Abstract
Abstract. Condensation trails (“contrails”) which form behind aircraft are estimated to cause on the order of 50 % of the total climate forcing of aviation, matching the total impact of all accumulated aviation-attributable CO2. The climate impacts of these contrails are highly uncertain, in part due to the effect of overlap between contrails and other cloud layers. Although literature estimates suggest that overlap could change even the sign of contrail radiative forcing (RF), the impacts of cloud–contrail overlaps are not well understood, and the effect of contrail–contrail overlap has never been quantified. In this study we develop and apply a new model of contrail radiative forcing which explicitly accounts for overlap between cloud layers. Assuming maximum possible overlap to provide an upper bound on impacts, cloud–contrail overlap is found to reduce the shortwave-cooling effect attributable to aviation by 66 % while reducing the longwave-warming effect by only 37 %. Therefore, on average in 2015, cloud–contrail overlap increased the net radiative forcing from contrails. We also quantify the sensitivity of contrail radiative forcing to cloud cover with respect to geographic location. Clouds significantly increase warming at high latitudes and over sea, transforming cooling contrails into warming ones in the North Atlantic corridor. Based on the same data, our results indicate that disregarding overlap between a given pair of contrail layers can result in longwave and shortwave radiative forcing being overestimated by up to 16 % and 25 %, respectively, with the highest bias observed at high optical depths (> 0.4) and high solar zenith angles (> 75∘). When applied to estimated global contrail coverage data for 2015, contrail–contrail overlap reduces both the longwave and shortwave forcing by ∼ 2 % relative to calculations which ignore overlap. The effect is greater for longwave radiation, resulting in a 3 % net reduction in the estimated RF when overlap is correctly accounted for. This suggests that contrail–contrail overlap radiative effects can likely be neglected in estimates of the current-day environmental impacts of aviation. However, the effect of contrail–contrail overlap may increase in the future as the airline industry grows into new regions.
Highlights
Condensation trails (“contrails”) are ice clouds which form in aircraft engine exhaust plumes
We evaluate the general effect of overlap on contrail radiative forcing (RF) through a parameterized analysis
Contrail overlap has the greatest effect on the net RF when contrails are located in hot, equatorial areas with high albedo, as is the case in low-latitude desert areas such as the Sahara. This results in a maximum net contrail RF reduction by contrail–contrail overlapping in the tropics (TROP), where we find a reduction from an average sensitivity of 1.6 W/m2 for two “independent layers” to an average sensitivity of 0.6 W/m2 for two “overlapping layers”
Summary
Condensation trails (“contrails”) are ice clouds which form in aircraft engine exhaust plumes. Previous studies have found the latter effect to be dominant, at night, when the cooling effects associated with reductions in incoming shortwave radiation do not exist (Liou, 1986; Meerkötter et al, 1999) The difference between these two effects is the net contrail radiative forcing (RF) (Penner et al, 1999; IPCC 2013). The net radiative-forcing impacts of contrails have been quantified using both global climate models (e.g., Chen and Gettelman, 2013; Ponater et al, 2002) and dedicated modeling approaches such as the Contrail Cirrus Prediction Tool (CoCiP) (Schumann, 2012) and the Contrail Evolution and Radiation Model (CERM) (Caiazzo et al, 2017).
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