# Evaluation of the Landsat-8 Albedo Product across the Circumpolar Domain
## 2 Materials and Methods
### Ground-Based Tower Albedo  
Validation of the circumpolar Landsat albedo product across the study domain was undertaken through the acquisition of ground tower data from various networks and collaborators as shown in Figure 1. A comprehensive list of the tower sites used in this validation effort can be found in Table 1. These sites cover a broad latitudinal range and a diversity of terrestrial biomes to serve as a proxy of the circumpolar region. In the United States, a great number of the validation sites are part of the National Ecological Observation Network (NEON) operated by Batelle and funded through the National Science Foundation [48; 49; 50]. For this work terrestrial NEON sites located north of 40 degrees in latitude were selected and all available data from 2015 to 2018 were considered. The NEON towers record incoming shortwave, outgoing shortwave, incoming longwave, and outgoing longwave radiation by a 4-component sensor located at the top of the tower infrastructure using a Hukseflux NR01 Net Radiometer. The data are published as one- and thirty-minute averages of 1 Hz observations. Shortwave tower albedo values were calculated as the average of the ratio of outgoing shortwave to incoming shortwave radiation in a one-hour window around the Landsat acquisition time (typically between 10:00 AM and 11:00 AM local time). The data were screened for quality assurance according to the NEON parameters and only high-quality retrievals were considered [51]. Figure 1: Validation tower sites used in this assessment with the red line indicating the product extent above 40\({}^{\circ}\)N.  
In addition to the NEON tower locations, data from the BSRN and SURFRAD networks were also used in this validation effort. The Baseline Surface Radiation Network (BSRN) is an international effort coordinated by the Data and Assessments Panel from the Global Energy and Water Cycle Experiment (GEWEX) under the World Climate Research Programme (WCRP) [52]. The United States supports the BSRN initiative through the maintenance and publication of the NOAA's Surface Radiation Budget Network (SURFRAD). As in the NEON data, the ratio of outgoing shortwave to incoming shortwave radiation during a one-hour window around the Landsat acquisition time is used to represent albedo at the time of Landsat acquisition. To expand the biomes covered in this validation effort, tower data from the Greenland Summit Research Station 2015 and 2017 was used. This tower has been established and maintained by the Earth System Research Laboratories (ESRL), a research branch of the National Oceanic and Atmospheric Administration (NOAA). The ESRL tower measured shortwave upwelling and downwelling radiation and is reported at hourly time steps. Lastly, data from the Sodankyla boreal forest site operated by the Finish Meteorological Institute from 2015-2019 were QA screened and used for valuation efforts [53]. When comparing satellite products with single tower measurements it is crucial to consider the spatial representativeness of the tower measurement. Tower measurements depend on the height of the tower and a spatial representativeness analysis is required to determine just how much of the surrounding landscape conforms to the area in the tower albedo footprint. The albedoeters in this validation effort were assumed to have nominal directional errors of 10% and a half field of view (\(FOV\)) \(\sim\)81\({}^{\circ}\). As such, the instrument footprints (g) were calculated with the mounted height (h) of the downwelling radiometer such that:  
\[\mathrm{g}\ =2\mathrm{h}\ \tan(\mathrm{FOV}^{\circ})\]  
The representativeness at each tower site was assessed at 500 m, 1 km and 2 km and representativeness was ranked by season and conditions [47] Site representativeness was assessed using geostatistical attributes which describe the landscape variability. Specifically, the still values presented in Table 1, are measured as the value where the variogram function flattens at increasing distance.