Resolving cirrus optical depth biases between active (CALIOP) and passive (MODIS,IIR) using IR retrievals

The successful launch of CALIPSO and CloudSat in June 2006 has provided for the first time global, vertically resolved observations of both clouds and aerosols. These active sensors provide nearly coincident observations with the MODIS and AIRS on the Aqua platform. The combination of the active and passive cloud observations has provided a truly new capability to better understand the composition and distribution of clouds and aerosols. A recent IR radiative closure analysis using co-located single layer cirrus retrievals from MODIS and CALIOP has found a bias between the two (MODIS larger than CALIOP by a significant amount). As part of this effort we developed and implemented an IR cirrus OT retrieval designed to be a reference for to compare the CALIOP and MODIS retrievals. Recent reprocessing of selected MODIS data at the SSEC atmospheric PEATE has enabled generation of retrievals with a modified version of Collection 5 ice cloud Look Up Tables (LUTs) that eliminate a forward scattering numerical artifact and reduce the retrieved optical thickness. The main questions that where investigated in this effort was to: 1) Investigate uncertainties in the single scatter calculations used as part of the MODIS retrieval of cloud optical depth and effective radius. 2) Investigate biases in the CALIOP retrieval of cirrus optical depth resulting from an incorrect multiple scattering correction and/or lidar ratio used as part of the retrieval process