Modification of spectral UV irradiance by clouds

Abstract

[1] Approximately 10,000 UV irradiance spectra resulting from 2 years of continuous measurements in Germany were used as a database to analyze the effect of cloudiness on spectral UV irradiance. Values of spectral cloud modification factors (CMF) were derived by modeling a corresponding clear-sky irradiance spectrum for every UV measurement under cloudy conditions. The total set of CMF values was used to train neural networks using different sets of input data ( parameter records) to describe the clouds, resulting in different, optimized, algorithms (CMF parameterizations). These different CMF parameterizations were evaluated by asking how the quality of the derived CMFs depended on the information content of different parameter records. It was shown that a visual description of cloudiness is not adequate to determine CMFs for an actual case (deviations of 50% and more), even if it was known whether or not the solar disk was obscured by clouds. Improvements for the determination of actual CMFs are possible, with deviations mostly below 15% if the parameter record comprises an actual broadband irradiance measurement. It was shown that such a CMF parameterization is able to provide a good estimation of actual CMFs, also for places with a different cloud climatology. The sensitivity of CMFs to wavelength and solar zenith angle was investigated on the basis of the derived CMF parameterizations. The relations found depend on the kind of CMF parameterization, i.e., the parameter record. In particular the separation of those cases when the solar disk is visible from those cases when the solar disk is obscured may lead to different dependencies of CMFs on solar zenith angle and wavelength.

Reconstruction of erythemal UV-doses for two stations in Austria: a comparison between alpine and urban regions

“…Kaurola et al, 2000;Schwander et al, 2002;Koepke et al, 2006). European Union's Action COST 726 identified global irradiance in combination with cloud modification factors (CMFs) as the best way to describe cloud attenuation and cloud effects on solar UV (Koepke et al, 2006).…”

Cloud observations for the statistical evaluation of the UV index at Toowoomba, Australia

“…A method to reconstruct the UV in the Netherlands has been reported that employs the reduction by cloud in the global broadband solar radiation to determine the UV exposures (den Outer 2005). Schwander et al (2002) have integrated pyranometer broadband total solar irradiance and illuminance, measured by a Luxmeter, with the variables of SZA, ground albedo and cloud parameters. These were determined from visual observations of total cloud cover, cloud type (low, medium high and high) and solar disk obstruction, to determine the cloud modification factor (CMF) for the UV.…”

Solar radiation and human health

“…The clear-sky irradiance is either taken from measurements or it is modelled. The latter is preferred since appropriate corresponding values (at least in SZA, ozone amount, aerosol properties and surface albedo) are often not available [42]. The CMF is expected to be close to 1.0 for clear sky and no aerosols.…”

“…This hypothesis, based on heat protection is not supported by physical arguments since a black body absorbs more energy, and, consequently, becomes warmer than a white body. 42 …”