Constraining MODIS snow albedo at large solar zenith angles: Implications for surface energy budget in Greenland

An understanding of the surface albedo of high latitudes is crucial for climate change studies. MODIS albedo retrievals flagged as high quality compare well with in situ Greenland Climate Network (GC-Net) measurements but cover too little area to fully characterize Greenland's albedo in non-summer months. In contrast, poor quality MODIS retrievals provide adequate spatio-temporal coverage, but are not recommended for use at large solar zenith angles (SZAs) where they have a systematic low bias. We introduced an empirical adjustment to the poor quality data based on high quality reference albedos and constrained by GC-Net data and theory, and used the adjusted data to improve estimates and fill-in gaps of the yearround, Greenland-wide, albedo and surface energy budget. For observations made with SZAs between 55° and 75°, the mean differences (MODIS minus GC-Net) between our adjusted 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 unadjusted MODIS albedo and GC-Net measurements. The adjusted MODIS snow albedos are usually between 0.75 and 0.87 over dry snow when SZA is larger than 55°, and they reduce unrealistic seasonal and meridional trends associated with MODIS retrievals at large SZA, defined as SZA > 55° and 70°, respectively, for lowand high-quality retrievals. The impact of the adjusted albedo on the surface energy budget, relative to the unadjusted albedo from all MODIS data, is least (-0.7±0.1W/m) in June, and greatest (-6.2±0.9 W/m) in September for the black-sky albedo (BSA). The mean annual absorbed solar radiation (ASR) reduction by the adjusted MODIS albedo in Greenland from 2003 to 2005 is 3.1±0.2 and 4.3±0.2 W/m for BSA and white-sky albedo (WSA), respectively, about 8.0±0.5% and 10.8±0.4% of ASR based on the raw BSA and WSA. The ASR reduction by the adjusted blue-sky (actual) albedo is between 2.9 and 4.5 W/m, enough to annually melt 27.1 to 41.7 cm snow water equivalent (SWE), or to sublimate 3.2 to 4.9 cm SWE. The ASR difference between the adjusted MODIS BSA and CERES albedo in March from 2003-2005 is only -0.1± 0.9 W/m, much less than the difference (4.9±1.4 W/m) between the unadjusted MODIS BSA and CERES. The albedo adjustments exceed the likely direct anthropogenic radiative forcing experienced by Greenland due to greenhouse gases or aerosols. The proposed adjustment preserves most of the zonal and meridional structure of raw MODIS albedo, and extends its usefulness as a cryospheric climate record in times and regions of Greenland with large SZA.