Aircraft-produced Ice Particles (APIPs) in Supercooled Clouds and the Probable Mechanism for their Production

Abstract

This paper presents the results of studies of aircraft-produced ice particles (APIPs) in supercooled fog over Mono Lake, California. The King Air 200T cloud physics aircraft of the University of Wyoming and three other aircraft (a Piper Aztec, a Cessna 421-C, and a T-28) were involved in the tests. The King Air served as the monitoring aircraft when the other aircraft were tested and as both the test and monitoring aircraft when it was tested. The studies demonstrated that the King Air produces APIPs. The ice crystals, in concentrations up to several hundred per liter, are initially quite small and of almost uniform size, and they grow to larger nearly uniform sizes with time. APIPs production is most likely at low ambient temperatures and high power settings, and when the gear and flaps are extended. APIPs were not detected from the other aircraft. The Piper Aztec and Cessna 421 aircraft were tested on days on which an APIPs signature was produced by the King Air. The T-28 aircraft was tested when the fog-top temperature was greater than ? 6°C and neither the T-28 nor the King Air produced APIPs under these conditions. Homogeneous nucleation appears to be responsible for the observed APIPs signature, although the exact mechanism for nucleation is not known. In addition, there is the suggestion that a weaker APIPs signature may be generated by heterogeneous nucleation, when the cooling in the prop-tip vortex falls short of that thought necessary for homogeneous nucleation (i.e., ? ? 39°C).