In Situ Verification of Aircraft Icing Conditions Involving Needle Crystals and Supercooled Water in Winter Storms

Clear icing conditions involving supercooled drizzle drops and raindrops (in contrast with supercooled cloud water droplets) are often deemed the most hazardous because large liquid drops impact the wing beyond the thermal ice protection section and cause runback icing with a substantially increased drag. A suite of thermodynamic/cloud microphysical/radar/lidar measurements on the Canadian National Research Council Convair-580 have been used for the verification of in situ icing conditions in winter storms in two aircraft campaigns (BAIRS I in 2013 and BAIRS II in 2017) under FAA sponsorship. The aircraft flights were conducted in the vicinity of NEXRAD dual-polarimetric radars in Buffalo and Binghamton, New York. Multiple clear icing scenarios have been documented with both aircraft and radar and are characterized by triple intersections of the 0 ^oC isotherm in local temperature profiles. These three intersection altitudes delimit four distinct microphysical zones, which from top to bottom are (1) cold (i.e., below freezing) zone in which supercooled drizzle, graupel, and unrimed ice have been variously identified, (2) melting zone characterized by partially melted ice, and raindrops and enhanced radar reflectivity (“bright band”) and with a temperature inversion in its lower portion, (3) a cold zone with large (>1 mm) supercooled drops and refreezing processes, and (4) a warm (above freezing) zone near the surface and below the cloud base. Both aircraft spirals and porpoising maneuvers through these zones have been effective in sampling the in situ microphysics. Ingestion of clean maritime air in these multiple case studies may be favorable to the formation of the supercooled drizzle drops. Comparisons will be shown between the dual-polarimetric-radar-based hydrometeor classifications and the in situ aircraft measurements in situations of this kind on February 27, 2013 and on January 24 and February 7, 2017.