Response of FSSP-100 and PVM-100A to Small Ice Crystals

Abstract

Abstract Correction factors Cf are derived for ice-crystal volume and effective radius Re, measured by Forward Scattering Spectrometer Probe (FSSP) and Particulate Volume Monitor (PVM) that are known to overestimate both parameters for nonspherical particles. Correction factors are based on ice-crystal volume and the projected area of randomly oriented model ice crystals with column, rosette, capped-column, and dendrite habits described by Takano and Liou. In addition, Cf are calculated for oblate and prolate spheroids. To test Cf, both probes are compared to small, predominately solid hexagonal ice-crystal plates and columns generated in the Colorado State University (CSU) Dynamic Cloud Chamber (DCC). The tendency of heat released by the PVM (placed inside the chamber) to evaporate ice crystals and the smaller upper size range of the PVM than the size range of the FSSP caused large differences in the probes’ outputs for most comparisons in the DCC. Correction factors improved the accuracy of Re measured by the FSSP for the solid hexagonal crystals, and both probes produced similar results for the projected area and ice water content when crystal sizes fell within the probes’ size ranges. The modification for minimizing ice-crystal shattering and the application of Cf for forward scatter probes such as the FSSP suggests the probes’ improved usefulness for measuring small ambient ice crystals.