Mapping 532 nm lidar ratios for CALIPSO-classified marine aerosols using MODIS AOD constrained retrievals and GOCART model simulations

Le résumé fourni par la source

The NASA-CNES Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission provided a spaceborne global record of atmospheric aerosol and cloud profiles from June 2006 to June 2023. As an elastic backscatter lidar, the CALIPSO Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) typically required an assumption of the aerosol lidar ratio (extinction-to-backscatter ratio; S a ) to retrieve aerosol extinction and column-integrated aerosol optical depth (AOD). In all previous versions of its data products, the CALIPSO extinction algorithms first determine the aerosol types then assign one S a value globally for each aerosol type (e.g., 23 sr for marine at 532 nm). One of the major changes for the final CALIPSO data products release (Version 5, or V5) is the implementation of regional and seasonal S a tables for CALIOP-classified “marine” aerosols. In this study, we describe the process of creating the tables using 12 years (June 2006–August 2018) of Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) total column AODs to constrain collocated CALIOP backscatter profiles in a Fernald inversion scheme and infer S a (at 532 nm), focusing solely on the CALIOP “marine” aerosol type. The Goddard Chemistry Aerosol Radiation and Transport (GOCART) global aerosol model is used to estimate sea salt volume fraction (SSVF) that are collocated with the constrained S a retrievals. Patterns of smaller SSVF (< 65 %) and larger constrained Sa (> 40 sr) are found near land masses, while larger SSVF (> 95 %) and smaller constrained S a (< 30 sr) are generally observed in the remote oceans. The inverse empirical relationship found between modeled SSVF and constrained S a over global oceans yields values of ∼ 21 sr for SSVF of 100 % (i.e., “pure” marine) and ∼ 58 sr for SSVF of 0 % (i.e., the absence of marine aerosol). This relationship is applied to develop regional and seasonal hybrid (retrieval and model-assisted) climatological S a maps for CALIOP-classified marine aerosols; i.e., when MODIS-constrained results are not available, the model-assisted values are used. These hybrid S a maps are subsequently used to retrieve new CALIPSO Level 2 (L2) aerosol extinction profiles and column AODs in the V5 release. For a 4-month (January, April, July, and October 2015) analysis, the V5 L2 CALIPSO AODs compared better to CALIPSO Ocean Derived Column Optical Depth (ODCOD) than the CALIPSO Version 4.51 (V4.51) standard AODs in several regions, most notably the Bay of Bengal/Arabian Sea, where smoke/pollution typically mixes with marine aerosols. Also, the V5 CALIPSO AODs likely provide a lower AOD bias and root-mean-square-error than V4.51 AODs relative to coastal and island Aerosol Robotic Network (AERONET) AODs, as found in a validation study using data from June 2006 through October 2022. The technique described in this study contributes to CALIPSO's final V5 data products release and provides critical S a information for future spaceborne elastic backscatter lidars.

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