Contrail clouds

released 04.28.11

The climate research community suspects that aviation impacts global climate by changing cloud cover, but the extent of this effect is very uncertain. That's the focus of a Stanford University project led by Mark Jacobson and Sanjiva Lele. Their team, including PhD candidate Alexander Naiman, is studying the impact of commercial aviation on climate.

The direct effect of aircraft on clouds can be seen by looking up on the right day: Under certain atmospheric conditions, airplanes produce condensation trails, or contrails, which are manmade ice clouds. There is also an indirect effect, in that the exhaust products from jets can induce cirrus cloud formation where clouds might not have formed otherwise. Increasing cloud cover in these ways affects climate by changing the radiative balance, but the change must be carefully studied to determine whether it warms or cools. Clouds can have either a cooling or warming effect depending on their specific properties, and this is where the uncertainty lies.

To understand the cloud properties better, Naiman is conducting research using NCSA's Abe, running a Large Eddy Simulation (LES) using a highly scalable parallelized code developed at Stanford. The LES models the formation and development of contrails, simulating both fluid dynamics and ice microphysics. The simulations start with the wake of a commercial airliner at a cruise condition and continue for 20 minutes of simulation time. The end result is a data set that provides 3D fields of ice size and spatial distribution that allows the calculation of radiative properties of the contrail. To date, simulations have varied conditions such as aircraft type, ambient humidity, and wind shear. Future work is planned to extend simulation times out to several hours, investigating the transition from linear contrails to diffuse cirrus clouds.

In addition to calculating the properties of individual contrails under a range of conditions, this work has also led to the development of a parameterized model of contrail dynamics. The simple parameterization is used as a subgrid scale model within a large-scale atmospheric simulation, predicting the evolution of individual contrails based on parameters provided by the large-scale simulation. The large-scale simulation is being used to improve estimates of the overall effect of aviation on climate.

Project results have been published in Atmospheric Chemistry and Physics in 2010, the Journal of Computational Physics in 2011, and also presented in 2009, 2010, and 2011 at meetings of the American Institute of Aeronautics and Astronautics, the American Physical Society Division of Fluid Dynamics, and the American Geophysical Union.

Contrails form in aircraft wakes, where the aerodynamics of flight that produce lift also create pairs of strong, counter-rotating vortices. This series of images shows the evolution of the vortex cores (opaque red and blue surfaces) and how they affect the distribution of jet exhaust material (transparent green/blue surface) that forms the contrail.

At 65 simulation seconds, the vortices are well defined and parallel to the flight direction.

An instability causes small perturbations of the parallel vortices to grow, and the vortices eventually link to form vortex loops. At 120 simulation seconds, a loop has formed and is beginning to expand in the spanwise direction.

At 210 simulation seconds, the loop has become very distorted due to mutual induction of different portions of the vortices. The chaotic interaction of the vortices then leads to quick dissipation of the organized system. Even after the vortices are gone, their effect can be seen in the periodic, puffy pattern characteristic of thick contrails.

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