Evaluation of global AEROCOM aerosol optical properties against satellite MODIS aerosol products

1. Evaluation of global AEROCOM aerosol optical properties • against satellite MODIS aerosol products • V. Pappas1, N. Hatzianastassiou1, C. Papadimas1, C. Matsoukas2, S. Kinne3, I. Vardavas4 • Laboratory of Meteorology, Physics Department, University of Ioannina, Greece, • Department of Environment, University of the Aegean, Mytilene, Greece, • Max-Planck Institute for Meteorology, Hamburg, Germany • Physics Department, University of Crete, Crete, Greece 1. Introduction-Motivation 3. Spatial variability 5. Radiative forcing (RF) (plots for [RFAEROCOM-RFMODIS/RFMODIS]) 7. Spectral variation of AOD, g, ω Aerosol optical properties are difficult to measure globally using ground-based instruments, due to limited number of stations. At the same time, their retrieval through satellites is subject to certain uncertainties and limitations. AEROCOM (Kinne et al., 2006) is a blend of ground-based observations and global modeling output, offering aerosol optical properties globally at 1°x1° resolution. In the present study AEROCOM is evaluated against MODIS retrievals,by comparing aerosol optical depth (AOD), single scattering albedo (ω) and asymmetry parameter (gaer), as well as radiative forcing simulations. For the simulations, a spectral radiation transfer model has been used (Hatzianastassiou &Vardavas, 1999). The vertical distribution of AEROCOM (model ECHAM output) is also compared with CALIOP version 2 data. AEROCOM data include AERONET quality observations from 1998 to 2007. These data are available as a climatological set and not as a time series. Hence, for the comparison, the mean value of MODIS data from 2000 to 2007 has been used. All AEROCOM data are freely available at the ftp site : ftp://ftp-projects.zmaw.de/aerocom/climatology/ • 4 sites characteristic of different types of aerosols (pollutants, sea salt spray, biomass, dust) have been selected. • Generally AOD compares well with AEROCOM underestimating. • ωis available by MODIS only over land • g is available by MODIS only over oceans (TOTAL) • AEROCOM overestimates AOD in areas with large aerosol load. • Most grids of larger AEROCOM AOD are over land, implying an overestimation of AOD in AEROCOM or an underestimation of MODIS land values. • AEROCOM overestimates sea salt AOD at the latitude band between 45 ° and 55° S, probably due to large value of prescribed effective radius (Kinne et al., 2006). Aerosol Optical Depth Atmospheric forcing relative difference (%) Aerosol Optical Depth Aerosol Optical Depth Wavelength (nm) Wavelength (nm) 8. Conclusions • Summer and spring are the seasons with the best agreement for AOD, while the worst is during autumn. • Radiative forcings using AEROCOM data have smaller magnitude than those based on MODIS, mainly because of smaller AEROCOM AODs. However, differences at surface and in atmosphere cancel out so that the difference at TOA is small (3%). • In places where AEROCOM AOD is larger than MODIS, Net Radiative forcing estimated by AEROCOM is larger. • In the global perspective, RFTOA only differs by 3.1%. • On a global annual mean basis, AEROCOM overestimates g andωby 16.6% and 4.1%, respectively. • Vertical distribution of AEROCOM aerosols is similar to the one of CALIOP version 2 data. This is expected to be improved with the implementation of CALIOP version 3 data by the AEROCOM team. • In the total, AEROCOM is a valuable tool for studies using aerosol optical properties, given the relatively small differences in magnitude and spectral profiles of AOD, SSA and gaer with MODIS. The good performance of AEROCOM maximizes its usefulness based on advantages like the separation between natural and anthropogenic aerosols and the spectral resolution of the 1°x1°aerosol optical properties. 4. Seasonal variability 4. Conclusions • For the AEROCOM RF computations, AEROCOM AOD/SSA/gaer have been used, while for MODIS RF, MODIS AOD and GADS SSA/gaer have been used. • Very good agreement of global mean TOA forcing, but large difference in S. Hemisphere oceans with AEROCOM underestimating it. • Overestimation of normalised RF (radiative forcing efficiency) up to 50 Wm-2 in areas with industrial type of aerosol . Underestimation in the largest part of oceans. • For both datasets, the season with the largest AOD values is spring and summer. However, while for MODIS the highest values appear during spring with summer coming second , for AEROCOM this is reversed. • The differences between the two datasets are largest in autumn. 2. Global and hemispherical view of AOD AEROCOMAOD: 0.144 MODISAOD: 0.162 AEROCOMAOD: 0.137 MODISAOD: 0.164 MODIS MODIS AEROCOM AEROCOM 6. Vertical distribution MODIS • 50 % of total columnar AOD is due to aerosols below 1.50 / 1.44 km for AEROCOM /CALIOP. • 85 % of total columnar AOD is due to aerosols below 2.73 / 3.2 km for AEROCOM/CALIOP. AEROCOM References AEROCOMAOD: 0.123 MODISAOD: 0.151 AEROCOMAOD: 0.117 MODISAOD: 0.144 • Kinne et al., 2006: An AeroCom initial assessment optical properties in aerosol component modules of global models. Atmos. Chem. Phys., 6, 1-20 • Hatzianastassiou, N., and I. Vardavas, 1999: Shortwave radiation budget of the Northern Hemisphere using International Satellite Cloud Climatology Project and NCEP/NCAR climatological data. J. Geophys. Res., 104, 24401-24421. AEROCOM AODs are systematically smaller than MODIS, with slightly larger/smaller differences in winter/summer.