11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Tuesday, 4 June 2002: 9:45 AM
Ice nucleation experiments at simulated cloud conditions (Formerly paper P5.17)
Ottmar Möhler, Forschungszentrum Karlsruhe, Karlsruhe, Germany; and S. Schaefers, O. Stetzer, U. Schurath, A. Mangold, and M. Krämer
Poster PDF (1.0 MB)
Ice nucleation plays a key role in the formation of cirrus clouds or the initiation of precipita-tion in lower tropospheric clouds. It can occur either by homogeneous freezing of solution droplets below about -35°C, or be heterogeneously induced by so-called ice nuclei. A quan-titative description of the ice nucleation processes is crucial for a better understanding of for-mation, life cycles, and optical properties of clouds. The large coolable and evacuable aerosol chamber AIDA is used as a cloud chamber to study processes of ice formation at simulated tropospheric cloud conditions. Particle freezing and growth is initiated by adiabatic expansion which leads to volume cooling and thus ice and water supersaturation at constant wall temperature. A wide range of cooling rates can be es-tablished during ice activation by combining selected pumping speeds with fast expansion into an evacuated volume connected to the cloud chamber. The impact of ageing processes on ice nucleation is investigated in repeated cycles of ice activation and evaporation. Intensity and depolarisation of forward- and back-scattered laser radiation is measured, caused by par-ticles in a small scattering volume far from the walls. The depolarisation signal is highly sen-sitive to the formation of a-spherical ice particles. The number size distribution of interstitial aerosol and activated ice particles is measured with an optical particle counter. Various insoluble aerosol components can be generated and added to the chamber. In a first set of experiments, the activation of pure soot aggregates from a graphite spark generator was investigated at temperatures between 190 K and 255 K. An overview of experimental meth-odology and first results will be presented.

Supplementary URL: http://imk-aida.fzk.de