On the lifecycle of individual contrails and contrail cirrus

Kurzfassung

The lifecycle of individual (initially line-shaped) contrails behind aircraft and of contrail cirrus (aged contrails mixed with other ice clouds) is described. The full contrail lifecycle is covered, from ice formation for given water, heat and particulate emissions, changes in the jet, wake and dispersion phases, until final sublimation or sedimentation. Contrail properties are deduced from various in-situ, remote sensing and model studies. Aerodynamically induced contrails and distrails are explained briefly. Contrails form both in clear air and inside cirrus. Young contrails consume most of the ambient ice supersaturation. Optical properties of contrails are age and humidity dependent. Contrail occurrence and radiative forcing depends on the ambient Earth-atmosphere conditions. Contrail cirrus seems to be optically thicker than assessed previously and may not only increase cirrus coverage but also thicken existing cirrus. Some observational constraints for contrail cirrus occurrence and radiative forcing are derived. Key parameters controlling contrail properties, besides aircraft and fuel properties, ambient pressure, temperature and humidity, are the number of ice particles per flight distance surviving the wake vortex phase, the contrail depth, and particle sedimentation, wind shear, turbulence and vertical motions controlling contrail dispersion. The climate impact of contrails depends among others on the ratio of shortwave to longwave radiative forcing (RF) and on the efficacy with which contrail RF contributes to surface warming. Several open issues are identified, including re-nucleation from residuals of sublimated contrail ice particles.