Contrail Cirrus Radiative Forcing for Future Scenarios

Abstract:

We estimate contrail cirrus radiative forcing (RF) within the climate model ECHAM5-CCMod. The new extension of the contrail cirrus parameterization, including a microphysical 2-moment scheme, allows a more realistic representation of microphysical processes and therefore of the microphysical and optical properties of contrail cirrus. In particular at the beginning of the contrail cirrus life cycle it appears to be crucial to estimate the contrail cirrus volume and the dilution connected with its growth. Water deposition is limited using an estimate for the diffusional growth of ice crystals. This is in particular important in later stages of the contrail cirrus life cycle. The model can resolve the temporal evolution of contrail cirrus properties, starting with many small crystals and a peak in ice water content after a few hours, resembling eventually natural cirrus. We perform simulations for the year 2002 (AERO2k), a base inventory for the year 2006 (AEDT) and for several scenarios for the year 2050 (AEDT) allowing the isolation of effects due to increases in air traffic volume and efficiency and soot reductions. The global RF of contrail cirrus for the year 2002 and 2006 is estimated to be 35.4mW/m² and 56.1mW/m², respectively. A strong increase of RF simulated for the year 2050 is the result of an increased air traffic volume and a shift towards higher altitudes. Particularly in the tropics the relation of RF to flight distance increases. In comparison climate change has a very small effect on contrail cirrus RF due to compensation in different regions. The strong increase in contrail cirrus RF for the future air traffic scenario cannot be compensated by a decrease in initial ice crystal numbers due to reduced soot emissions. Contrail cirrus RF appears to be nonlinearly dependent on the initial ice crystal concentration, with larger soot reductions having a large impact on contrail cirrus optical depth and life times.