Abstract
The top-of-atmosphere (TOA) albedo is one of the key parameters in determining the Arctic radiation budget, with continued validation of its retrieval accuracy still required. Based on three years (2007, 2015, 2016) of summertime (May–September) observations from the Clouds and the Earth’s Radiant Energy System (CERES) and the Multi-angle Imaging SpectroRadiometer (MISR), collocated instantaneous albedos for overcast ocean and snow/ice scenes were compared within the Arctic. For samples where both instruments classified the scene as overcast, the relative root-mean-square (RMS) difference between the sample albedos grew as the solar zenith angle (SZA) increased. The RMS differences that were purely due to differential Bidirectional Reflectance Factor (BRF) anisotropic corrections ( σ A D M ) were estimated to be less than 4% for overcast ocean and overcast snow/ice when the SZA ≤ 70°. The significant agreement between the CERES and MISR strongly increased our confidence in using the instruments overcast cloud albedos in Arctic studies. Nevertheless, there was less agreement in the cloud albedos for larger solar zenith angles, where the RMS differences of σ A D M reached 13.5% for overcast ocean scenes when the SZA > 80°. Additionally, inconsistencies between the CERES and MISR scene identifications were examined, resulting in an overall recommendation for improvements to the MISR snow/ice mask and a rework of the MISR Albedo Cloud Designation (ACD) field by incorporating known strengths of the standard MISR cloud masks.
Highlights
The top-of-atmosphere (TOA) albedo, which is defined as the fraction of incoming solar irradiance that is scattered back to space by the earth-atmosphere system, is one of the key quantities in determining the global energy balance [1,2]
Given that the Multi-angle Imaging SpectroRadiometer (MISR) adopts the Clouds and the Earth’s Radiant Energy System (CERES) angular distribution models (ADMs) for clear-sky albedo retrievals but applies the radiative transfer model (RTM)-based approach for cloud albedo estimates, we focused on quantifying the instruments overcast albedo retrieval differences that were functions of both the surface type and solar zenith angle
Note that the much smaller RMS difference compared to the overcast ocean case did not mean a closer agreement of the overcast albedo retrieval algorithm between the CERES and MISR for sea ice and fresh snow
Summary
The top-of-atmosphere (TOA) albedo, which is defined as the fraction of incoming solar irradiance that is scattered back to space by the earth-atmosphere system, is one of the key quantities in determining the global energy balance [1,2]. The white reflective surfaces of snow-covered ice and bare ice gradually form relatively dark ponded sea ice with some of it getting replaced by the dark ocean This change is seen as Arctic darkening as observed from space when uncompensated by other changes, such as an increase in cloudiness. Pistone et al [5] argued that the shortwave radiative forcing at the TOA associated with Arctic darkening was equivalent to 25% of the global CO2 direct forcing during the study period (1979–2011) Such a large sensitivity of the Arctic climate to the ice-albedo feedback raises the importance of obtaining reliable estimates of the Arctic TOA albedo from the Earth-orbiting satellite sensors
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