Efficient numerical orientation average for calculations of single-scattering properties of small atmospheric ice crystals

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Description

Cloud phase and composition have substantial impacts on vertical profiles of radiative heating. However, reliable retrievals of these quantities from remote sensing measurements are difficult. Furthermore, retrieval techniques using satellite radiance measurements are limited to at most a few viewing angles. During the 2004 Mixed-Phase Arctic Clouds Experiment (M-PACE) over the North Slope of Alaska (NSA) site, the Atmospheric Radiation Measurement Climate Research Facility’s Unmanned Aerospace Vehicle’s (UAV) diffuse field camera (DFC), consisting of a pair of nadir-mounted digital cameras with hemispheric field-of-view lenses with 620–670 nm and 1580–1640 nm pass filters, was mounted on the Proteus aircraft and measured cloud radiance and irradiance fields. In this study, DFC data are used to derive the complete directional dependence of cloud reflectance and hemispherical directional reflectance factor (HDRF), which is related to cloud phase and composition. The observed HDRF are compared with those calculated for water, ice, and mixed-phase clouds using the radiative transfer code libRadtran that uses the discrete ordinate radiative transfer solver DISORT version 2.0. Distinct features in the calculated HDRF with for varying phases (i.e., water, ice, or mixed), compositions (i.e., sizes and shapes of ice crystals), and cloud optical depths are examined to find the best match compared with the observed HDRF. For mixed-phase cloud simulations the fraction of solid ice and liquid water and the vertical inhomogeneity of clouds are varied to determine which best matches the observed HDRF; similarly, the idealized ice crystal habit that best matches the observed HDRF is identified for ice clouds.