Total Column Optical Depths Retrieved from CALIPSO Lidar Ocean Surface Backscatter

Abstract. This paper introduces the new Ocean Derived Column Optical Depth (ODCOD) algorithm. ODCOD is now being used to retrieve column optical depths from the 532 nm measure­ments acquired by the Cloud-Aerosol Lidar with Orthogonal Polari­zation (CALIOP) onboard the Cloud-Aerosol Lidar Infrared Pathfinder Satellite Observations (CALIPSO) spacecraft. ODCOD retrieves total column optical depths using the lidar backscatter signal return from the ocean surface, together with collocated wind speed estimates from Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA‑2). An advantage of ODCOD retrievals is that the column optical depths include contributions from particulates throughout the entire column including regions with attenuated backscatter below the CALIOP layer detection thresholds. In contrast, the standard CALIOP processing only estimates optical depths for clouds and aerosols detected by the CALIOP layer detection scheme. In this paper we describe the ODCOD algorithm, develop uncertainty estimates, and characterize the ODCOD retrievals relative to existing datasets. The paper presents detailed comparisons of ODCOD retrievals to collocated measurements from Langley Research Center’s airborne high spectral resolution lidars (HSRL), daytime estimates derived from Moderate Resolution Imaging Spectro­radio­meter (MODIS), and daytime and nighttime optical depths estimates from the Synergized Optical Depth of Aerosols (SODA) algorithm. ODCOD aerosol-only optical depth estimates are higher compared to airborne HSRL measurements by 0.009 ± 0.043 (median ± median absolute deviation) or 6 % ± 27 % relative difference, lower than MODIS by ‑0.009 ± 0.041 (8.0 % ± 34 % relative difference), higher in the daytime than SODA by 0.004 ± 0.035 (12 % ± 34 % relative difference), and higher in the nighttime by 0.027 ± 0.034 (20 % ± 33 % relative difference). In addition to being a new method of retrieving column optical depth, ODCOD’s estimates are independent from the standard CALIOP optical depth retrieval algorithms and have potential for further advances in the CALIPSO data record both to validate CALIOP estimates and as a potential column constraint for future improvements to extinction retrievals.