A balloon-borne continuous cloud particle replicator for measuring vertical profiles, of cloud microphysical properties: Instrument design, performance, and collection efficiency analysis

The performance of a balloon-borne Formvar replicator that measures continuous profiles of cloud microphysical properties is investigated. The replicator measures the small particles (<100-μm diameter) that are undetectable or poorly resolved by aircraft measurements from optical imaging probes and provides high-resolution information on ice crystal shapes. During ascent on a balloon, the replicator samples and preserves plastic replicas of cloud particles collected on a filmstrip coated with Formvar; an attached radiosonde provides simultaneous thermodynamic measurements. Replicator data are digitized at a resolution of 1.4-3.4 μm, sufficient to observe ice crystal habits and even details of ice crystal structure. Image processing and analysis yields particle size and habit distributions and particle cross-sectional areas, from which other bulk microphysical properties are calculated. Cloud particles at least as small as 7 μm in diameter are collected. The size-dependent collection efficiency E_c for this replicator is determined using a numerical fluid dynamics model and validated using wind tunnel measurements. The E_c for droplets increases slowly from 3% at 5 μm to 11% at 20 μm, then increases rapidly to 59% at 40 μm, and increases slowly for larger particle sizes. An assessment of uncertainties suggests that the E_c can be applied over the entire range of collectable particle sizes, provided the imaged replicator data are averaged and analyzed across the full width of the collector if small particles are present. The E_c differs substantially from the available theoretical treatments that use approximations of the flow around simple collector shapes, suggesting that a detailed analysis of the E_c is necessary on a case-by-case basis for impactor-type particle collectors that contain a sample opening. A summary of general design criteria for particle impactors is given, oriented toward enhancing the E_c of small particles.