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

Abstract

Abstract. Heterogeneous ice formation induced by volcanic ash from the Eyjafjallajökull volcano eruption in April 2010is 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.

Sensitivities of Lagrangian modelling of mid-latitude cirrus clouds to trajectory data quality

“…For comparison to our model data we use the highresolution temperature data from the SUCCESS campaign, which took place over the central and western United States (Toon et al, 1998), and from the MACPEX campaign, which took place mainly over the Gulf of Mexico and the southeastern United States (Rollins et al, 2014). Both campaigns show significantly higher intensity for all frequencies, as will be further analysed below.…”

“…Red curve: average of online trajectories. Solid and dashed black lines: PSDs of vertical velocities measured during SUCCESS (Toon et al, 1998) and MACPEX (Rollins et al, 2014), respectively. Blue dashed and dashed-dotted lines: data from the ALPEX campaign for active and quiet days, respectively (Kuettner, 1981).…”

“…Hoyle et al, 2005;Brabec et al, 2012;Rolf et al, 2012;Engel et al, 2013;Cirisan et al, 2014), because the cooling rate in the very moment of the nucleation event affects the nucleation rate (in particular for homogeneous nucleation). These temperature fluctuations remain largely unresolved in state-of-the-art NWP models due to their limited spatial resolution.…”

“…While temperature variations at spatial scales of several tens of kilometres can be captured by regional NWP models, vertical velocity and temperature fluctuations at smaller spatial scales remain unresolved due to limited spatial resolution. Several studies have resorted to including the unresolved vertical motions in cirrus cloud modelling by superimposing artificial fluctuations on the trajectory data (Hoyle et al, 2005;Brabec et al, 2012;Rolf et al, 2012;Spichtinger and Krämer, 2013;Cirisan et al, 2014;Murphy, 2014;Dinh et al, 2015). Amplitudes and frequency distributions of the unresolved motions are typically taken from field measurements, which are unrelated to the investigation at hand.…”

“…The box model simulations are usually conducted along backward trajectories, which provide the required temperature and pressure history of the air parcels (e.g. Jensen et al, 1994a, b;Haag and Kärcher, 2004;Hoyle et al, 2005;Comstock et al, 2008;Brabec et al, 2012;Rolf et al, 2012;Jensen et al, 2013;Cirisan et al, 2014). The Zurich Optical and Microphysical Model (ZOMM; Luo et al, 2003a, b), the Model for Aerosol and Ice Dynamics (MAID; Bunz et al, 2008) and the Advanced Particle Simulation Code (APSC; are some of the models used.…”