Measuring the multiple scattering correction for CALIPSO using combine MODIS infrared and CALIOP observations

The combination of active and passive measurements provides a truly new capability to better characterize the accuracy of cloud retrievals and better understand the global composition and distribution of clouds. As with any new observing system there are challenges that need to be addressed to fully utilize the observations. Once calibrated the fundamental observations from CALIOP are the vertically resolved attenuated backscatter (at both 532 nm and 1064 nm) and volume depolarization ratio (calibrated ratio of the cross- and co-polarized signals) at 532 nm. Although very useful for qualitative assessments, attenuated backscatter does not provide quantitative characterization of cloud radiative forcing. To use the lidar observations for quantitative studies the cloud extinction profile needs to be retrieved. For a single channel lidar such as CALIPSO inverting the attenuated backscatter to retrieve the cloud extinction profile requires constraining the extinction to backscatter ratio (the lidar ratio) and correcting for multiple scattering which results from photons which remain in the receiver FOV as part of the energy scattered in the forward diffraction peak. The multiple scattered contributions are primarily a function of the receiver FOV and the particle size. For space based lidar's the contribution from multiple scattering is significant due to the large distance between the instrument and target (clouds) resulting in the receiver footprint at the cloud top being large (~80 meters for CALIOP). The current V3 CALIPSO retrievals assume a constant multiple scatting correction of 0.6 that is derived based on theoretical calculations. This paper will investigate the feasibility of directly estimating the multiple scattering correction using collocated MODIS and CALIPSO observations. Using the MODIS infrared observations we will retrieve the IR extinction optical thickness for collocated CALIPSO fov that have sufficient molecular returns below the cloud base so that the two way attenuation can be directly measured. The ratio between the IR retrieved extinction and the constrained CALIPSO attenuation provides an estimate of the multiple scattering contributions for CALIPSO as the IR retrieval is independent of multiple scatting. Using this method we will present a global distribution of the estimated multiple scattering factor for CALIPSO.