Using MISR to account for effects of sastrugi in the CERES clear-sky permanent snow ADMs

Sastrugi are surface roughness elements common on Antarctica that can significantly alter the bi-directional reflectance distribution of snow. This can lead to biases in the estimate of top-of-atmosphere (TOA) reflected shortwave (SW) fluxes when inverting radiances using an empirical angular dependence model (ADM), such as those used in processing the Clouds and Earth's Radiant Energy System (CERES) measurements. A recent study has shown that the presence of sastrugi can result in monthly-mean grid-box biases of between ±15 Wm-2 in reflected 24-hour energy-weighted SW fluxes at the TOA. The Multi-angle Imaging SpectroRadiometer (MISR) on NASA's Terra satellite makes near-instantaneous radiance measurements of the same location from 9 angles in the along track direction, making it uniquely suited to examine the angular effects of sastrugi. In this study we use the Single Scanner Footprint MISR (SSFM) dataset, which applies the CERES point-spread function to co-incident MISR measurements. Using statistical relationships between measurements from MISR's cameras we create a set of adjustment factors. These adjustment factors are then applied to CERES measurements to create a set of ADMs that more accurately capture the effect of sastrugi on the anisotropy of snow reflectance. Applying this new set of ADMs to CERES measurements significantly reduces the effect of sastrugi on instantaneous albedo estimates. This, in turn, results in a reduction of the monthly-mean 24-hour energy-weighted SW flux grid-box biases to between 10 Wm-2 and -5 Wm-2. Removing the bias caused by sastrugi in the CERES TOA SW flux will allow for more accurate comparisons between CERES data and climate model output over Antarctica.