Abstract
Persistent contrails and contrail cirrus are responsible for a large part of aviation induced radiative forcing. A considerable fraction of their warming effect could be eliminated by diverting only a quite small fraction of flight paths, namely those that produce the highest individual radiative forcing (iRF). In order to make this a viable mitigation strategy it is necessary that aviation weather forecast is able to predict (i) when and where contrails are formed, (ii) which of these are persistent, and (iii) how large the iRF of those contrails would be. Here we study several data bases together with weather data in order to see whether such a forecast would currently be possible. It turns out that the formation of contrails can be predicted with some success, but there are problems to predict contrail persistence. The underlying reason for this is that while the temperature field is quite good in weather prediction and climate simulations with specified dynamics, this is not so for the relative humidity in general and for ice supersaturation in particular. However we find that the weather model shows the dynamical peculiarities that are expected for ice supersaturated regions where strong contrails are indeed found in satellite data. This justifies some hope that the prediction of strong contrails may be possible via general regression involving the dynamical state of the ambient atmosphere.
Highlights
Persistent contrails contribute substantially to the climate effect of aviation
The condition for contrail formation is that the so called Schmidt-Appleman criterion [4] is fulfilled. It says that the mixture consisting of the expanding exhaust gases and ambient air must transiently become water saturated during the expansion of the plume. This can be cast into the physical terms of the maximum temperature, Tmax, at which contrail formation is still possible and the relative humidity RHmax the mixture obtains at the moment when it has temperature Tmax
In order to assess whether a promising strategy to eliminate a considerable share of contrail’s warming impact on climate is ready to be realized, namely to avoid the formation of Big Hits, which are contrails with a large positive individual radiative forcing [15], we tested whether a state-of-the-art weather prediction model, represented by the ERA-5 reanalysis, is able to predict reliably (i) the formation of contrails, (ii) their persistence, and (iii) the magnitude of their instantaneous radiative forcing
Summary
Persistent contrails contribute substantially to the climate effect of aviation (for recent estimates see, e.g., [1,2,3]). The instantaneous RF of a single contrail (iRF) can, be three orders of magnitude larger with both negative (cooling) and positive (warming) sign, such that the value reported by the Intergovernmental Panel on Climate Change (IPCC) and in the estimates mentioned above is just the small net effect that remains when large negative and positive contributions almost balance. A.; Thouret, V.; Nedelec, P.; Smit, H.; Helten, M.; Kley, D.; Karcher, F.; Simon, P.; Law, K.; Pyle, J.; et al Measurement of ozone and water vapor by Airbus in-service aircraft: The MOZAIC airborne program, An overview. K.; Schumann, U.; Helten, M.; Smit, H.; Marenco, A. A distribution law for relative humidity in the upper troposphere and lower stratosphere derived from three years of MOZAIC measurements
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