Characterization of midlatitude cirrus clouds with airborne lidar - investigating an indirect aviation effect

Kurzfassung

In view of global warming, science is prompted with the task to investigate all potential impacts on climate. Cirrus clouds that consist of small ice crystals exhibit a considerable climate impact due to their large extent. They lead to high uncertainties in climate prediction, as it is unclear how their impact is modified in a changing climate and by anthropogenic activities. The evermore growing air traffic causes the formation of condensation trails that can persist for long times under suitable conditions and add to the general cirrus cover. Besides this direct effect from aviation, the concept of an indirect effect on natural cirrus clouds has been proposed decades ago. Several modeling studies have investigated this indirect effect, where aircraft exhaust particles contribute to the background aerosol load in the upper troposphere and act as ice nuclei during natural cirrus formation. Yet, it has not been verified in field studies until now. Model estimations of the radiative forcing, delivered contradicting results from no significant effect to a range of −350 mW m^−2 to 90 mW m^−2. These uncertainties stem from the absence of observational evidence that is needed to specify the relevant model scenarios. In this work northern hemisphere midlatitude cirrus clouds are characterized by an airborne lidar and investigated in the context of an indirect effect. Two groups of clouds are identified based on their optical properties. They form two modes in their particle depolarization ratio at δ^part = 0.4 and δ^part = 0.5. This indicates fundamental differences in the cloud microphysics of both groups. The group with elevated depolarization ratios is associated with the indirect aviation effect. Other possible influences are excluded. This allows first insights into the indirect effect. Clouds in the affected group show lower ice supersaturations, larger particle sizes and lower number concentrations. All this can be interpreted as the traces of heterogeneous freezing due to aircraft exhausts. Whole clouds are affected on a large scale. Their frequent occurrence and their unclear but potentially large climatic impact demands further investigations. From the results of this work, measurement strategies are deduced that will help to gather further observational evidence in future. They should focus on nucleation areas in air traffic and unpolluted regions.