Open Access

Ralf Sussmann, Klaus M. Gierens

• Vortex-regime evolution of contrails is investigated by focusing on the role of ambient humidity. Lidar cross-section measurements and observational analysis are combined with numerical simulations of fluid dynamics and microphysics. Contrail evolution behind four-turbofan aircraft is classified into three different scenarios. In the case of ice-subsaturated air, a visible pair of wingtip vortices is formed that disappears at the end of the vortex regime. In case of ice supersaturation, a diffuse secondary wake evolves above the wingtip vortices. It is due to detrainment of ice particles growing by sublimation of ambient humidity. A vertical wake-gap opens between the wingtip vortices and the secondary wake. It is due to subsaturated air moving upward along the outer edges of the sinking vortex tubes accumulating around the upper stagnation point of the vortex system. The vertical wake-gap preferably occurs in the wake of heavy (four turbofans) aircraft, since the vortices behind lightVortex-regime evolution of contrails is investigated by focusing on the role of ambient humidity. Lidar cross-section measurements and observational analysis are combined with numerical simulations of fluid dynamics and microphysics. Contrail evolution behind four-turbofan aircraft is classified into three different scenarios. In the case of ice-subsaturated air, a visible pair of wingtip vortices is formed that disappears at the end of the vortex regime. In case of ice supersaturation, a diffuse secondary wake evolves above the wingtip vortices. It is due to detrainment of ice particles growing by sublimation of ambient humidity. A vertical wake-gap opens between the wingtip vortices and the secondary wake. It is due to subsaturated air moving upward along the outer edges of the sinking vortex tubes accumulating around the upper stagnation point of the vortex system. The vertical wake-gap preferably occurs in the wake of heavy (four turbofans) aircraft, since the vortices behind light aircraft migrate down too slowly. The secondary wake is composed of nonspherical particles larger than the ones in the wingtip vortices which are spherical particles and/or particles smaller than ≈0.5 μm. In most cases the secondary wake is the only part of a contrail that persists after vortex breakdown. This is because the ice in the vortex tubes evaporates due to adiabatic heating as the vortices travel downward. Only in the rare case of higher ambient ice supersaturation (>2%) do both parts of a contrail contribute to the persistent ice cloud. The number of ice crystals initially formed is typically reduced by a factor of 200 by evaporation (60% ambient humidity). This leads to a high population of interstitial particles. The results imply that formation of persistent contrails can be minimized by technical means.…