Abstract

Abstract. The impact of a potential future fleet of supersonic aircraft on contrail coverage and contrail radiative forcing is investigated by means of simulations with the general circulation model ECHAM4.L39(DLR) including a contrail parameterization. The model simulations consider air traffic inventories of a subsonic fleet and of a combined fleet of sub- and supersonic aircraft for the years 2025 and 2050, respectively. In case of the combined fleet, part of the subsonic fleet is replaced by supersonic aircraft. The combined air traffic scenario reveals a reduction in contrail cover at subsonic cruise levels (10 to 12 km) in the northern extratropics, especially over the North Atlantic and North Pacific. At supersonic flight levels (18 to 20 km), contrail formation is mainly restricted to tropical regions. Only in winter is the northern extratropical stratosphere above the 100 hPa level cold enough for the formation of contrails. Total contrail coverage is only marginally affected by the shift in flight altitude. The model simulations indicate a global annual mean contrail cover of 0.372% for the subsonic and 0.366% for the combined fleet in 2050. The simulated contrail radiative forcing is most closely correlated to the total contrail cover, although contrails in the tropical lower stratosphere are found to be optically thinner than contrails in the extratropical upper troposphere. The global annual mean contrail radiative forcing in 2050 (2025) amounts to 24.7 mW m−2 (9.4 mW m−2) for the subsonic fleet and 24.2 mW m−2 (9.3 mW m−2) for the combined fleet. A reduction of the supersonic cruise speed from Mach 2.0 to Mach 1.6 leads to a downward shift in contrail cover, but does not affect global mean total contrail cover and contrail radiative forcing. Hence the partial substitution of subsonic air traffic leads to a shift of contrail occurrence from mid to low latitudes, but the resulting change in contrail-induced climate impact is almost negligible.

Highlights

  • Contrails are line-shaped ice clouds which form in the wake of jet aircraft

  • The Special Report on “Aviation and the Global Atmosphere” by the Intergovernmental Panel on Climate Change (IPCC, 1999) estimated the global radiative forcing (RF) of contrails in 1992 to be in the range of 0.02 W m−2 which amounts to 40% of the total RF caused by aviation

  • IPCC (1999) stated that the partial replacement of a subsonic fleet by supersonic aircraft may lead to a 50% higher aircraft-induced RF in 2050 than for a subsonic fleet

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Summary

Introduction

Contrails are line-shaped ice clouds which form in the wake of jet aircraft. Like natural cirrus clouds, they have an influence on the earth’s climate system. Sausen et al (2005) provided an updated estimate of aviation RF for the year 2000, mainly based on results of the EU project TRADEOFF. The Special Report on “Aviation and the Global Atmosphere” by the Intergovernmental Panel on Climate Change (IPCC, 1999) estimated the global radiative forcing (RF) of contrails in 1992 to be in the range of 0.02 W m−2 which amounts to 40% of the total RF caused by aviation. These results indicate a global RF of (line-shaped) contrails of 0.01 W m−2 which is approximately a factor of three to four smaller than the IPCC-based estimate scaled to 2000 (0.034 W m−2). The IPCC (2007) provided an estimate of the global contrail RF in 2005 of 0.01 W m−2 with a range from 0.003 W m−2 to 0.03 W m−2

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