PVM and FSSP Response to Small Ice Crystals

Forward-scattering aircraft probes including the PVM-100 and the FSSP-100 are known to produce an overestimated volume (Vd) and effective radius (Re) for aspheric particles such as ice crystals. The motivation to again look at the response of these probes to small ice crystals comes from the knowledge of the optical importance of small ambient crystals, especially in cold cirrus, and from the expected improvement of the probes' performance given modification to their inlet geometry that minimizes ice-crystal shattering.

An earlier study in the DCC (Dynamic Cloud Chamber) of Colorado State University is re-examined to evaluate the response of the PVM and FSSP to small ice crystals generated in this chamber. The ice crystals consisted mostly of hexagonal plates and columns, and also consisted of some plates with dendritic features, and some crystals that were quasi-spherical. Dimensions of the crystals, measured manually with a microscope, formed the basis of calculating reference values of V and Re to which PVM and FSSP measurements were compared. Only several of the 18 experiments run in the DCC produced reasonable agreement between PVM, FSSP, and the V reference, given that heat emitted by the PVM located in the DCC tended to evaporate ice crystals. After correction Re measured by the FSSP agreed well with the Re reference.

Measured Vd were also compared to V calculated for randomly oriented ice-crystals (described by Takano and Liou, 1995, JAS) consisting of plates, columns, dendrites, and rosettes. This resulted in a volume correction factor Cf = V/Vd as a function of crystal dimensions for those crystal habits that ranged between 0.05 - 0.75. A correction factor was also determined for Re. The DCC experiments are described, and algorithms for the dependence of Cf on ice-crystal dimensions are given in Gerber and DeMott (2014; JTECH, submitted).