Using MODIS to improve the CERES Sea Ice Angular Dependence Models

The Clouds and Earth's Radiant Energy System (CERES) instruments on NASA's Terra and Aqua satellites measure the shortwave (SW) radiance reflected from the Earth. In order to provide the top-of-atmosphere reflected SW flux we need to know the anisotropy of the radiance reflected from the scene. Sea Ice scenes are particularly complex due to the wide range of surface conditions that comprise sea ice. For example, the anisotropy of snow-covered sea ice is quite different to that of sea ice covered with melt-ponds. To provide a consistent scene classification we have developed the Sea Ice Brightness Index (SIBI). The SIBI is defined as one minus the normalized difference between the clear-sky 0.469 micron and 0.858 micron bands from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. The SIBI value is close to 1.0 for bright snow-covered sea ice scenes. It decreases as snow and ice melt and reaches the lowest value of 0.2-0.3 for open water .

We use the SIBI to classify sea ice scenes in order to quantify the effect of surface brightness on the anisotropy. For clear-sky scenes we use 3 SIBI bins. For overcast scenes we use a monthly map of the SIBI to classify the surface below the clouds. For overcast scenes we also split the scenes into liquid and water clouds, and use a linear regression between the reflectance and optical depth in order to calculate the anisotropic factor used in the radiance-to-flux inversion. We evaluate the performance of the angular dependence models (ADM) by comparing their ability to predict the measured CERES radiances in latitude/longitude boxes. Using this method we find the RMS difference decreases from 8% to 7% in May and from 17% to 12% in July, when using the SIBI ADMs instead of the existing ADMs. Additional tests using multi-angle data from the MISR instrument show a reduction in the standard deviation of instantaneous fluxes retrieved from different viewing zenith angles from 7.3%, using the existing ADMs, to 6.6% when using the SIBI based ADMs. This suggests that using the SIBI to classify sea ice scenes will lead to improvements in the CERES SW fluxes.