The clearness index is also called global transmissivity

2. Material and methods
2.1. MERRA re-analysis
The MERRA data set [22] has a resolution of 0.5° × 0.65° with 72 vertical levels from ground to 0.01 hPa. The radiative transfer model for shortwave radiation (CLIRAD-SW) is described in Ref. [9]. MERRA includes an odd-oxygen family transport model providing the ozone concentration necessary for solar absorption. Production and loss of ozone as well as other optically active species are specified from climatology of the Goddard two-dimensional chemistry and transport model [12]. The hourly SSI estimates are horizontally interpolated using a bilinear interpolation technique, to the measurement site from the closest four surrounding grid Lactacystin with a weighting factor that is inversely proportional to the distance. Daily irradiation is computed by summing the hourly SSI estimates after multiplying them by the number of seconds in 1 h.
2.2. ERA-interim re-analysis
The ERA-Interim data set [11] has a resolution of 0.75° × 0.75° and counts 60 vertical levels from ground to 0.1 hPa. Ref. [11] noted an overestimation of 2 W m−2 of the incoming radiation at the top of the atmosphere. The radiative transfer model uses the prognostic water vapour and cloud variables (cloud cover, cloud condensed water) from the meteorological model and climatologic values for aerosols, carbon dioxide, trace gases and ozone. The SSI is estimated every 3 h. Similarly to MERRA, the SSI is bi-linearly interpolated to the measurement site. Daily irradiation is computed by summing the eight available SSI estimates after multiplying them by the number of seconds in 3 h.