Rangno, A. L., and P. V. Hobbs, 1983: Production of ice particles in clouds due to aircraft penetrations. J. Climate Appl. Meteor., 22, 214-232.
The hypothesis of Woodley and associates is that these ice particles are nucleated homogeneously in the adiabatically-cooled zones caused by nearly-supersonic airflow around the propeller blade tips. The degree of cooling is a complicated function of propeller design, rotation rate, and aircraft drag. The MOLAS investigators arranged for several different aircraft to participate in the experiments in order to explore a range of these parameters.
Through winter 1996-1997, these experiments were conducted in a shallow layer of supercooled fog that occasionally forms over Mono Lake, California, during winter months. These fogs occur under inversion conditions, so the coolest air is at the surface, the coolest air is warm enough that ice does not appear naturally, and there is no question that ice particles appearing in the fog are caused by the penetrating aircraft. Early results are presented in:
Woodley, W. L., T. J. Henderson, B. Vonnegut, G. Gordon, R. Beidenthal,
and S. M. Holle, 1991: Aircraft-produced particles (APIPs) in supercooled
clouds and the probable mechanism for their production. J. Appl.
Meteor., 30, 1469-1489.
Participation of the armored T-28 was first arranged for the winter 1994-1995, and arrangements continued for three subsequent winters. For the first 3 years, the T-28 was moved to Carson City, Nevada, in the fall to be ready for operations when fog was forecast. The T-28 pilot, upon notification, would travel to Nevada via public carrier and perform his mission.
MOLAS operations during these years involved multiple aircraft. The T-28 was equipped with a canister of sulfur hexaflouride, an artificial trace gas, to release as it penetrated the cloud. The instrumented King Air research aircraft operated by the University of Wyoming would then penetrate the T-28 wake to investigate the cloud and iceparticle population, there. The King Air carried an extensive suite of airborne particle samplers and a sulfur hexafluoride analyser. The tracer augmented the ability of the investigators to know exactly when the wake was penetrated.
Given the difficulties of forecasting the conditions for fog, and of arranging prompt travel of the T-28 pilot and the Wyoming King Air to Reno, Nevada, during holiday and ski seasons, there were no operations for the first 3 winters. For winter 1997-1998, alternate arrangements were made to conduct the two-aircraft experiment in northeastern Colorado in elevated supercooled stratus clouds. A successful flight was conducted on 16 February 1998. There was no signature of APIPs in the limited conditions flown (~ -9 C, full power, flaps partially extended).
In addition, on this 1998 flight, testing was conducted of cloud seeding flares designed to produce a hygroscopic aerosol. The microphysical signature observed was ambiguous.