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Contrails and Climate ? Understanding Vertical Dispersion of Contrails

Contact: Ralf Sussmann

 

The most evident effect of air traffic upon the atmosphere is certainly its formation of (long living) condensation trails (contrails). These artificial clouds appear when the hot and humid exhaust gas from the jet engines reaches a water-supersaturated state during its mixing with the cold ambient air. This then allows the water vapour to condense and freeze to form ice crystals. Generally, persistent contrails receive more interest than those that are short living (lifetime ,1 min), because only persistent contrails can significantly impact on the radiative energy balance of the Earth.

Understanding the mechanisms determining growth in the vertical and horizontal directions is crucial for quantifying the impact upon regional radiation balance. Vertical extension is ruled by the dynamics of the downward trailing vortex pair that forms behind cruising aircraft due to pressure differences at the wings ("vortex regime", i.e., 10–100 s behind aircraft). Horizontal growth is then determined by vertical shear of horizontal wind speed and turbulence in the "dispersion regime".

Therefore, vortex-regime evolution of contrails is investigated by focusing on the role of ambient humidity. Cross-section measurements with the Garmisch scanning aerosol lidar and observational analysis are combined with numerical simulations of fluid dynamics and microphysics. Contrail evolution behind four-turbofan aircraft is classified into three different scenarios.

 

Contrail Evolution Scenario
i) In the case of ice-subsaturated air, a visible pair of wingtip vortices is formed that disappears at the end of the vortex regime. ii) In case of ice supersaturation, a diffuse secondary wake evolves above the wingtip vortices. It is due to detrainment of ice particles growing by sublimation of ambient humidity. A vertical wake-gap opens between the wingtip vortices and the secondary wake as shown by the lidar measurement:

 

Contrail Lidar Measurement

 

It is due to subsaturated air moving upward along the outer edges of the sinking vortex tubes accumulating around the upper stagnation point of the vortex system as shown by the numerical simulation:

 

Numerical Simulatiuon    Numerical Simulatiuon
The vertical wake-gap preferably occurs in the wake of heavy (four turbofans) aircraft, since the vortices behind light aircraft migrate down too slowly. The secondary wake is composed of nonspherical particles larger than the ones in the wingtip vortices which are spherical particles and/or particles smaller than 0.5 mm. In most cases the secondary wake is the only part of a contrail that persists after vortex breakdown. This is because the ice in the vortex tubes evaporates due to adiabatic heating as the vortices travel downward. iii) Only in the rare case of higher ambient ice supersaturation (.2%) do both parts of a contrail contribute to the persistent ice cloud. The number of ice crystals initially formed is typically reduced by a factor of 200 by evaporation (60% ambient humidity). This leads to a high population of interstitial particles. The results imply that formation of persistent contrails can be minimized by technical means.

Reference

 

This paper obtained the science award of the year 2000 of the German Aerospace Center (DLR):

Sussmann, R. and Gierens, K.: Lidar and numerical studies on the different evolution of vortex pair and secondary wake in young contrails, J. Geophys. Res., 104, 2131-2142, 1999.

Related papers:

Sussmann, R.: Vertical dispersion of an aircraft wake: Aerosol-lidar analysis of entrainment and detrainment in the vortex regime, J. Geophys. Res., 104, 2117-2129, 1999.

Sussmann, R. and K.M. Gierens: Differences in early contrail evolution of two-engine versus four engine aircraft: Lidar measurements and numerical simulations, J. Geophys. Res., 106, 4899-4911, 2001.

Sussmann, R., Optical properties of contrail-induced cirrus: discussion of unusual halo phenomena, Appl. Opt., 36, 4195-4201, 1997.