Albedo-Driven Radiative Forcing Dynamics from Seasonal Melt of the Greenland Ice Sheet

A combination of low snowfall totals and earlier than average spring warming have resulted in an early and extensive 2019 melt season in Greenland. Melting snow and the exposure of bare ice decreases surface albedo by between 30 and 40 percent, resulting in increased radiative forcing. Albedo is a fundamental geophysical variable in determining land surface energy budgets, and the earlier and longer melt season in Greenland, observed both by satellite and in situ measurements over the past several decades, has the potential to cause increased local surface radiative forcing by reducing reflected solar radiation. Albedo measurements from the MODIS, VIIRS, Landsat-8 OLI, and Sentinel-2 MSI sensors provide a clear and comprehensive data set of arctic seasonal melt cycles, with daily albedo retrievals provided by the operational MODIS and VIIRS albedo products (MCD43, VNP43), and higher spatial resolution albedo measurements provided by Landsat-8 and Sentinel-2. The longer, denser time series provided by daily MODIS/VIIRS enables decadal-scale time series analysis, while the recent Landsat-8/Sentinel-2 products expose fine-scale spatial heterogeneity, providing for a more nuanced estimate of radiative forcing. MODIS and Landsat measurements of ice surface temperature are also available for comparison.

The daily MCD43 and VNP43 products rely on the semi-empirical linear RossThick-LiSparse Reciprocal BRDF model to characterize surface anisotropy, using a 16 day moving window to accumulate multi-angle observations, with an emphasis on the day of interest for the retrieval of the BRDF for each pixel, which is used to calculate Black sky (directional) and white sky (diffuse) albedos. Albedo is calculated for Landsat-8/Sentinel-2 pixels by integrating the coarser-resolution MODIS or VIIRS BRDF with the finer-resolution directional Landsat-8 or Sentinel-2 surface reflectances, based on the premise that the albedo to nadir reflectance ratio is comparable over spectrally similar target areas covered by the two sensors. Furthermore, improved fine scale albedo retrieval is now also possible over snow covered areas due to the higher radiometric fidelity of Landsat-8 and Sentinel-2 relative to earlier Landsat sensors. This research explores temporal and spatial trends of the Greenland ice sheet melt dynamics.