*2.1. Ground Measurements*

Measurements between year 2012 and 2016 at 20 tower sites from the FLUXNET, SURFRAD and BSRN networks were used in this study. The sites were located over 5 continents as follows: Europe, North America, South America, Australia and Antarctica, as shown in Figure 1. Table 1 lists the key characteristics of the 20 selected sites, including their associated network, geographic coordinates and land cover type. Shortwave radiation is measured by albedometers at these selected tower sites. A broadband (shortwave) albedometer essentially consists of two pyranometers, which measure the total downward and upwelling radiation. Diffuse radiation is measured by an independent shaded pyranometer using a sun tracker to shield the sensor from direct sunlight [16]. Data from the SURFRAD and BSRN sites were taken between year 2012 and 2016 at 1-min resolution, and data from FLUXNET sites were taken between 10-min and 1-h resolution, depending on the site location. We select for presentation one station from each network including two from the USA and one from Australia representing, tundra with snow/ice, grasslands and evergreen broadleaf respectively.

**Figure 1.** Geographical distribution of selected sites (Google Earth).
