Ice formation in ash-influenced clouds after the eruption of the Eyjafjallajkull volcano in April 2010

The influence of volcanic ash on heterogeneous ice nucleation in tropospheric clouds is investigated on the basis of 90 observed cloud cases. The clouds were observed with polarization lidars at the two central-European EARLINET stations Leipzig (51.3°N, 12.4°E) and Maisach (48.2°N, 11.3°E, 25 km northwest of Munich), Germany, in volcanic aerosol layers which originated from the strong eruptions of the Icelandic Eyjafjallajkull volcano in April 2010. Case studies of evolving boundary layer cumuli and long-lasting free tropospheric cloud events with unusual behavior (mixed-phase cloud complex, cirrus deck) are discussed. A clear impact of ash is observed. The ice nuclei concentration derived from the lidar observations has been estimated to range from 2-20 per liter in the boundary layer and from 100-300 per liter at cirrus level. The statistical analysis based on the 90 evaluated cloud cases revealed that all observed cloud layers with cloud top temperatures of below -15°C contained ice. Typically (under non-volcanic aerosol conditions) such a high fraction of ice-containing clouds is not reached before temperatures decrease below -25°C over central Europe.