Airplanes have rainbows in tow

Aerodynamic Contrails: Researchers Reveal New Source of Manmade Concentration

Sometimes visible in the sky: aerodynamic contrails © Jeff Well / DLR
Read out

Contrails are not equal to contrails. German scientists have now described for the first time how contrails arise not only from the exhaust fumes of the engines but, under certain conditions, also over the wings of aircraft. To what extent this new class of contrails - rainbow-colored ice clouds - has an impact on climate change is unclear, as it has not been included in climate models so far.

The cause of the initially gossamer contrails, which can only be observed under certain conditions directly behind an aircraft at cruising altitude, is the rapid pressure drop over the wings of an aircraft. As a result, the temperature of the air drops within a few hundredths of a second and tiny ice particles are formed, provided there is sufficient moisture in the air.

Genesis mechanism revealed

Scientists of the German Aerospace Center (DLR) in Oberpfaffenhofen were able for the first time to conclusively explain the formation mechanism of the aerodynamic contrails. "We have developed a physical model with which we can explain the observations of aerodynamic contrails very precisely, " says Professor Bernd Kärcher from the DLR Institute of Atmospheric Physics.

Simplified, the researchers describe the formation of the aerodynamic condensed clouds as follows: The uniform flow over the wings produces the smallest ice particles in a uniform size, which, if sufficient water vapor is present in the atmosphere, initially grow uniformly.

This even growth is also the reason for the rainbow colors of the contrail: Right behind the plane, the ice particles are still relatively small and reflect mainly blue light, the larger the ice particles are - the farther they are from the aircraft - the longer-wave is the light, that they reflect. display

According to the scientists, the color of the cone cloud changes from blue to green and yellow to red. After about 175 meters, the contrail appears white, because the ice particles are gradually getting into the turbulence of the wake vortices behind the aircraft. There, the ice particles no longer grow uniformly, they reflect the light diffusely in all wavelengths due to their different sizes and thus appear white.

New challenge for climate models

All vapor trails caused by soaring aircraft are ice clouds called cirrus. They increase the cloud coverage, alter the natural cirrus growth and thus influence the climate. Aerodynamic contrails represent a hitherto unnoticed anthropogenic (man-made) cloud source, which, just like contrails from the engine exhaust gases, contributes to the climate impact of aviation.

This type of contrails has always existed, but now climate models have to be used to determine how great their effect actually is, says K rcher. The aim of the researchers is to integrate aerodynamic contrails into a climate model that determines the global climate impact of aviation. As part of the DLR project "climate-compatible air transport system", the scientists are also seeking to find ways to minimize the impact of aviation on the air, for example through suitable flight routes or aircraft constructions,

Often especially in the tropics

Conventional contrails usually form when the ambient air of the aircraft is cooler than minus 40 degrees Celsius. This condition is often fulfilled on cruising routes in the middle latitudes and over polar caps.

In contrast, the atmosphere must be much warmer than minus 40 degrees Celsius for the formation of climate-effective aerodynamic contrails. Only then is sufficient water vapor present in the air so that ice particles can grow to a significant size. The DLR researchers therefore assume that in the subtropics and tropics, where the atmosphere is warmer and wetter overall, aerodynamic contrails frequently occur.

Since very high growth rates are currently forecast for air routes in these regions, it is important to estimate the climate impact of this anthropogenic source of high pollution.

(German Aerospace Center (DLR), 03.04.2009 - DLO)