In-Situ Verification of Remote Aircraft Icing Detection Using S-Band Polarization Radar Measurements

The presence of pure super-cooled large drops (SLD) or mixed phase (ice and liquid) conditions can cause aircraft icing hazards. Accretion of liquid water on the aircraft surface depends on liquid water content, droplet size and the collection efficiency of the surface. In regions where temperatures are below 0 degree 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 freeze when they come into contact with the wings of an aircraft, disrupting the airflow and reducing the aerodynamic performance of the wings. The presence of SLW is difficult to detect using single polarization Doppler radar measurements. The ability to detect and track SLW remotely from the ground would result in greatly increased aviation safety near airports.

Several techniques using millimeter wavelength cloud radar to remotely detect regions of aircraft icing have recently been proposed. However, measurements using short wavelength radars at Ka or X-band frequencies may not be useful in the presence of precipitation sized particles due to Mie scattering and severe attenuation in rain. Therefore, additional methods of remote SLW detection are under investigation. Recent studies have shown the utility of polarimetric radars to distinguish hydrometeor particle types. The NCAR S-Pol polarimetric radar was recently operated during the Mesoscale Alpine Program (MAP) and the Severe Thunderstorm and Electrification and Precipitation Study (STEPS) in coordination with aircraft equipped with microphysical instrumentation. A fuzzy logic-based hydrometeor identification algorithm was implemented using real-time S-Pol observations. When available, the brightband was used to determine the height of the 0 degree C isotherm in the cloud layer. In this paper, we investigate the detection of SLW using ground-based S-band polarization radar measurements by direct comparison of the aircraft microphysical data to appropriate S-Pol data and hydrometeor classification. This is achieved with recently developed software that matches the aircraft data to the radar data in both time and space within user-specified parameters. This allows large data sets to be objectively compared relatively quickly. Results of the radar/aircraft data comparison during periods of icing will be presented.