First correlated measurements of the shape and scattering properties of cloud particles using the new Particle Habit Imaging and Polar Scattering (PHIPS) probe

Abstract

Studying the radiative impact of cirrus clouds requires the knowledge of thelink between their microphysics and the single scattering properties of thecloud particles. Usually, this link is created by modeling the opticalscattering properties from in situ measurements of ice crystal sizedistributions. The measured size distribution and the assumed particle shapemight be erroneous in case of non-spherical ice particles. We present here anovel optical sensor (the Particle Habit Imaging and Polar Scattering probe, PHIPS) designed to measure the 3-D morphology and thecorresponding optical and microphysical parameters of individual cloudparticles, simultaneously. Clouds containing particles ranging in size froma few micrometers to about 800 μm diameter can be systematicallycharacterized with an optical resolution power of 2 μm and polarscattering resolution of 1° for forward scattering directions(from 1° to 10°) and 8° for side and backscatteringdirections (from 18° to 170°). The maximum acquisition rates for scattering phase functionsand images are 262 KHz and 10 Hz, respectively. Some preliminary resultscollected in two ice cloud campaigns which were conducted in the AIDA cloudsimulation chamber are presented. PHIPS showed reliability in operation andproduced comparable size distributions and images to those given by othercertified cloud particles instruments. A 3-D model of a hexagonal ice plateis constructed and the corresponding scattering phase function is comparedto that modeled using the Ray Tracing with Diffraction on Facets (RTDF)program. PHIPS is candidate to be a novel air borne optical sensor forstudying the radiative impact of cirrus clouds and correlating the particlehabit-scattering properties which will serve as a reference for othersingle, or multi-independent, measurements instruments.