| This paper describes the algorithms and data used to construct a worldwide Linke turbidity factor (TL, for an air mass equal to 2)database. Two main steps had to be performed to obtain the information: 1. Assembling estimates of TL and 2. fusing different background layers for the construction of the TL maps. The estimates of TL have two forms: either they stand for specific geographical locations and have a high accuracy, or they are available as gridded data averaged over large areas. Point information was gathered from measured time series of hourly beam and daily global radiation, which were transformed to TL. From publications and networks like AERONET other turbidity quantities were obtained and transformed to TL. The basic gridded data are the maps of daily global irradiation supplied by the NASA-Langley Research Center. The included monthly clear sky irradiations were converted to TL with the same method as for the ground sites. Further gridded information was taken from NOAA pathfinder aerosol data and NASA NVAP. An algorithm was devised to fuse these two types of data and to produce gridded maps in a canonical projection and 5' arc angle cells. These final maps should reproduce the values observed at specific locations. The root mean square error of the interpolation is 0.73 TL units. |
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