Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 5 2004 10.5194/acpd-4-6213-2004 http://www.atmos-chem-phys-discuss.net/4/6213/2004/ http://www.atmos-chem-phys-discuss.net/4/6213/2004/acpd-4-6213-2004.html http://www.atmos-chem-phys-discuss.net/4/6213/2004/acpd-4-6213-2004.pdf 6213 6238 2004-10-06 Improved sky imaging for studies of enhanced UV irradiance J. M. Sabburg sabburg@usq.edu.au C. N. Long Centre for Astronomy, Solar Radiation and Climate, Faculty of Sciences, University of Southern Queensland, Toowoomba, Australia Pacific Northwest National Laboratory, Richland, WA, USA A recent World Meteorological Organisation report discussed the importance of continued study of the effect of clouds on the solar UV radiation reaching the earth’s surface. The report mentions that the use of all-sky imagery offers the potential to understand and quantify cloud effects more accurately. There are an increasing number of studies investigating the enhancement of surface UV irradiance, and UV actinic flux, using automated CCD and sky imagers. This paper describes new algorithms applicable to a commercially available all-sky imager (TSI-440), for research investigating cloud enhanced spectral UV irradiance. Specifically, these include three new algorithms relating to cloud amount at different spatial positions and the visible brightness of clouds surrounding the sun. A possible relationship between UV enhancement and the occurrence of near-sun cloud brightness is reported. It is found that a range of wavelength dependent intensities, from 306 to 400 nm, can occur in one day for UV enhancements. Evidence of a decreasing variation of intensity with longer wavelengths is also presented.