Interaction of Aerosols with Clouds and Radiation

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High uncertainties in future climate predictions arise from insufficient knowledge of the interaction of clouds with visible (solar) and infrared (terrestrial) radiation. The optical properties and lifetime of clouds are strongly influenced by the ability of atmospheric aerosol particles to act as cloud condensation nuclei (CCN) or ice nuclei (IN). This so-called indirect aerosol effect has been recognized as one of the greatest source of uncertainty in assessing human impact on climate. Up to now, the climate relevant properties of clouds and their formation processes are still poorly understood, particularly those of mixed-phase clouds where supercooled cloud droplets and ice crystals coexist. Previous research has found that the cloud radiative properties strongly depend on the cloud ice mass fraction, which is influenced by the abundance of IN. Increased IN concentrations are also thought to enhance precipitation, thus causing a decrease in cloud lifetime and cloud cover, resulting in a warming of the atmosphere. Burning questions in this context are:- Which aerosol particles act as IN in our atmosphere ?- By which detailed mechanisms do atmospheric aerosols contribute to the formation of ice ?To explore and reduce these uncertainties, one major goal of this project is to develop a new inlet for the measurement of cloud droplets and ice crystals. This inlet will also allow the extraction of small ice particles in mixed-phase clouds for the physico-chemical characterization of tropospheric IN. The inlet will represent a novel tool for the in-situ investigation of clouds and will deliver information that is not available by means of any other existing inlet. Another focus will be the combination of ground-based and remote-sensing observations of mixed-phase clouds. This will contribute to better parameterization of radiative properties of mixed-phase clouds in climate models. The newly designed inlet will be an integral part of these measurements. These studies will be performed at the Jungfraujoch, one of the world’s most prominent high Alpine research stations located at 3580 m altitude in the middle of Switzerland. This unique location offers the possibility to perform these studies in mixed-phase clouds that are representative for the current European background. The proposed research will be performed in a collaborative effort of several Swiss and a German institute. It is in line with the expertise of the involved institutions, i.e. the Laboratory of Atmospheric Chemistry of the Paul Scherrer Institut (aerosol/cloud research), the PMOD/WRC in Davos and MeteoSwiss in Payerne (radiation) as well as the Institute for Meteorology and Climate Research at the Karlsruhe Institute of Technology (cloud microphysics and optics).