Ice Crystal Roughness Proves Important to Scattering of Sunlight
Measurements from an ARM airborne research campaign are used to identify the key ice crystal parameters that impact scattering of solar radiation at near-forward angles.
The portion of solar radiation that appears to originate from a small disk around the sun is called circumsolar radiation or solar aureole. This radiation arises from near-forward scattering of direct solar radiation by atmospheric particles with sizes comparable to or larger than the wavelength (i.e., larger than 1 µm). The larger the particle is compared to the wavelength of radiation, the more scattering is concentrated at near-forward angles. Consequently, the amount of circumsolar radiation varies widely depending on the geographical, seasonal, and diurnal variation of airborne particles. The overarching goal of this research is to understand how ice clouds influence the downwelling solar radiances within a few degrees from the direction of the sun.
Scientists used detailed measurements of ice crystals from an aircraft research campaign sponsored by the DOE Atmospheric Radiation Measurement (ARM) facility to study how ice crystals affect scattering of solar radiation. Of all parameters considered, assumptions about ice crystal roughness (or non-ideal features in general) were found to be most important for near-forward solar scattering. Ice crystal size distribution was also found to be important for the angular distribution of circumsolar radiance. The radiative transfer modeling tools developed for this work, as well as the increased understanding of the importance of ice crystal roughness on solar radiation, might be of interest for the design of concentrating solar power systems and for the interpretation of data from instruments intended to measure the direct solar radiation.
The impact of ice clouds on solar disk and circumsolar radiances is investigated using a Monte Carlo radiative transfer model. The monochromatic direct and diffuse radiances are simulated at angles of 0 to 8° from the center of the sun. Input data for the model are derived from measurements conducted during the 2010 Small Particles in Cirrus (SPARTICUS) campaign together with state-of-the-art databases of optical properties of ice crystals and aerosols. For selected cases, the simulated radiances are compared with ground-based radiance measurements obtained by the Sun and Aureole Measurements (SAM) instrument. First, the sensitivity of the radiances to the ice cloud properties and aerosol optical thickness is addressed. The angular dependence of the disk and circumsolar radiances is found to be most sensitive to assumptions about ice crystal roughness (or, more generally, non-ideal features of ice crystals) and size distribution, with ice crystal habit playing a somewhat smaller role. Second, in comparisons with SAM data, the ice cloud optical thickness is adjusted for each case so that the simulated radiances agree closely (i.e., within 3 %) with the measured disk radiances. Circumsolar radiances at angles larger than ≈3 degrees are systematically underestimated when assuming smooth ice crystals, whereas the agreement with the measurements is better when rough ice crystals are assumed. The results suggest that it may well be possible to infer the particle roughness directly from ground-based SAM measurements. In addition, the results show the necessity of correcting the ground-based measurements of direct radiation for the presence of diffuse radiation in the instrument’s field of view, in particular in the presence of ice clouds.
University of Helsinki
This work was supported by the Academy of Finland Centre of Excellence (grant no. 272041), the Maj and Tor Nessling Foundation (201600119), and the Office of Biological and Environmental Research (BER) of the US Department of Energy (DE-SC0008500, DE-SC0014065, and DE-SC0016476 through UCAR subcontract 217-90029). Data were obtained from the Atmospheric Radiation Measurement (ARM) program archive, sponsored by the US DOE, Office of Science, BER, Environmental Sciences Division.
Haapanala P, P Raisanen, GM McFarquhar, J Tiira, A Macke, M Kahnert, J DeVore, and T Nousiainen. 2017. “Disk and Circumsolar Radiances in the Presence of Ice Clouds.” Atmospheric Chemistry and Physics 17(11):6865-6882. [DOI: 10.5194/acp-17-6865-2017]