Comparison of land surface albedo between MODIS and ground-based measures at the Thule High Arctic Atmospheric Observatory (THAAO) in Pituffik, Greenland

The land surface albedo is one of the key parameters and a driver in climate and weather models since it regulates the shortwave radiation absorbed by the Earth’s surface. In the Polar Regions, the high albedo of snow and ice helps to maintain low surface temperature by reflecting most of the incident solar energy. As the temperature increases and the snow and ice melt, the absorption of solar radiation increases, leading to more warming and melting. This positive ice-albedo feedback is partially responsible for the amplified warming in the Arctic compared to lower latitudes.

In remote areas, where in-situ instruments are absent, satellites are crucial to measure surface albedo changes.

 In this work, a comparison of satellite and in-situ measurements of surface albedo is conducted. The area of interest selected is around the Thule High Arctic Atmospheric Observatory (THAAO, https://www.thuleatmos-it.it) on the North-western coast of Greenland (76.5°N, 68.8°W), where the measurements of downwelling and upwelling solar irradiance have been started in 2016. The used radiometers are regularly calibrated, and corrections for thermal offset are applied.

Albedo determinations based on  MODIS observations from both Terra and Aqua (MODIS MCD43A3 dataset), consisting of daily values with a spatial resolution of 500 m, have been compared with the ground-based measurements.

The analysis has been carried out through five successive steps: the choice of the size of the area for averaging the satellite data; the application of data selection methods; the determination of Blue Sky Albedo by weighting Black and White Sky Albedo; the selection of clear sky conditions based on in-situ measurements; and the comparison of the ground-based and satellite albedo measurements for different areas (1 km x 1 km, 2 km x 2 km and 4 km x 4 km centred at THAAO and including only land surface) and sky conditions (all-sky and cloud-free)

Moreover, filters based on the quality flag have been applied to select the highest-quality data.

The albedo measurements were compared only for cloud-free cases selected using the in-situ solar irradiance measurements (332 cases out of 2922 in the period July 2016 – October 2023).

The results show an underestimation of albedo measurements from satellite compared to the ground-based measurements at the THAAO over a large part of the period considered. The best agreement is found in the summer when there is no snow around the observatory, and the mean measured albedo value is 0.202. The mean bias during this season is around 0.004 for cloud-free conditions and 0.016 for all sky conditions. In spring, when the in-situ albedo values are highly variable between (0.350 and 1), the mean bias is around 0.051 for cloud-free conditions and 0.076 for all sky conditions.