Verified Detection of Supercooled Large Droplets with Dual-Polarized, Millimeter-wave Radar

Supercooled large cloud droplets (SLDs), those of 50-500 micron diameter, have been previously identified as creating a particularly severe aircraft icing hazard, due to their ability to penetrate the slip stream of an aircraft wing and freeze on the surface as rough ice. Several polarization states have been evaluated with NOAA/ETL's Ka-band (8.66 mm wavelength) radar to distinguish hazardous SLDs from the various types of cloud ice particles that commonly occur but do not cause icing. Most recent tests focused on depolarization ratio, DR, obtained from transmitting a 45 -slant, quasi-linear polarization state and receiving the corresponding co- and cross-polarization signals. Scattering calculations have demonstrated that this polarization state has a significant advantage over the standard horizontal linear state because of the increased power received in the cross-polarized received signal, which is considerably weaker than that received co-polar channel, but required to differentiate particle type. Another of several advantages is that the slant polarization state is comparatively insensitive to variations in settling of orientation of ice particles, which causes uncertainty is horizontal DR measurements. Current and previous tests have isolated warm large droplets from ice particles, and that in itself demonstrated the capability of this kind of dual-polarization radar to differentiate between them. However, supercooled large droplets are rare events and are not often present during short field experiments. Now, under FAA sponsorship, verification of SLD detection with the radar has been found in data taken during the 1999 Mt. Washington Icing Sensors Project (MWISP). The radar DR signature of the large water droplets is independent of their temperature. However, their detection in the radar DR signature, with size and supercooling confirmed with in-situ observations, provided reassurance that millimeter-radar identification of actual icing hazards can be accomplished . The expected radar signature for water droplets was observed in a sub-freezing cloud while 150 m mean diameter droplets (SLDs) were observed with CRREL's 2DGC PMS cloud particle probe at Mt. Washington Observatory (MWO). Other supporting data included visual observation of fog at the summit, substantial icing measured with the MWO Rosemount icing probe, substantial cloud liquid water measured with ETL's microwave radiometer, and sub-freezing temperatures verified with an NCAR CLASS sounding. The case is presented and further supported by measurements that show how the various individual types of ice particles were well-differentiated among themselves and from SLDs.