Aviation-induced cirrus cover detected from diurnal cycle over North Atlantic
August 29, 2012
Aviation-induced cirrus cover detected from diurnal cycle over North Atlantic
Figure 1: Air traffic distribution in the North Atlantic (colored). The box over the North Atlantic identifies the region investigated. It lies within the circle of Meteosat visibility (figure by DLR).
Figure 2: Cirrus cover (white) over the North Atlantic as derived with the MeCiDa method from Meteosat data. The scenes show an example with observed cirrus cover before and after passage of the eastbound traffic on a specific day. The noon scene shows additional cirrus cover, obviously caused by aviation. The grey polygon includes the North Atlantic flight corridor considered (figure by DLR).
Figure 3: Diurnal cycles of air traffic density (red, right axis) and cirrus cover (black, left axis) in western and eastern parts of the North Atlantic traffic corridor (figure by DLR).
Linear cirrus clouds of ice particles, so-called contrails, are often visible behind cruising aircraft. In cold and humid air masses they grow and persist and often become indistinguishable from natural cirrus clouds after some time. Contrails have been identified from their linear structure in satellite data. However, the total amount of cirrus cover could only be estimated from model simulations. For the first time the cover by aviation-induced cirrus clouds was observed from Meteosat satellite observations using traffic data provided by EUROCONTROL.
Air traffic over the North Atlantic follows a systematic time pattern (see animation). Every morning hundreds of airliners cruise from North America to Europe and return to North America in the early afternoon. The same pattern has been detected in the diurnal cycle of cirrus cover. Scientists of DLR, the Germany Aerospace Center, have used this pattern to quantify the amount of aviation-induced cirrus cover and its dynamical time scales for the first time.
The geostationary Meteosat Second Generation (MSG) satellites, established under cooperation between EUMETSAT and the European Space Agency (ESA), carries the Spinning Enhanced Visible and InfraRed Imager (SEVIRI), which has the capacity to observe the Earth in 12 spectral channels. Cirrus cover was derived from infrared data of the SEVIRI Instrument every 15 minutes with about 5 km horizontal resolution for eight years of observations. For this purpose a new Meteosat Cirrus Detection Algorithm MeCiDA2 is used which was developed jointly with the University Munich. Air traffic density data were provided by the European air traffic control center EUROCONTROL with high spatial and temporal resolution for several weeks in 2004.
The satellite measurements and traffic data were used to derive the mean diurnal cycles of cirrus cover and air traffic density. Both show a double wave in time with maxima at early morning and early afternoon.
The double wave for cirrus follows the double wave for air traffic with a delay of a few hours. Such a delay time is to be expected and corresponds to the time during which contrails spread to maximum width and thickness. Because of different flight directions, the maxima are closer together in the east part of the North Atlantic than in the west part. This east-west double wave pattern is unique for aviation and therefore serves as fingerprint to identify aviation contributions to cirrus. Cover and air traffic can be related by a linear response model. From the data and the model a mean aviation-induced cirrus cover of 1-2 % is derived with typical delay times of 2-4 h. The derived cover is larger than estimated so far with models. The cover amplitude and the delay time can be used to test the modeling of contrail dynamics.
The results have just been published in a paper of Geophysical Research Letters. In a follow-on paper, the authors quantify also the climate impact caused by these additional cirrus clouds. Results will be reported in a few months when the next paper is published. The results show warming and cooling effects depending on daytime, weather, and underground. This offers new chances for mitigation of the aviation climate effect.
