Uncertainty analysis for estimates of the first indirect aerosol effect Y. Chen and J. E. Penner University of Michigan, Department of Atmospheric, Oceanic and Space Sciences, Ann Arbor, USA
Abstract. The IPCC has stressed the importance of producing unbiased estimates of the
uncertainty in indirect aerosol forcing, in order to give policy makers as
well as research managers an understanding of the most important aspects of
climate change that require refinement. In this study, we use 3-D
meteorological fields together with a radiative transfer model to examine
the spatially-resolved uncertainty in estimates of the first indirect
aerosol forcing. The global mean forcing calculated in the reference case is
-1.30 Wm-2. Uncertainties in the indirect forcing associated with
aerosol and aerosol precursor emissions, aerosol mass concentrations from
different chemical transport models, aerosol size distributions, the cloud
droplet parameterization, the representation of the in-cloud updraft
velocity, the relationship between effective radius and volume mean radius,
cloud liquid water content, cloud fraction, and the change in the cloud drop
single scattering albedo due to the presence of black carbon are calculated.
The aerosol burden calculated by chemical transport models and the cloud
fraction are found to be the most important sources of uncertainty.
Variations in these parameters cause an underestimation or overestimation of
the indirect forcing compared to the base case by more than 0.6 Wm-2.
Uncertainties associated with aerosol and aerosol precursor emissions,
uncertainties in the representation of the aerosol size distribution
(including the representation of the pre-industrial size distribution), and
uncertainties in the representation of cloud droplet spectral dispersion
effect cause uncertainties in the global mean forcing of 0.2~0.6 Wm-2.
There are significant regional differences in the uncertainty
associated with the first indirect forcing with the largest uncertainties in
industrial regions (North America, Europe, East Asia) followed by those in
the major biomass burning regions.
Citation: Chen, Y. and Penner, J. E.: Uncertainty analysis for estimates of the first indirect aerosol effect, Atmos. Chem. Phys., 5, 2935-2948, doi:10.5194/acp-5-2935-2005, 2005.