GACP Projects
FORM A
GACP ACCOMPLISHMENT REPORT
Name: Anthony D. Del Genio
Address: NASA GISS, 2880 Broadway, New York, NY 10025
TITLE: An Investigation of the Indirect Effect of Aerosols on Climate: Coupled Chemistry-Climate Modeling and Satellite Validation
GOALS: We will attempt to constrain the indirect effects of aerosols on clouds and climate through a combined modeling and satellite data analysis approach.
OBJECTIVES: A sulfur chemistry model, and eventually interactive source/transport models for other aerosol types, will be coupled to the GISS GCM and the indirect effects (both radiative and microphysical) of anthropogenic aerosols calculated. Available aerosol and cloud in situ and satellite data sets will be analyzed to detect signatures of the indirect effect and used to validate the coupled model. Sensitivity of the model-data agreement to aspects of the cloud parameterization that influence cloud liquid water path and lifetime, and to assumptions linking aerosol chemical and cloud microphysical properties, will be used to estimate the uncertainty in the calculated indirect effects.
APPROACH: We will combine those data sets that measure cloud droplet number concentration directly, rather than inferring it from measured CCN concentrations and supersaturation assumptions, to develop an optimum sulfate-droplet number relationship for the coupled model. We will alter the GISS autoconversion parameterization to admit variable number concentration, test the sensitivity of the microphysical indirect effect to different autoconversion parameterizations, and develop a parameterization suitable for the GCM grid scale to constrain the microphysical effect. We will determine the correlation between sulfate and cloud optical thickness variations in several locations to assess the relative roles of indirect effects, in-cloud production, and wet deposition processes. We will compare model-predicted effective radius, column number concentration, and droplet size - cloud optical thickness correlations to ISCCP estimates. We will analyze aerosol optical thickness climatologies for evidence of regional anthropogenic trends and look for evidence of cloud property changes over the same time period.
TASKS COMPLETED: In the past year source/transport models for organics, black carbon and sea salt were added to the existing coupled climate-sulfur chemistry model. Organic and black carbon mass are obtained from the IPCC emissions for carbonaceous aerosols. Biomass and fossil fuel emissions are from Liousse et al. (1996, JGR 101, 19411) and Penner et al. (1993, Atmos. Env. 27A, 1277), respectively. Natural emissions from terpenes are from Guenther et al. (1995, JGR 100, 8873); a 10% yield rate of OM from terpenes is assumed, giving a natural OM source strength between those of Liousse et al. and Andreae and Crutzen (1997, Science 276, 1052). We explored the sensitivity of the black carbon distribution to different treatments of its solubility to estimate its impact on the direct effect; however, we do not yet consider its role in the indirect effect. Sea-salt concentrations are those obtained from the modeling study of Gong et al. (1997, JGR 102, 3805) and are prescribed as monthly mean fields in 6 size bins, the first 4 of which (sizes < 2 m m) contribute to the indirect effect. Organic and black carbon and sea-salt prescribed in the first model layer are then advected using a quadratic upstream scheme. Cloud properties and aerosols are linked in the GCM diagnostically using a multiple regression equation of observed droplet numbers against sulfate, organic, and sea-salt mass based on field observations from Borys et al. (1998, JGR 103, 22073) and Leaitch et al. (1996, JGR 101, 29123). The second indirect effect is also simulated using the autoconversion parameterization of Tripoli and Cotton. We have compared simulated and ISCCP-derived particle size-cloud optical depth relationships and explored the physical mechanisms responsible for them. We have examined the detailed vertical distribution of low-level cloud tops in the GCM and in ISCCP data to understand how errors in simulated cloud location affect estimates of the indirect effect. Finally, we have compared AVHRR aerosol optical depths, sulfate emissions inventories and ISCCP low cloud optical thickness distributions for East Asia and the ocean areas downwind to look for a signal of possible anthropogenic aerosol impacts.
RESULTS: The aerosol mass distributions that determine the direct and indirect effects in the model are shown in Figure 1. The model ratio of carbonaceous to sulfate aerosols increases with altitude over many oceanic regions, especially in summertime, consistent with TARFOX data; however, over land and during other seasons the ratio generally decreases with altitude. Present-day direct radiative forcing, assuming an external mixture of aerosol species, is estimated by the GCM as +0.35, -0.35, and -0.65 W/m**2 for black carbon, organic carbon, and sulfate, respectively. The combined first and second indirect effect, including all aerosol species except black carbon, is -4.8 W/m**2. However, this result is quite sensitive to the GCM's simulation of the vertical distribution of low-level clouds. The GCM places too many of its low cloud tops near the surface (32, 29, and 39% at p = 934, 854, and 720 mb, respectively), where aerosol concentration is greatest, relative to ISCCP observations (16, 26, and 58%,). A sensitivity experiment in which clouds are suppressed at 934 mb and enhanced at 854 and 720 mb, removing about half the discrepancy with ISCCP, yields a global indirect effect of -2.5 W/m**2 with an otherwise identical model (Figure 2 ). The GCM is able to reproduce the ISCCP-derived tendency for cloud optical thickness — particle size correlations to switch from positive for thin clouds to negative for thick clouds. The latter may be evidence of an indirect effect made detectable by the onset of precipitation, which limits dynamics-based liquid water fluctuations that obscure aerosol effects in thin clouds. AVHRR optical depth trends downwind of China during the 1980's are consistent with sulfate emission trends over China during the same decade. However, ISCCP low cloud types in this region do not show noticeable trends that can be attributed to this aerosol increase.
Figure captions:
Fig. 1. One year annual mean aerosol mass in m g/m**3 for sulfate, organic mass and sea salt (size 0.3-2 m m) in the first GCM layer (959 mb).
Fig. 2. One year annual mean aerosol indirect effect (combined radiative and cloud lifetime effects) for a sensitivity test in which cloud amount in the first model layer was artificially reduced to better approximate ISCCP observations. The global mean is –2.47 W/m**2.
FUTURE PLANS: In the coming year we will incorporate the indirect effect due to black carbon into the existing GCM and estimate the magnitude of the total indirect effect relative to other climate forcings for the 1951-2000 time period. We will modify the existing cloud cover parameterization to include a more physically-based prediction of cloud formation based on layer stability, which should reduce the excessive layer 1 cloudiness and the magnitude of our simulated indirect effect. We will analyze pixel-level ISCCP estimates of optical thickness, droplet effective radius, and column droplet concentration and segregate them by cloud top level to isolate model errors in cloud-aerosol interaction from errors in cloud physical thickness. We will also perform regional comparisons of cloud column susceptibility to retrievals by Q. Han. We will also modify the predicted cloud radiative properties (following the work of Cairns) and the Tripoli-Cotton autoconversion parameterization to include the effect of subgrid liquid water variations.
FORM B
GACP SIGNIFICANT HIGHLIGHTS
See the description of project results and Figures 1,2 above.
FORM C
GACP BIBLIOGRAPHY
Papers in refereed journals and books
Koch, D., D. Jacob, I. Tegen, D. Rind and M. Chin, 1999: Tropospheric sulfur simulation and sulfate direct radiative forcing in the GISS GCM. J. Geophys. Res., 104, 23799-23822.
Koch, D., 2000: The transport and direct radiative forcing of carbonaceous and sulfate aerosols in the GISS GCM. Submitted to J. Geophys. Res.
Lohmann, U., G. Tselioudis and C. Tyler, 2000: Why is the cloud albedo-particle size relationship different in optically thick and optically thin clouds? Geophys. Res. Letters, 27, 1099-1102.
Tegen, I., D. Koch, A.A. Lacis and M. Sato, 2000: Towards a global aerosol climatology: Preliminary trends in tropospheric aerosol amounts and corresponding impact on radiative forcing between 1950 and 1990. J. Geophys. Res., in press.
Printed technical reports and non-refereed papers
Liu. J., 1999: Satellite observations of recent east Asian aerosol and cloud optical thickness trends. Masters thesis, Columbia University Dept. of Earth and Envir. Sci..
Brenguier, J.-L., L. Schüller, H. Pawlowska, D. Roberts, J. Feichter, J. Snider, U. Lohmann, S. Menon and S. Ghan, 2000: Report on the First PACE (Parameterization of the Aerosol Indirect Climatic Effect) Meeting, European Comm. Rep. EVK2-CT-1999-00054.
Oral presentations and posters
Menon, S., A. Del Genio, D. Koch and G. Tselioudis, 1999: An estimate of the aerosol indirect effect using a coupled chemistry-climate model and comparisons with satellite data. Fall AGU Meeting, San Francisco.
Menon, S., 2000: Cloud-aerosol interaction in the GISS GCM. First PACE Workshop, NASA/GISS, May 1-5.
Del Genio, A.D., 2000: Cloud-radiation parameterization in the GISS GCM. First PACE Workshop, NASA/GISS, May 1-5.
Brenguier, J.-L., J. Feichter, S. Ghan, U. Lohmann, S. Menon, H. Pawlowska, D. Roberts, L. Schüller and J. Snider, 2000: Parameterization of the indirect effect of aerosols on climate: From ACE-2 Cloudy Column to PACE. 13th Intl. Conf. on Clouds and Precipitation, Aug. 14-18, Nevada.
