Evaluating Aerosol Optical Properties in the NASA MERRA-2 Reanalysis Using NASA Lidar Observations

Since about 2000 NASA has produced a number of aerosol lidar data sets from ground-based (MPLNET), airborne (CPL, HSRL, DIAL), and space-based (CALIOP, CATS) platforms that complement passive ground-based and satellite observations during Earth Observing System (EOS) period by providing measurements of particulate backscatter, extinction, and depolarization ratio at various wavelengths (355, 532, and 1064 nm). Here, we utilize these observations to evaluate simulated aerosol optical properties in the recent NASA Goddard Earth Observing System version 5 (GEOS-5) Modern Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2, 1980 - Present). Aerosols in MERRA-2 are simulated using an online version of the Goddard Chemistry, Aerosol, Radiation, and Transport (GOCART) model that simulates dust, sea-salt, sulfates, black carbon, and organic carbon. MERRA-2 assimilates both meteorology and bias-corrected column aerosol optical thickness (AOT) from spaceborne AVHRR, MODIS Aqua/Terra, and MISR sensors, as well as AERONET ground based sun photometers. While MERRA-2 is constrained in the total aerosol loading through assimilation of AOT, the aerosol vertical distribution and composition are sensitive to assumptions in the model physics, particle size distribution, and preconfigured lookup tables of aerosol optical properties. Aerosol lidar data sets provide a unique opportunity to refine the assumptions about aerosol optical properties assigned to the aerosol species simulated in MERRA-2. After identifying case studies within the NASA lidar record that characterize individual aerosol types (e.g. dust) simulated in MERRA-2, simulated aerosol backscatter, extinction, and depolarization properties for that type are compared to corresponding observed quantities to identify and correct biases in the assumed aerosol optical properties and particle size distributions used in MERRA-2.