Toward an Optimal Retrieval of Cloud Optical Property Retrievals from Satellite Imagers Over Snow/Ice-Covered Surfaces

Regional sensitivity of climate models to changes in global radiative forcing appears to be greatest in the Arctic, where sea ice and snow cover are highly variable seasonally and interannually. Clouds play a significant role in the Arctic climate through ice-albedo feedback processes. Because of the often small contrast between clouds and snow or ice in the visible and infrared windows, it has been difficult to monitor cloud properties accurately in polar regions on a long-term basis. This difficulty is mitigated to a certain degree during sunlit conditions by using near-infrared (NIR) channels available on recent research satellites. The MODerate-resolution Imaging Spectroradiometer (MODIS) and other newer imagers, such as VIIRS and the GOES-16 ABI, have several relevant NIR channels that can be exploited to retrieve cloud properties over snowy surfaces including sea ice. The Clouds and the Earth’s Radiant Energy System (CERES) uses the MODIS 1.24-µm channels on Terra and Aqua to retrieve cloud optical depth over snow and ice for its Edition 4 processing. Cloud effective particle size is retrieved using the 3.8-µm channels in all cases. While some results appear to be quite accurate compared to surface-based retrievals, there remain some large uncertainties and improvements are needed. Long-term monitoring of clouds is possible using older Advanced Very High Resolution Radiometer (AVHRR) data on the NOAA and MetOp series of operational satellite combined with newer sensors such as MODIS and VIIRS. The AVHRR measures reflected radiation at 0.65 and 0.87 µm and either at 1.6 or 3.8 µm. This channel complement limits the retrieval potential for clouds over snow. To better understand the limitations and develop an optimal retrieval system for a given set of imager channels, in particular the newer ones, this paper performs retrievals over snow covered surfaces using Aqua/Terra MODIS and GOES-16 data to study the sensitivity and accuracies of optical depths and other parameters retrieved using the 0.65, 0.87, 1.24, 1.6, 2.1, and 2.25-µm channels. Additionally, infrared only techniques are explored to retrieve optical depths for thinner clouds. The results are compared to independent measurements taken at surface sites and from CloudSat and CALIPSO. The overall goal is to develop optimal retrieval systems for both historical and current satellite observing systems, especially for the next edition of CERES.