Using polarimetric remote sensing measurements to estimate ice particle size, optical depth and ice water path during CRYSTAL-FACE

In situ observations made during the CRYSTAL-FACE field experiment have indicated that ice crystals have smaller sizes and are more reflective than is commonly assumed in most current climate models. The size of the particles appears to be principally determined by temperature with the smallest particles being found at the coldest temperatures. Previous analyses of polarimetric measurements in non-absorbing bands have suggested that either bubble inclusions (inhomogeneous hexagonal mono-crystals) or distortions of the hexagonal crystal shape (distorted chain aggregates) are responsible for the observed general absence of haloes, smooth angular variation of reflectance and brightness of ice clouds. In this paper we use multi-angle measurements made by the Research Scanning Polarimeter (RSP) to examine the polarized and unpolarized reflectance of cirrus clouds in bands where ice is non-absorbing (670 and 865 nm) and absorbing (1590, 1880 and 2250 nm). During CRSYTAL-FACE the RSP scan was biased so that the view angle range was from 0 to 75 degrees to the rear of the Proteus aircraft and from 0 to 45 degrees to the front and was oriented to scan along the groundtrack of the aircraft. This allowed observations of a single target over a wide scattering angle range particularly when consecutive flight legs could be combined which allows for basic discrimination of crystal habit using the non-absorbing bands, similar to previous studies (at least in the gross sense of being able to separate columns from plates from distorted crystals from spheroidal shapes). However, compared with non-absorbing bands, the reflectance in absorbing bands is different depending on whether ice particles are geometrically distorted or contain air bubble inclusions because the path length of light inside an ice crystal is quite short which limits scattering off the bubble inclusions. Consequently the retrieved particle size is also sensitive to whether ice crystals are modeled as being distorted or containing air bubbles. We examine how the RSP size retrievals, with an appropriate vertical weighting determined by Green's function calculations, compare with in situ measurements and examine the angular and spectral polarized and unpolarized residuals from the retrievals. This allows us to identify the most appropriate crystal habit for use in the remote sensing of cirrus clouds formed by convection over land, such as those observed during CRYSTAL-FACE, and provide best estimates for the particle size, optical depth and ice water path determined using solar reflectance measurements.