PNNL

Abstract

Current global aerosol models use different physical and chemical schemes and 4 parameters, different meteorological fields, and often different emission sources. Since 5 the physical and chemical parameterization schemes are often tuned to obtain results that 6 are consistent with observations, it is difficult to assess the true uncertainty due to 7 meteorology alone. Under the framework of the NASA global modeling initiative (GMI), 8 the differences and uncertainties in aerosol simulations (for sulfate, organic carbon, black 9 carbon, dust and sea salt) solely due to different meteorological fields are analyzed and 10 quantified. Three meteorological datasets available from the NASA DAO GCM, the 11 GISS-II’ GCM, and the NASA finite volume GCM (FVGCM) are used to drive the same 12 aerosol model. The global sulfate and mineral dust burdens with FVGCM fields are 40% 13 and 20% less than those with DAO and GISS fields, respectively due to its larger 14 precipitation. Meanwhile, the sea salt burden predicted with FVGCM fields is 56% and 15 43% higher than those with DAO and GISS, respectively, due to its stronger convection 16 especially over the Southern Hemispheric Ocean. Sulfate concentrations at the surface in 17 the Northern Hemisphere extratropics and in the middle to upper troposphere differ by a 18 factor of 3 between the three meteorological datasets. The agreement between model 19 calculated and observed aerosol concentrations in the surface source regions is similar for 20 all three meteorological datasets. Away from the source regions, however, the 21 comparisons with observations differ greatly for DAO, FVGCM and GISS, and the 22 performance of the model using different meteorological datasets varies depending on the 23 site and the compared species. Sensitivity simulations with the NASA GEOS-4 24 assimilated fields show that the inter-annual variability of aerosol concentrations can be higher than a factor of 2 depending on the location and season, which is generally, however, smaller than the differences due to using different meteorological datasets. Global annual average aerosol optical depth at 550 nm is 0.120-0.131 for the three meteorological datasets. However, the contributions from different aerosol components to this total optical depth differ significantly, which reflects differences in the aerosol spatial distributions. The global annual average anthropogenic and all-sky aerosol direct forcing at the top-of-the atmosphere is estimated to be -0.75, -0.35, -0.40 W m-2 respectively for DAO, FVGCM, and GISS fields. Regional differences can be much larger (by a factor of 4-5) in the tropics over the ocean and in the polar regions.