Contrails - Research, comments and links

Changes in cirrus cloudiness

CHANGES IN CIRRUS CLOUDINESS AND THEIR RELATIONSHIP TO CONTRAILS
Patrick Minnis NASA Langley Research Center, Hampton, VA 23681., J. Kirk Ayers, Rabindra Palikondra, David R. Doelling, Ulrich Schumann, and K. Gierens.

Condensation trails, or contrails, formed in the wake of high-altitude aircraft have long been suspected of causing the formation of additional cirrus cloud cover in otherwise clear skies. Contrail-generated cirrus clouds alter the radiation budget in the same manner as natural cirrus clouds of equivalent optical depth and microphysical properties. Because commercial air traffic is increasing globally, any radiative effects from contrails may also be on the rise. To estimate the maximum possible impact of contrail-generated cirrus on the Earth�s radiation budget, trends in cirrus cloud amounts are correlated with the density of high altitude air traffic using both surface and satellite-generated cirrus cloud amounts. A minimum estimate is derived using the coverage based on linear contrail observations. Cloud radiative forcing is estimated by assuming that the contrail-generated cirrus clouds have the average microphysical properties derived from contrails with satellite data. The results provide a range of estimates that can be used to evaluate the current state of contrail effects and to determine the potential for future radiative forcing. Increases in cirrus cloudiness are well correlated with air traffic patterns with the radiative forcing being greatest over the USA. These estimates suggest that contrails may be an important component of climate change, at least at the regional scale.

12th Symposium on Global Change Studies and Climate Variations

Large-Eddy-Simulation of Contrails

Engine water vapor emissions by aircraft are expected to be of significant importance because they may lead to an increase in high altitude ice clouds (Gra�l, 1990; Liou et al., 1990). Ice clouds enhance reflection of sunlight as well as thermal emission. The net effect is uncertain because the competition of solar albedo and IR greenhouse effects involving ice clouds is dependent on such factors as cloud position, cloud thickness and microphysical properties. Contrails are expected to enhance the greenhouse effect, owing to their small thickness.

The objectives of our study have been firstly to investigate the secondary flow structure within clouds to understand the processes that cause contrails to spread out to form cirrus clouds. Secondly, to examine the respective roles of external conditions, such as temperature, humidity, static stability and baroclinicity of the ambient atmosphere in the evolution of contrails and to determine the importance of physical processes such as radiation on the development of contrails. The approach undertaken is to use a state of the art LES-model (Chlond, 1992, 1994, 1997) that incorporates a detailed description of all relevant physical processes.

To set up the control run which serves as a reference case, the model is applied to conditions that are typical for those under which contrails could be observed in the atmosphere; i. e. we anticipate an air volume at a height of 10550 m (p00=250 hPa) which is supersaturated with respect to ice (rice=129%) and at a temperature of 220 K. The model was run assuming no mean vertical wind shear with the initial condition taken as an atmosphere at rest except for a constant background wind along the contrail. The initial potential temperature field was assumed to be horizontal homogenous and initially slightly stable throughout (G=2.5 K/km). Among the reference case 12 sensitivity runs have been performed to clarify the influence of external conditions and physical processes in the evolution of contrails. Based on these experiments the following conclusions could be drawn:

1. Persistent contrails can only form in an atmosphere that is supersaturated with respect to ice.

2. The turbulent eddies forming within the contrail are driven by buoyancy. In particular, latent heating due to condensation provides the significant energy source while destabilization due to differential radiative heating plays no role owing to the small optical depth of the contrail.

3. Contrail evolution is controlled primarily by the temperature, humidity and static stability of the environmental air. It turns out that increasing the relative humidity or warming the ambient atmosphere, respectively, will produce more vigorous eddies in the contrail. The physical reason for the enhanced energy levels in both cases is that the greater the relative humidity and the higher the temperature of the ambient air, respectively, the greater is the conditionally available potential energy that could be converted into kinetic energy. On the other hand, reducing the relative humidity or cooling the ambient atmosphere, respectively, produces less vigorous eddies. In both cases the conversion rate from water vapor to ice, and hence the latent heat release is much less than in the reference case owing to the initial decrease of total water content. The effect of stratification is as follows: In a neutrally stratified atmosphere enhanced energy levels are observed because the eddies have to perform no work against buoyancy forces. On the other hand, in a more stable stratified atmosphere, stable stratification counteracts vertical motions so that mixing is much slower in the vertical than in the horizontal direction. Finally, the baroclinicity of the atmosphere (i. e. the vertical wind shear of the horizontal mean flow) is only a control parameter of secondary importance.

4. The initial ice particle size and radiative processes are of minor importance (at least during the first 30 minutes simulation period) in the evolution of contrails.

Literature:

Chlond, A. (1992): Three-dimensional simulation of cloud street development during a cold air outbreak. Bound. Layer Meteor., 58, 161-192)

Chlond, A. (1994): Locally modified version of Bott's advection scheme. Monthly Weather Review, 122, 111-125.

Chlond, A. (1997): Large-eddy-simulation of contrails. Submitted to Journal of the Atmospheric Sciences.

Gra�l, H. (1990): Possible climatic effects of contrails and additional water vapour. In: U. Schumann (ed.): Air traffic and the enviroment. Lect. Notes in Engrg., Vol 60, Springer-Verlag, Berlin, 124-137.

Liou, K. N., S. C. Ou and G. Koenig (1990): An investigation of the climatic effect of contrail cirrus. In: U. Schumann (ed.): Air traffic and the enviroment. Lect. Notes in Engrg., Vol 60, Springer-Verlag, Berlin, 154-169. 

Exhaust from long distance air traffic is polluting the upper troposphere and the lower stratosphere with an estimated growth rate of 3-4 % per year. In the traffic corridor across the North Atlantic about 40 % of the fuel is burned above the tropopause. From satellite images an additional contrail cloud cover of up to 2 % has been estimated for this region, but values on the order of 10 % have been suggested for regional scale corridors.

Contrails may enhance the greenhouse effect, but microphysical and optical properties as well as the size and global distribution of contrails are not adequately known for conclusive model calculations.

In this study a groundbased scanning lidar system is used to investigate the vortex regime (10-100 sec), the dispersion regime (100 - 1000 sec), and the diffusion regime of persistent contrails with respect to geometric and optical parameters

The evolution of persistent contrails has modeled over time-scales of 15-180 minutes using a large-body simulation model with detailed microphysics. Model results have been compared to satellite and in situ measurements of persistent contrails from the SUCCESS experiment. In particular, we simulated the evolution of the persistent contrail observed on May 12, 1996. In simulations with large ambient supersaturations and moderate wind shear, crystals with length > 200 microns are generated within 35 minutes by depositional growth. In situ measurements in the May 12 contrail case showed that these large crystals did in fact form. The large crystals fall rapidly and the contrail horizontal extent increases due to the wind shear (see Figure 1). Strong radiative heating (with rates up to 30 K/day) drives a local updraft and lofts the contrail core several hundred meters. The observed rate of contrail spreading and maintenance of optical depths larger than 0.1 can be explained simply by growth and precipitation of ice crystals nucleated during the initial contrail formation if the environmental humidity is high enough (relative humidity with respect to ice > 125%). This result is consistent with observed high humidities in regions where the persistent contrails formed on May 12. Also, the simulations indicate that the humidity must be high throughout a depth of at least several hundred meters below the contrail to allow the crystals to continue growing as they fall.

Research Staff
Eric Jensen

Point of Contact
Eric Jensen, (650) 604-4392
ejensen@sky.arc.nasa.gov

Spreading and Growth of Contrails in a Sheared Environment

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Last Modified: 10/28/98