Radiative properties of persistent contrails

The sensitivity of the instantaneous radiative forcing of persistent linear contrails to their physical properties is assessed in a sophisticated off-line two-stream radiative transfer model. We prescribe one altitude for the 1992 global contrail cover, with a physical thickness of 1 km, and assume natural cirrus properties (ice water content and effective size), derived from in situ measurements, which we allow to vary as a function of ambient temperature. Three representative cruise altitudes and three ice water content ranges are compared. The results show that the IWC variability that aged contrails may present under these assumptions has a more modest impact on their estimated RF than what one would expect, and that the consequences of larger contrail ice water contents would include an enhanced dependence of their radiative impact on season and altitude.

The altitude dependence experiments showed that low cruise altitudes during July showed negative net radiative forcings of a few mWm^-2 over most of the continental Northern Hemisphere, while the cruise altitude with the highest traffic produced values in excess of 2 Wm^-2 over the Central Europe and Northeastern USA in January. The seasonal dependence showed that larger ice water contents enhance the contrail's long-wave radiative forcing more than the shortwave one in January but not in July, translating in the net radiative forcing being almost a factor of 4 larger in January.

The contrail global mean net radiative forcing dependence on the ice water content variability covered a range between 4 and 32 mWm^-2, suggesting that the latest IPCC value of 10.0 mWm^-2, which is based on studies using fixed contrail micro-physics and optical depths on the lower end of our predicted range, could be reversed if measurements of larger optical depths in persistent contrails are found to be more representative.