Ice Cloud Optical Depth From MODIS Cirrus Reflectance

An algorithm has been developed to infer ice cloud optical depth from the isolated cirrus reflectance derived from Moderate Resolution Imaging Spectroradiometer (MODIS) observations. The present method is a modification of a previous study, which, due to limitations in the assumed ice particle habit distribution, limited ice cloud optical depth retrieval to the tropics. Here, the bulk scattering properties of ice clouds are updated, utilizing the new ice crystal size and habit distributions developed for the latest MODIS collection 5 operational cloud retrieval algorithm. The cirrus reflectance parameter, which is derived from reflectance measurements in the 0.66- and 1.375-mum spectral bands, is highly sensitive to ice cloud optical depth, allowing for optical depth retrieval. In the retrieval, an effective particle diameter of 50 mum is used, representing the peak of the global distribution of the effective particle sizes from the MODIS operational cloud products. The applicability of the algorithm is illustrated using daily cirrus reflectance data from the MODIS level-3 data set. The present method, which derives the contribution to optical depth by ice clouds only, is complementary to the operational MODIS ice cloud retrieval, which provides a total column optical depth.