Wang et al. (2024): Radiative Effect of Two Contrail Cirrus Outbreaks Over Western Europe Estimated Using Geostationary Satellite Observations and Radiative Transfer Calculations

The article written by Xinyue Wang, Kevin Wolf, Olivier Boucher,and Nicolas Bellouin can be found at:
https://doi.org/10.1029/2024GL108452

Contrails are formed in the atmosphere by aircraft engine exhaust. Depending on the temperature and the humidity of the atmosphere, some contrails can persist and remain for several hours disturbing the Earth’s radiative budget. The latest studies have reported the overall impact of contrails to be warming but with uncertainties. Especially when we focus on contrail outbreaks, which is the main contributor to contrail induced radiative effects, their forcing exhibits variations in sign and magnitude and needs to be accurately quantified.

Xinyue Wang and her colleagues combined satellite data, weather data and air traffic data to detect two contrail cirrus outbreaks over Western Europe in clear sky conditions. Infrared images were used to identify and track contrails with a 15 min frequency. Then, cloud properties, which are retrieved from geostationary satellite observations, are used with a radiative transfer code to calculate the radiative forcing of these contrail-cirrus outbreaks. In this particular case, we can see that contrail cirrus show various signs of radiative forcing, resulting in alternative cooling and warming effects. During daylight hours, the cooling effect prevails, but there are also moments that longwave warming dominates especially over land surface. The absolute value of the radiative forcing in W/m² is also very high. For comparison, the forcing of the CO₂ since 1750 calculated by the IPCC is 1,5 W/m², which is 10 to 15 times less than the maximum instantaneous radiative forcing that a contrail can have. The difference is the timescale: contrails have a very strong impact for a few hours, whereas CO₂ remains in the atmosphere for centuries.

The animated images below represent contrails cirrus observed using satellite data. The left panel shows the dust RGB composited from infrared images. In the middle, wind data and the ice supersaturated area for contrail formation and persistence are superimposed. The right panel shows the calculated radiative forcing. The year, month, day and hour in UTC are shown in the title.

To lower the climate impact of aviation, contrail avoidance strategies such as aircraft rerouting are being investigated. The method used in this study to quantify the hourly radiative impact forcing of contrail outbreaks could be applied for verifying the climate benefits of these strategies.