Dehydration effects from contrails in a coupled contrail–climate model

Abstract

Abstract. The uptake of water by contrails in icesupersaturated air and the release of water after ice particle advection and sedimentation dehydrates the atmosphere at flight levels and redistributes humidity mainly to lower levels. The dehydration is investigated by coupling a plumescale contrail model with a global aerosol-climate model. The contrail model simulates all the individual contrails forming from global air traffic for meteorological conditions as defined by the climate model. The computed contrail cirrus properties compare reasonably with theoretical concepts and observations. The mass of water in aged contrails may exceed 10 6 times the mass of water emitted from aircraft. Many of the ice particles sediment and release water in the troposphere, on average 700 m below the mean flight levels. Simulations with and without coupling are compared. The drying at contrail levels causes thinner and longer-lived contrails with about 15 % reduced contrail radiative forcing (RF). The reduced RF from contrails is on the order of 0.06 W m −2 , slightly larger than estimated earlier because of higher soot emissions. For normal traffic, the RF from dehydration is small compared to interannual variability. A case with emissions increased by 100 times is used to overcome statistical uncertainty. The contrails impact the entire hydrological cycle in the atmosphere by reducing the total water column and the cover by high-and low-level clouds. For normal traffic, the dehydration changes contrail RF by positive shortwave and negative longwave contributions on the order of 0.04 W m −2 , with a small negative net RF. The total net RF from contrails and dehydration remains within the range of previous estimates.

Variability in Contrail Ice Nucleation and Its Dependence on Soot Number Emissions

“…At temperatures close to the contrail formation threshold, maximum plume supersaturation limits the ice nucleation since only the larger soot or ambient particles entrained into the plume can activate into water droplets (Kärcher et al, 2015). Initial ice crystal number in global climate models has been calculated either from the water vapor available for deposition, assuming a fixed ice crystal size (Chen & Gettelman, 2013), or prescribed using a constant initial value inferred from in situ measurements (Bock & Burkhardt, 2016b) or set equal to engine soot number emissions accounting for different aircraft types (Schumann et al, 2015). The method of Chen and Gettelman (2013) introduces a dependency on the atmospheric state that only remotely relates to the nucleation process within contrails.…”

Reassessing properties and radiative forcing of contrail cirrus using a climate model

“…They estimated contrail cirrus radiative forcing including the feedback on natural clouds for the year 2002 to be 31mW/m 2 , exceeding their own radiative forcing estimate of line‐shaped contrails by a factor of roughly nine, comparable and even slightly larger than the forcing due to the accumulated CO 2 emitted by aviation from the start of air traffic. The global radiative forcing of contrail cirrus has been studied with different model setups, ranging from studies recognizing contrail ice crystal formation simply as an input into the natural cloud scheme to coupling a plume‐scale contrail model to a global climate model, to online simulations of the life cycle of contrail cirrus in a climate model [ Burkhardt and Kärcher , ; Chen et al , ; Schumann et al , ]. Associated radiative forcing estimates are 12 and 63mW/m 2 for the year 2006 [ Chen et al , ; Schumann et al , ], respectively, and 39mW/m 2 for the year 2002 [ Burkhardt and Kärcher , ].…”

Sensitivity of surface temperature to radiative forcing by contrail cirrus in a radiative-mixing model

“…Long-lived contrails of significant optical thickness (> 0.1) are estimated to cover about 0.2-0.5 % of the Earth, with higher values in northern midlatitudes Schumann et al, 2015;Bock and Burkhardt, 2016).…”

“…Various models to represent contrails in three-dimensional atmospheric global circulation models have been developed (Ponater et al, 1996;Rind et al, 2000;Ponater et al, 2002;Marquart et al, 2003;Rap et al, 2010b;Burkhardt and Kärcher, 2011;Jacobson et al, 2011;Olivié et al, 2012;Chen and Gettelman, 2013;Schumann et al, 2015;Bock and Burkhardt, 2016), with different treatments of traffic, subgrid-scale contrail formation, and optical properties. Some of these models were run with atmosphere-ocean coupling (Rind et al, 2000;Ponater et al, 2005;Rap et al, 2010a;Huszar et al, 2013;Jacobson et al, 2013).…”