Correcting the MODIS snow albedo bias in Greenland based on in situ GC-Net measurements: Implications for the surface energy budget

The shortwave broadband MODIS albedo has higher uncertainties over snow-covered land and at large Solar Zenith Angle (SZA). We have documented a systematic negative bias for SZA>55-60° in Greenland. This study uses in situ data from the Greenland Climate Network (GC-Net) to characterize and empirically correct the MODIS snow albedo (MCD43C) bias at large SZA and thereby remediate the implied bias in the surface energy budget. For observations made with SZAs between 55° and 75°, the mean differences (MODIS minus GC-Net) between the corrected MODIS albedo and GC-Net measurements are -0.02 and -0.03 at Saddle and Summit, respectively, compared to -0.05 and -0.08 between the original MODIS albedo and GC-Net measurements. The corrected MODIS snow albedos are larger than 0.8 and less than 0.87 over dry snow when SZA is larger than 55°, and reduce or eliminate artificially seasonal and meridional trends associated with MODIS retrievals at large SZA. The albedo correction's impact on the surface energy budget is least (-0.5 W/m) in June, and greatest(6.2 W/m) in September for the black-sky albedo (BSA). The mean annual absorbed solar radiation (ASR) reduction by the corrected MODIS albedo in Greenland from 2003 to 2005 is 3.0 and 4.5 W/m for BSA and white-sky albedo (WSA), respectively, about 7.8% and 11.2% of ASR based on the raw MODIS albedo. The ASR reduction by the corrected blue-sky (actual) albedo is between 3.0 and 4.5 W/m, enough to annually melt 28.5 to 40.2 cm snow water equivalent (SWE), or to sublimate 3.4 to 4.7 cm SWE. The snow surface ASR difference between the corrected MODIS BSA and the CERES albedo in March 2005 is only 0.73 W/m, much less than the difference (5.2 W/m) between the raw MODIS BSA albedo and CERES. These albedo adjustments exceed the likely direct anthropogenic radiative forcings experienced by Greenland due to greenhouse gases or aerosols. The proposed correction preserves most of the zonal structure of the raw MODIS albedo, and extends its usefulness as a cryospheric climate record in times, seasons, and regions of Greenland with large SZA.