Aircraft-produced Ice Particles in a Highly Supercooled Altocumulus Cloud

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Abstract
On 25 October during the 1986 Project FIRE Intensive Field Observations experiment, a NCAR King Air mission conducted over a ground-based polarization lidar site at Wausau, Wisconsin, sampled a highly supercooled (−28° to −31°C) altocumulus perlucidus layer. Ground-based photography shows the concurrent formation of a contrail, which glaciated and gradually spread until producing an ice crystal optical display. According to wind-advected flight tracks, the aircraft-produced ice particles (APIP)-affected cloud volume was later sampled during zenith lidar measurements, indicating a ∼1.0 m s−1 horizontal dispersion rate, and accidentally repenetrated by the aircraft during a spiral descent. In situ data obtained during the legs generating the APIP measured average liquid-water contents of 0.02–0.05 gm−3, mean droplet diameters of 12–15 μm, and droplet concentrations of 20–25 cm−3. The resampling of the APIP region yielded ice crystal concentrations of 242 I−1 at the original leg altitude, and 36 1−1 within an apparent crystal fallstreak. The background altocumulus ice particle concentration averaged ∼41−1. The APIP mechanism most likely to explain the observations involves the homogeneous freezing of contrail and natural cloud droplets induced by the rapid cooling behind the King Air propeller tips. In view of the wide use of aircraft of this type in basic and applied cloud physics research, caution should be exercised in interpreting ice particle data obtained when the likelihood of flight track resampling exists.

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