The Impact of Synoptic Variability and Aviation Soot Emissions on Contrail Cirrus Radiative Forcing

Contrail cirrus constitute the largest known aviation related radiative forcing component. Contrails form when water saturation is reached during the mixing of hot and moist plume with ambient air and persist in ice supersaturated air. Emitted soot and entrained ambient aerosol particles activate into water droplets and subsequently freeze to form contrail ice crystals. Contrail cirrus increase cirrus cloudiness, modify the upper tropospheric water and heat budget and warm on average the atmosphere. Aviation growth rates are large, making contrail cirrus radiative forcing an increasingly important part of climate change. We have implemented a contrail cirrus parameterization within a climate model treating contrail cirrus as an independent cloud class. Contrail ice nucleation, ice crystal loss within the contrails vortex phase, spreading of the young contrails and microphysical processes within the contrail are parameterized. We study life cycles of contrail cirrus clusters and their dependence on aviation soot emissions and synoptic variability. Furthermore, we study properties of contrail cirrus when prescribing a whole air traffic inventory. Ice nucleation rates within contrails and the ice crystal loss in the contrail’s vortex phase, when engine plumes are trapped within the downward travelling vortices, are generally dependent on aircraft soot emissions and the atmospheric state. In the mid latitudes at flight levels ice nucleation is mainly controlled by soot emissions, whereas vortex survival rates depend on the atmospheric state as well. Changes in soot emissions cause changes in ice crystal number concentration within young contrails that affect microphysical process rates significantly. Nevertheless, synoptic variability has an even stronger impact on microphysical process rates. Higher soot emissions cause in the mid latitudes on average lower sedimentation rates, higher contrail cirrus optical depth and larger contrail cirrus lifetimes. Synoptic variability has a large impact on microphysical process rates with large scale ice supersaturated areas supporting large scale and long lived contrail cirrus outbreaks. These contrail cirrus outbreaks explain a large fraction of the contrail cirrus radiative impact. In this synoptic regime, reductions in soot emissions have a very large impact on optical properties and life times of contrail cirrus whereas short lived contrail cirrus clusters and their properties change little due to soot emission reductions.