paper we present preliminary results of the study to determine if polarimetric radars are capable of detecting regions of aircraft icing hazards.