On dehydration effects from contrails in a coupled contrail-climate model

Abstract

The uptake of water by contrails in ice-supersaturated air and release of water after ice particle advection and sedimentation is investigated using a coupled climate-contrail model. Simulations with and without coupling arc compared. In the coupled mode, the contrails make the upper troposphere slightly drier which in turn causes thinner contrails. As a consequence, the radiative forcing (RF) by contrails computed in the coupled mode is slightly smaller (about 15 %) than without coupling. As a further effect, many of the contrail ice particles forming in the upper troposphere sediment and release water at lower altitudes, on average 700 m below the mean flight levels. This causes a further drying of the upper troposphere but adds humidity at lower levels. As a consequence, contrails change the entire hydrological cycle in the atmosphere. Contrail formation and contrail ice particle sedimentation causes a reduced total water column and rcduccd cover of high and low-level clouds. The quantitative effects arc small compared to climate noise in the cou- plcd model. Significant changes were found in the simulations only for strongly increased (factor 100) air traffic emissions. Scaled to nominal emissions, the dehydration causes a negative net RF in the global model of order -0.01 W m"2. In total, the RF by contrails is reduced because of humidity exchange between contrails and the background atmosphere and because of dehydration of the upper troposphere. This model study was performed with a 3 times larger soot emission index than in previous studies. The change in soot emission increases the contrail RF which offsets the forcing caused by water coupling and dehydration in the model. Hence, the net contrail-induced RF result is similar to that from previous studies (0.04-0.08 W m"2).