Particle Impact and Breakup in Aircraft Measurement

Abstract

Measurements of cloud particle properties from aircraft by optical and impact techniques are subject to artifacts following particle breakup prior to detection. The impact kinetic energy to surface energy ratio (L) provides a breakup criterion at L ≥ 7 for water and ice with major fragmentation for L > 100. This applies to optical imaging probes for particle concentration, size, and projected area spectra measurement. Uncertainty arises should impacting particles shatter and disperse, defeating the intent of the original measurements. Particle shatter is demonstrated in Formvar replicas (University of North Dakota Citation) and video records of particle approach and impact on the Cloudscope (NCAR C-130, NASA DC-8) at airspeeds of 130 and 200 m s?1. Sufficient impact kinetic energy results in drop splash and ice shatter, with conversion to surface energy and ultimately thermal energy through viscous dissipation and ice defect production occurring down to the molecular scale. The problem is minimized in design by reducing the regions responsible for particle breakup to a minimum and locating sensors in regions inaccessible to shatter fragments.