Dependence of Circumsolar Radiation on Ice Cloud Properties

In the presence of ice clouds, circumsolar radiation (i.e., that portion of radiation scattered into the region of the solar disk and aureole) can be considerable. Although the role of ice crystal shapes and sizes on their single- and multiple-scattering properties has been studied in much detail, the impact of crystal shape and size on the circumsolar radiation has received much less attention. As the measured direct radiation is often contaminated by some circumsolar radiation, the contribution of which would be beneficial to quantify and correct for, it is important to quantify how much the crystal shapes can influence the angular profile of the circumsolar radiation.

In this study, the impact of ice crystal sizes and shapes on downwelling monochromatic radiance is investigated as a function of angle from the Sun at a wavelength of 670 nm. The radiances are simulated by using a forward Monte Carlo model, which uses non-truncated treatment for the phase functions and accounts for the width of the solar disk as well as the limb darkening effect. The input size and shape distributions were measured with in-situ probes installed on the Stratton Park Engineering Company (SPEC Inc.) Learjet during the United States Department of Energy's Atmospheric Radiation Measurement program's 2010 Small Particles in Cirrus (SPARTICUS) field campaign conducted over the Southern Great Plains site. For sensitivity studies, the in-situ measured size distributions are combined with a single idealized habit at a time (column, plate, bullet rosette or aggregate), instead of the in-situ measured shape distributions used in the control case. Size-shape distributions are combined with existing databases of single scattering properties of idealized ice crystals to produce the bulk optical properties of the cloud needed as input to the Monte Carlo model. The simulated radiances are also compared against ground-based very high dynamic range measurements of disk and aureole radiances (as a function of angle from the center of the Sun out to 8°). Reasons for discrepancies between observations and model calculations are discussed.