Inhomogeneous Cirrus Clouds during the AIRTOSS Campaign

The aircraft campaign AIRTOSS in May and September 2013 provided measurement data of cirrus clouds over North Sea and Baltic Sea in various meteorological situations. The measurements were carried out with a Learjet and a towed sensor shuttle below the aircraft. This configuration allows us to obtain almost horizontally collocated measurements at different vertical levels (inside and outside clouds). Microphysical properties of cirrus clouds, as ice water content, ice crystal number concentrations, diameter and shape of ice crystals were measured.

In this study, we concentrate on a rare case of an in situ generation of a thin and sparse cirrus cloud, as can be seen on METEOSAT images. For this case, the issue about the main formation mechanism (homogeneous vs. heterogeneous or both) will be addressed. In a first step the meteorological conditions leading to the cirrus formation are analyzed using meteorological analyses as obtained from the European Centre for Medium-Range Forecasts (ECMWF). The ECMWF wind fields are then used to calculate backward trajectories with the Lagrangian analysis tool LAGRANTO. From these investigations the large-scale/mesoscale motions are derived and analyzed. Finally, the meteorological analyses and measurements (temperature, wind, humidity) are used as initial conditions for cirrus cloud simulations. We used the model EULAG as large eddy simulation (LES) model, including a state-of-the-art ice microphysics scheme for 2D and 3D idealized and quasi-realistic simulations. In order to address the impact of dynamics vs. aerosols (i.e. heterogeneous nucleation), we investigated different environmental conditions. The microphysical and macrophysical properties of the simulated cloud are finally compared to the measurements, in order to get some information about the most probable scenarios.