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GACP Projects

Estimating the Indirect Aerosol Effect Considering Different Aerosol Species

Ulrike Lohmann, PI
Johann Feichter, Co-I
Julian Wilson, Co-I

Abstract: A research program devoted to study cloud-aerosol-radiation interactions has been carried out. In a first attempt to understand the interactions between sulfate aerosols and cloud droplets Lohmann and Feichter (JGR 102, 12685-13700, 1997) simulated the indirect sulfate aerosol effect by using a coupled cloud microphysics - sulfur cycle model inside the ECHAM general circulation model (GCM). This enabled us to make the first ever estimate of this effect due to both prolonging cloud lifetimes by changes in the precipitation efficiency, and changes in cloud albedo. Also, we clearly demonstrated the uncertainties of the indirect sulfate aerosol effect due to parameterizations in cloud physics. Until now, we are using a simple relationship between the mass of sulfate aerosols and the number of cloud droplets. However, much of the sulfate aerosol mass is formed within cloud droplets, where it may simply add mass to pre-existing cloud condensation nuclei (CCN), but not increase CCN number concentration, an effect we could not account for with this simple relationship. Also other aerosol components contribute to the indirect aerosol effect.

Therefore, in this proposed research we are attempting to parameterize cloud-aerosol-interactions more sophisticated. We are planning to use the ECHAM GCM to simulate different aerosol species, sulfate aerosols, carbonaceous aerosols, sea salt and dust. Prognostic equations will be solved for all sulfur species, black and organic carbon and cloud liquid water and ice. An aerosol transport module, developed for ECHAM as part of the SINDICATE project (CoIs Feichter and Wilson) will be available for our studies. This module solves prognostic equations for different aerosol size classes including number and mass of each class. The aerosol module was developed by Dr. Wilson, and will be implemented and tested in ECHAM in collaboration with Dr. Feichter. Additionally a prognostic equation for the number of cloud droplets will be introduced into the ECHAM GCM by Dr. Lohmann to realistically simulate the cloud droplet nucleation process. It will be parameterized as a function of the total number of aerosol particles and the vertical velocity. The evaluation of cloud and aerosol quantities will be done by Dr. Lohmann. After having shown that this sophisticated way of simulating aerosols and aerosol-clouds interactions results in a realistic present-day climate, we will use this tool to calculate the direct and indirect aerosol forcing as the difference in radiative forcing from experiments with present-day aerosol concentrations and with pre-industrial ones and perform sensitivity studies to obtain an uncertainty range.

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