This in turn would contribute to a cooling of the Earth’s surface and could have enormous consequences for climate. Consequently, ALK inhibitor special emphasis was given to investigations on whether drizzle is suppressed in ship tracks. In 2000 this second indirect effect, often called the cloud lifetime effect, was detected by both a field experiment and satellite measurements. Ferek et al. (2000) were able to show by radiometric measurements and radar observations that increased droplet concentrations in ship tracks, accompanied by smaller droplet sizes, significantly alter the liquid water path. In observations of the Tropical Rainfall Measuring Mission (TRMM) over

South Australia, Rosenfeld (2000) found the same result on the cloud scale: drop growth by collision is very effectively suppressed by anthropogenic aerosol particles originating from power plants, lead smelters and oil refineries. The same effect of cloud droplet size reduction together with a delay in the onset of precipitation was found over the Amazon during the Large-scale Biosphere-Atmosphere Experiment in Amazonia subproject on Smoke, Aerosols, Clouds, Rainfall and Climate (LBA-SMOCC), where it was shown

in detail what an enormous influence thick smoke from fires can have on cloud microphysics (Andreae et al. 2004). Other comprehensive field experiments which contributed considerably to knowledge about aerosol cloud interactions are the Smoke, Clouds, Radiation-Brazil (SCAR-B) experiment (Kaufmann et al. 1998), the Tropospheric Aerosol NLG919 clinical trial Urocanase Radiative Forcing Experiment (TARFOX), the Indian Ocean Experiment (INDOEX), the Aerosol Characterization Experiments (ACE-1) (Bates et al. 1998) and ACE-2 (Raes et al. 2000) and the Aerosol Characterization Experiment in Asia (ACE-Asia). The planning of these field campaigns was stimulated by the presence of global fields of aerosol optical thickness derived from satellites (e.g. Husar et al. 1997) as well as by global

model results (e.g. Langner & Rodhe 1991), which highlighted certain regions with conspicuously enhanced aerosol concentrations. One of these regions is the Indian Ocean. Here, INDOEX discussed another aspect of indirect aerosol effects: highly absorbing aerosol particles and their long-range transport over the ocean. Trade wind cumuli were moving within deep layers of dark haze. Based on these observations Ackermann et al. (2000) suggested that the reduction of tropical cloudiness by soot could represent another major effect of aerosols on clouds. Model calculations showed that the typical decrease in relative humidity during the daytime driven by solar heating with a maximum around noon is enhanced by the presence of absorbing haze in the boundary layer.