IMPACT OF SULFATE AEROSOLS ON ALBEDO AND LIFETIME OF CLOUDS - A SENSITIVITY STUDY WITH THE ECHAM4 GCM

A coupled sulfur chemistry-cloud microphysics scheme (COUPL) is used t o study the impact of sulfate aerosols on cloud lifetime and albedo. T he cloud microphysics scheme includes precipitation formation, which d epends on the cloud droplet number concentration (CDNC) and on the liq uid water content. On the basis of different observational data sets, CDNC is proportional to the sulfate aerosol mass, which is calculated by the model. Cloud cover is a function of relative humidity only. Add itional sensitivity experiments with another cloud cover parameterizat ion (COUPL-CC), which also depends on cloud water, and with a differen t autoconversion rate of cloud droplets (COUPL-CC-Aut) are conducted t o investigate the range of the indirect effect due to uncertainties in cloud physics. For each experiment, two simulations, one using presen t-day and one using preindustrial sulfur emissions are carried out. Th e increase in liquid water path, cloud cover, and shortwave cloud forc ing due to anthropogenic sulfur emissions depends crucially upon the p arameterization of cloud cover and autoconversion of cloud droplets. I n COUPL the liquid water path increases by 17% and cloud cover increas es by 1% because of anthropogenic sulfur emissions, yielding an increa se in shortwave cloud forcing of -1.4 W m(-2). In COUPL-CC the liquid water path increases by 32%, cloud cover increases by 3% and thus shor twave cloud forcing increases by -4.8 W m(-2). This large effect is ca used by the strong dependence of cloud cover on cloud water and of the autoconversion rate on CDNC, cloud water, and cloud cover. Choosing a different autoconversion rate (COUPL-CC-Aut) with a reduced dependenc e on CDNC and cloud water results in an increase of liquid water path by only 11% and of cloud cover by 1%, and the increase in shortwave cl oud forcing amounts to -2.2 W m(-2). These results clearly show that t he uncertainties linked to the indirect aerosol effect are higher than was previously suggested.