No abstract

Within the framework of the European Fifth Framework Project TRADEOFF, the impact of changing cruise altitudes on contrail coverage and corresponding radiative forcing was investigated. On the basis of the reference year 1992, a series of aircraft emissions inventories with changed flight altitudes was prepared. These emission scenarios provide flown distances, fuel consumption and NO x emissions on a three-dimensional grid. The vertical resolution of these inventories was significantly increased over that used in former inventories. With a downshift of cruise altitude by 2000 ft (Throughout this paper we denote flight levels in ft. 2000 ft convert to approximately 610 m.), 4000 ft, and 6000 ft global annual mean contrail coverage is reduced in an approximately linear manner, reaching a maximum decrease of almost 45 % for a 6000 ft lower cruise altitude. Contrary to this, a slight increase by 6 % of global annual mean contrail coverage resulted for a 2000 ft higher maximum flight altitude. Relative changes of corresponding radiative forcing were shown to be very similar to those of contrail coverage. For changes in contrail coverage and radiative forcing associated with changes in flight altitudes, a strong seasonal and regional variability was found. This study only considers contrail radiative forcing. Trade-offs from other aviation related radiative impacts, e.g., from CO 2 or O 3 , have not been studied. Zusammenfassung Im Rahmen des Projektes TRADEOFF wurde der Einfluss geänderter Flughöhen auf die Kondensstreifenbedeckung und den damit verbundenen Strahlungsantrieb untersucht. Für das Referenzjahr 1992 wurde eine Reihe von Luftverkehrsemissionskatastern mit geänderten Flughöhen entwickelt. Diese Emissionsszenarien stellen geflogene Distanzen, Treibstoffverbrauch und Stickoxidemissionen auf einem dreidimensionalen Gitter bereit. Gegenüber früheren Luftverkehrsemissionskatastern wurde die Vertikalauflösung beträchtlich erhöht. Die jahresgemittelte globale Kondensstreifenbedeckung nimmt ungefähr linear mit der Flughöhe ab, wenn letztere um 2000 Fuß, 4000 Fuß und 6000 Fuß reduziert wird, und erreicht eine maximale Abnahme von etwa 45 % für 6000 Fuß tiefere Flugniveaus. Im Gegensatz dazu führt eine Erhöhung der Flugniveaus um 2000 Fuß zu einer geringen Zunahme der jahresgemittelten Kondensstreifenbedeckung um 6 %. Die entsprechenden relativenÄnderungen des Strahlungsantriebes sind denen der Kondensstreifenbedeckung sehr ahnlich. Für dieÄnderungen der Kondensstreifenbedeckung und des Strahlungsantriebes ergibt sich eine hohe saisonale und regionale Variabilität. In der vorliegenden Arbeit wird nur der Strahlungsantrieb aufgrund von Kondensstreifen betrachtet, andere Beiträge zum Strahlungsantrieb des Luftverkehrs z. B. aufgrund von CO 2 oder O 3 wurden nicht untersucht.

[1] In equilibrium climate change simulations with a global climate model we estimate the climate sensitivity parameter to contrail cirrus to be 0.43 K/(Wm À2 ), only about 60% of the corresponding value for a CO 2 forcing. The spatial pattern of the surface temperature response is much smoother than the forcing pattern of contrails with little correlation between both. As the thermal inertia of the climate system causes a marked delay of the transient response, only about 30% of the equilibrium surface warming to be expected from some aircraft induced radiative forcing is actually realised for aviation increase rates typical for the 1990s. Our findings do not confirm recent results of a dominating influence of contrail cirrus on temperature change over the United States between 1975 and 1994.

[1] Selected results of the ECHAM4 general circulation model are compared with sophisticated radiative transfer model calculations in order to investigate the performance of the ECHAM4 radiation schemes with respect to the radiative forcing (RF) of contrails. The mean shortwave RF is found to be in accordance with radiative transfer model results. For the longwave part it is shown that the effective emissivity approach combined with the assumption of maximum-random cloud overlap used in the standard ECHAM4 leads to a considerable underestimation of the longwave contrail RF. Hence, this approach cannot be recommended for calculating the longwave RF of optically thin clouds in general. As an alternative, more adequate results of a modified version of the ECHAM4 longwave scheme are discussed.INDEX TERMS: 3359