Linke turbidity factor
|Last update: Mar. 2017|
The Linke turbidity factor (TL, for an air mass equal to 2) is a very convenient approximation to model the atmospheric absorption and scattering of the solar radiation under clear skies. It describes the optical thickness of the atmosphere due to both the absorption by the water vapor and the absorption and scattering by the aerosol particles relative to a dry and clean atmosphere. It summarizes the turbidity of the atmosphere, and hence the attenuation of the direct beam solar radiation. The larger TL, the larger the attenuation of the radiation by the clear sky atmosphere.
Publications, with - keyword "Linke Turbidity factor": WMO 1981; Kasten 1996, Remund et al. 2003, Diabaté et al. 2003, Rigollier et al. 2000, see also Holben et al. 2001.
The TL denotes the transparency of the cloudless atmosphere. If the sky were perfectly dry and clean, TL would be equal to 1. When the sky is deep blue, the TL is just above 1 and still very small. In summer, in Europe, the water vapour is often large and the blue sky is close to white. The TL is larger than 3. In turbid atmosphere, e.g. in polluted cities, the TL is close to 6 - 7.
Errors were assessed by cross-checking (LOOV-Leave One Out Validation) which consists in removing one station (out of approx. 220), making calculation for this site (using the elevation database and not the actual elevation) and computing the discrepancy. The bias was found to be 0.01 and the RMSE 0.73 as an annual average with slight deviations from month to month.
Twelve monthly maps of Linke turbidity factor values were created given by latitude and longitude. Latitude is positive North, longitude is positive eastwards of longitude 0. The data are in gridded, raw format, no header (tiff images), 1 byte per value (unsigned int encoding), 2160 rows and 4320 columns. Cell size is 5' (approx. 10 km at mid-latitude). Upper left corner is 90 N, 180 W. Then, point 90 N, 179.5 W etc. Lower right is 90S, 180 E.
Two representations for January