Lidar observation and model simulation of a volcanic-ash-induced cirrus cloud during the Eyjafjallajökull eruption

Atmos. Chem. Phys., 12, 10281–10294, 2012 www.atmos-chem-phys.net/12/10281/2012/ doi:10.5194/acp-12-10281-2012 © Author(s) 2012. CC Attribution 3.0 License. Atmospheric Chemistry and Physics Lidar observation and model simulation of a volcanic-ash-induced cirrus cloud during the Eyjafjallajökull eruption C. Rolf1 , M. Krämer1 , C. Schiller† , M. Hildebrandt1 , and M. Riese1 1 Forschungszentrum Jülich, IEK-7, Jülich, Germany † deceased Correspondence to: C. Rolf () Received: 11 May 2012 – Published in Atmos. Chem. Phys. Discuss.: 22 June 2012 Revised: 26 October 2012 – Accepted: 29 October 2012 – Published: 6 November 2012 Abstract. Heterogeneous ice formation induced by volcanic ash from the Eyjafjallajökull volcano eruption in April 2010 is investigated based on the combination of a cirrus cloud observed with a backscatter lidar over Jülich (western Germany) and model simulations along backward trajectories. The microphysical properties of the cirrus cloud could only be represented by the microphysical model under the assumption of an enhanced number of efficient ice nuclei originating from the volcanic eruption. The ice nuclei (IN) concentration determined by lidar measurements directly before and after cirrus cloud occurrence implies a value of around 0.1 cm−3 (in comparison normal IN conditions: 0.01 cm−3 ). This leads to a cirrus cloud with rather small ice crystals having a mean radius of 12 µm and a modification of the ice particle number (0.08 cm−3 instead of 3 × 10−4 cm−3 under normal IN conditions). The effectiveness of ice nuclei was estimated by the use of the microphysical model and the backward trajectories based on ECMWF data, establishing a freezing threshold of around 105 % relative humidity with respect to ice in a temperature range from −45 to −55 ◦ C . Only with these highly efficient ice nuclei was it possible for the cirrus cloud to be formed in a slightly supersaturated environment. 1 Introduction The Eyjafjallajökull volcano in Iceland ejected a large ash cloud during its eruptions in April 2010. The cloud spread out over central Europe in a period of 6 days and severely disrupted the air traffic. The ash cloud was observed from ground (e.g. Ansmann et al., 2010; Gross et al., 2012; Seifert et al., 2011) and aircraft (e.g. Johnson et al., 2012; Schumann et al., 2011) and is well documented in several special issues (ACP, JGR, and Atmospheric Environment). Two days after the first large eruption on 14 April, we detected the ash cloud with a backscatter lidar system over Jülich, western Germany (50◦ 540 N, 6◦ 240 E). Embedded in the ash plume a cirrus cloud is also observed. The volcanic ash event provides a good opportunity to investigate the impact of volcanic ash particles on cirrus cloud formation in the atmosphere. At the moment, there is a lack of observations, and the influence of volcanic ash on heterogeneous freezing is a matter of controversy. Some studies state that volcanic ash particles act as good ice nuclei (IN) (e.g. Isono et al., 1959; Durant et al., 2008; Fornea et al., 2009; Prenni et al., 2009), while others suggest that the volcanic ash particles have no further impact as IN (e.g. Langer et al., 1974; Schnell and Delany, 1976). The heterogeneous freezing efficiency of the Eyjafjallajökull ash particles has been investigated in two previous studies (Hoyle et al., 2011; Steinke et al., 2011). Both studies used particle probes from ground near the volcano and found only moderate effects on atmospheric ice formation. Another study by Bingemer et al. (2012) shows a large increase of the IN concentration during the Eyjafjallajökull events on two sites near the surface. All these studies are based on IN that are directly sampled from the ground or using filter probes. IN efficiency was analyzed in a laboratory simulation chamber under controlled conditions. Seifert et al. (2011) studied the influence of Eyjafjallajökull ash on cloud formation using a lidar and found periods with induced cirrus clouds embedded in ash layers. They showed the existence of very efficient IN, which form ice crystals in an environment that is relatively dry and Published by Copernicus Publications on behalf of the European Geosciences Union. 10282 only few percent supersaturated. The study by Seifert et al. (2011) focuses on a real atmospheric observation of an ashinduced cirrus cloud with a lidar, whereas the present paper also shows lidar observation extended by investigations on the microphysical properties of induced cirrus clouds by model simulations. The formation of the cirrus cloud is analyzed by microphysical simulations along backward trajectories. The simulation provides information on the microphysical properties of the ash-induced cirrus cloud and conditions for the development of such clouds. The lidar (short for light detection and ranging) measures optical properties (i.e. backscatter and extinction coefficient) and depolarization of particles at one wavelength with a high vertical resolution. In the depolarization channel, it is possible to distinguish various shapes of observed particles. Periods with or without volcanic ash occurred in accordance with the dynamic situation. The largest amount of ash was found above the planetary boundary layer, below seven km, in the free troposphere (Ansmann et al., 2010; Schumann et al., 2011). However, during some periods, our measurements show an increased depolarization and particle extinction signal at higher altitudes. This may have been due to pure volcanic ash, ice crystals or a mixture of the two. In this study, we investigate in detail one of the observed cirrus cloud embedded in a volcanic ash layer. First, the origin of the observed air mass is assigned by calculating ECMWF (European Centre for Medium-Range Weather Forecasts) backward trajectories. With our detailed microphysical box model MAID (Model for Aerosol and Ice Dynamics) (Bunz et al., 2008), we simulate the ice formation along these trajectories. Thus it is possible to distinguish observations from pure volcanic ash, natural cirrus, and induced cirrus clouds. Furthermore, microphysical and optical properties of the resulting ice crystals where investigated with this combination of lidar and model simulations. In the first two sections, the instrument and methodology is described, starting with the lidar instrument and the measurement technique, followed by a description of the ice model MAID, the calculation of backward trajectories, and their combination with MAID. In Section 3, the observation of the main Eyjafjallajökull volcanic ash cloud and the induced cirrus on 16 April 2010 is presented. The origin of the air masses is analyzed based on trajectory calculations. Subsequently, the IN concentration is estimated from lidar data. The simulation of induced cirrus with MAID, including two sensitivity studies, is described and discussed accordingly. 2 2.1 Instrumentation and methodology Lidar measurements (...truncated)