Comparison of an Aerosol Assimilation System of MODIS Radiances with AERONET retrievals.

We present results from a simple off-line assimilation system of the radiances from the 7 MODIS channels that sense atmospheric aerosols. We describe the assimilation cycle. The Goddard Chemistry and Aerosol Radiation Transport Model (GOCART), which is driven by assimilated meteorology, simulates five aerosol types: dust, seasalt, black carbon, organic carbon and sulfate. The forward model takes the aerosol information from the GOCART model and calculates radiances based on optical parameters of the aerosol type, satellite viewing angle and the particle growth from relative humidity. Because the GOCART model is driven by previously assimilated meteorology, these forward model radiances can be directly compared with the observed MODIS level2 radiances. The off-line assimilation system simply adjusts the aerosol loading in the GOCART model so that the observed minus forward model (O-F) radiances agree. Minimal change is made to the GOCART aerosol vertical distribution, size distribution and the ratio of the five different aerosol types. The loading in the GOCART model is updated with new MODIS observations every 6 hours. Since the previously assimilated meteorology provides surface wind speed, we account for radiance sensitivity to wind speed over rough ocean. Over land we use surface albedoes from the MODIS land team kindly provided by Eric Moody. Over ocean the assimilation aerosol optical depths (AOD) compare well with AERONET, over land less so. We compare our results with AERONET retrieved single scattering albedo and effective radius. We also investigate data retention issues in the assimilation. This research is part of an ongoing effort at NASA Goddard to integrate aerosols into the Goddard Modeling and Assimilation Office (GMAO) products.