Circumsolar radiation provides information on atmospheric ice crystals
The circumsolar region surrounding the solar disk is often much brighter than the rest of the sky, especially in the presence of high clouds consisting of ice crystals. Comparison of modelled and measured solar radiation in the circumsolar region indicates that most ice crystals either have rough surfaces or other irregularities.
Comparison of modelled and measured solar radiation in the circumsolar region indicates that most ice crystals either have rough surfaces or other irregularities. When the sky is covered by high clouds consisting of ice crystals, the circumsolar region surrounding the solar disk is much brighter than the sky farther away from the sun. The brightness of the circumsolar radiation results from light scattered by ice crystals. Ice crystals scatter much of the visible radiation in near-forward directions, within a few degrees from the sun. Measurements of the distribution of solar radiation near the sun can provide information on the properties of the ice crystals.
In a study coordinated by the Finnish Meteorological Institute and the University of Helsinki, a systematic investigation of how ice cloud properties influence the circumsolar radiation was carried out. Based on calculations with a radiative transfer model, the distribution of circumsolar radiation is influenced most strongly by the roughness (or other irregularities) of ice crystals, along with the ice crystal sizes. The model calculations were compared with measurements of circumsolar radiation with the Solar Aureole Measurement instrument, carried out in the USA during the SPARTICUS measurement campaign. In all cases considered, the agreement between model results and measurements was better when the ice crystals were assumed to have rough surfaces. This suggests that most atmospheric ice crystals have either rough surfaces or other irregularities, and that ideal, completely smooth hexagonal ice crystals are relatively rare.
The results of this research can be applied for developing remote sensing techniques of ice cloud properties based on solar radiation measurements from the surface. The results may also be utilized in the interpretation of measurements of direct solar radiation and in the design of concentrating solar energy applications.
This research was funded by the Academy of Finland, the Maj and Tor Nessling foundation, and the US Department of Energy. In addition to University of Helsinki and the Finnish Meteorological Institute, scientists from the USA, Germany and Sweden participated in the work.
More infomation:
Senior research scientist Petri Räisänen, tel. 029 539 2224, petri.raisanen@fmi.fi
Postodoctoral reseacher Päivi Haapanala, tel. 029 415 1681, paivi.haapanala@helsinki.fi
Haapanala, P., Räisänen, P., McFarquhar, G. M., Tiira, J., Macke, A., Kahnert, M., DeVore, J., and Nousiainen, T.: Disk and circumsolar radiances in the presence of ice clouds, Atmos. Chem. Phys., 17, 6865-6882, https://doi.org/10.5194/acp-17-6865-2017, 2017.
http://www.atmos-chem-phys.net/17/6865/2017/acp-17-6865-2017-discussion.html