On the spectral dependency of UV radiation enhancements due to clouds in Valdivia, Chile (39.8 degrees S)

Abstract

Data gathered with a five-channel radiometer are used to analyze the spectral composition of cloud enhancements and attenuations of UV (305, 320, 340, and 380 nm) and photosynthetically active radiation (PAR) (400-700 nm) at Valdivia, Chile (39.8 degrees S), during summer months and within 3 hours from solar noon. Variations are referred to clear-sky days occurring at not too distant dates or obtained as polynomial fit to cloud-free periods on partially cloudy days. Photographic images of the sky were taken simultaneously with part of the radiometric data. Frequency of occurrence and duration of enhancements were estimated. Most of the enhancements were associated with cumuliform clouds in dissipation stage while the cloud fringes were crossing over the Sun disk. Both attenuations and enhancements present spectral dependence: the first decreasing and the second increasing toward longer wavelengths. Typical magnitude of the enhancement of 1-min averages for PAR is close to 20%, with highest values exceeding 40%, and decreasing to 6% for 305 nm. Cases with well-defined cloud side reflections were not observed. A weak relation between cloud cover and enhancement magnitude was found: Larger cloud cover was associated with higher enhancements.