Influence of soot number emissions on contrail cirrus properties, life cycle and climate impact

Kurzfassung

Contrail cirrus modify upper tropospheric cloudiness significantly. Aircraft soot emissions have a large impact on microphysical process rates of contrail cirrus. Our objective is to analyze the influence of reduced soot particle emissions, as expected from biofuels or lean combustion, on contrail cirrus properties, life cycle and climate impact. Our study is based on a contrail cirrus parameterization, using a double moment microphysics scheme, within the ECHAM5 climate model. We perform idealized process studies prescribing constant initial ice crystal number concentrations and investigate life cycles of contrail cirrus clusters in different synoptic situations and flight levels. In long-lived and large-scale ice supersaturated areas, sedimentation of ice crystals into subsaturated areas accounts for 25 to 45% of the overall ice crystal loss limiting the lifetime of contrail cirrus. At low ice crystal number concentrations, ice crystals grow initially faster, sedimentation is increased and, later in the life cycle, ice crystal number concentrations are earlier so low that deposition on ice crystals is limited by diffusional growth time scales. The influence of soot emission reductions is largest in large-scale ice supersaturated areas: A reduction in initial ice crystal number concentration by 80 % leads to an increase in ice particle loss due to sedimentation by about 15%. This means that on average, contrail cirrus lifetimes appear to be shorter by 3 to 4 hours, the mean optical depth is significantly reduced and the maxima of total extinction of short wave radiation are decreased by a factor of 2 to 2.5.