SIMULATING THE GLOBAL ATMOSPHERIC RESPONSE TO AIRCRAFT WATER-VAPOR EMISSIONS AND CONTRAILS - A FIRST APPROACH USING A GCM

The effect of contrails and aircraft water vapour emissions on global climate is studied by means of a general circulation model (GCM). In a first approach water vapour emissions and mean contrail coverage with in the main flight routes are prescribed according to current observat ions in a simplified manner. A hierarchic experiment strategy with gra dual increase of the forcing is applied to identify the resulting clim ate signals. The water vapour increase to be expected from air traffic is too small to force a detectable radiative or climatic response. Th e sensitivity of the model climate to the occurrence of contrails appe ars to be higher. For mid-latitude summer conditions, the high cloud i ncrease experiments show a consistent temperature response pattern. Ho wever, its magnitude is statistically significant only for a mean cont rail coverage exceeding present-day amounts. Moreover, the magnitude o f the contrail climate signal is highly sensitive to the details of th e experimental setup due to several non-linearities of the cloud-radia tive interaction. Hence, the prescription of contrails in the GCM has to be as careful as possible for an optimal treatment of the problem. Respective recommendations are given.