GACP Projects
GACP Progress Report - Modeling
Name: Inna Tegen
The GACP modeling effort during the past year has concentrated on two areas: the development of a preliminary aerosol climatology for the years 1950 through 1990 and the participation in an aerosol model intercomparison.
A preliminary aerosol climatology has been developed from global transport models for a mixture of sulfate and carbonaceous aerosols from fossil fuel burning, including also contributions from other major aerosol types such as soil dust and sea salt. The aerosol distributions change for the period of 1950 to 1990 due to changes in emissions of SO2 and carbon particles from fossil fuel burning. The optical thickness of fossil fuel derived aerosols increased by nearly a factor of 3 during this period, with particularly strong increase in eastern Asia over the whole time period. In countries where environmental laws came into effect since the early 1980s (e.g. US and western Europe), emissions and consequently aerosol optical thicknesses did not increase considerably during this time, resulting in a shift in the global distribution pattern over this period. Plate 1 summarizes the global trend in monthly optical thicknesses for this time period. In addition to the optical thickness, aerosol single scattering albedos may have changed during this period due to different trends in absorbing black carbon and reflecting sulfate aerosols. However, due to the uncertainties in the emission trends, this change cannot be determined with any confidence. Radiative forcing of this aerosol distribution is calculated for several scenarios, resulting in a wide range of uncertainties for TOA forcings. Uncertainties in the contribution of the strongly absorbing black carbon aerosol leads to a range in top-of-atmosphere forcings of ca. -0.5 to +0.1 W/m2, while the change in aerosol distributions between 1950 to 1990 leads to a change of -0.1 to -0.3 W/m2 for fossil fuel derived aerosol with a 'moderate' contribution of black carbon aerosol. A description of this aerosol climatology and the initial findings about are summarized in a manuscript which was submitted to the Journal of Geophysical Research (Tegen et al., 1999). The results will be made available via the GACP Website, and will include model derived aerosol optical thicknesses and single scattering albedos. This climatology will be refined once better measurements of global aerosol properties become available, and via the input of the scientific community. Dorothy Koch at NASA GISS is currently working on including sea salt and carbonaceous aerosols as interactive tracers in the GISS GCM, which will allow a refinement of this aerosol climatology.
GISS participated in the intercomparison of aerosol transport/chemistry models organized by Joyce Penner. This intercomparison was primarily intended as a contribution to the IPCC report; however, the results will be available for GACP as well. This intercomparison project included modeling the cycles of sulfate, dust, sea salt, and carbonaceous aerosols using given emission fields. The results will be discussed at the second GACP science team meeting.
Plate 1 (PS): Seasonal variation of modeled tropospheric aerosol optical thickness between 1950 and 1990, shown separately for dust and non-dust aerosols. Shown also is the variation of the stratospheric aerosol as parameterized in the GISS GCM (Sato et al., 1993).
References
- Sato, M., J. Hansen, A. Lacis, M. P. McCormick, and J. Pollack (1993): 'Stratospheric aerosol optical depths, 1850-1990', JGR 98, 22,987-22,994
- Tegen, I., D. Koch, A. Lacis, and M. Sato (1999): 'Towards a global aerosol climatology: Preliminary trends in tropospheric aerosol amounts and corresponding impact on radiative forcing between 1950 and 1990', submitted to JGR.
