Modelling of contrail ice crystal formation with an improved representation of aerosol and ice microphysics

• Applicant: Dr. Andreas Bier
• Subject Area: Atmospheric Science
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 412514550

Contrail cirrus clouds (consisting of young and aged contrails) have a large contribution to the anthropogenic global warming caused by air traffic. The number of ice crystals formed by contrails has a very strong influence on the further life cycle, properties and radiative effect of contrail cirrus clouds. In the previous project, the main objective was to combine two complementary approaches for contrail formation. This means simulating the contrail formation behind commercial aircraft using Large Eddy Simulations (LES) and a sophisticated representation of contrail formation microphysics. In the previous project, the work focused on the extension of the particle-based LCM with regard to the contrail formation microphysics. The ice crystal formation on soot particles and later also on background particles mixed into the exhaust plume was integrated in order to simulate contrail formation behind both conventional passenger aircraft and aircraft with hydrogen combustion. The corresponding studies were carried out with a LCM-based box model using a multi-trajectory approach. On the other hand, there were difficulties in simulating a meaningful turbulent jet expansion with the LES model EULAG, so that the intended 3D simulations with contrail formation are still pending. While this open goal is still being pursued in the follow-up project, two other essential aspects regarding the extension of the contrail formation microphysics in the LCM are intended: 1.) The radiative forcing of contrails formed in natural cirrus clouds has not been considered yet in air traffic mitigation assessments. So far, there is only one study that has analyzed the formation of contrails in natural cirrus clouds in a regional climate model. In this project, the contrail formation in pre-existing cirrus should be simulated with the LCM and the influence of the existing cirrus on contrail formation should be investigated. Finally, the overall quantities of contrail and cirrus compared to the cirrus before will be analyzed for different cirrus properties, atmospheric conditions and soot particle number emissions. 2.) In the future, engine soot particle emissions should be significantly reduced through various political regulations and mitigation measures. Therefore, it is expected that contrail formation on ultrafine volatile particles (UFPs) will play an increasing role. For example, substantial soot number emission reductions were recently measured behind engines with lean burn combustion, but the observed contrail ice crystal numbers remained very similar compared to the classic rich burn combustion case. This indicates significant ice crystal formation on UFPs for soot-poor emissions. Therefore, UFPs should be included into the LCM in addition to soot and background particles. Besides the investigation of their activation and freezing behavior for different exhaust emission scenarios, the coagulation between UFPs (and UFPs and soot) should be implemented.

• DFG Programme: Research Grants
• International Connection: USA
• Co-Investigator: Dr. Simon Unterstraßer
• Cooperation Partner: Professor Dr. Piotr Smolarkiewicz