JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, D11201, doi:10.1029/2011JD017019, 2012 Contrails developed under frontal influences of the North Atlantic B. A. Laken,1,2 E. Pallé,1,2 D. R. Kniveton,3 C. J. R. Williams,4 and D. A. Kilham5 Received 14 October 2011; revised 24 April 2012; accepted 24 April 2012; published 1 June 2012. [1] Satellite imagery reveals a visually striking pattern of persistent line-shaped contrails located to the Northwest of the British Isles on 1 September 2007, just before sunrise. These contrails formed over the heavily trafficked eastbound North Atlantic Track (NAT) flight paths, as they intersected an area of prefrontal mixing. The high relative humidity with respect to ice within the prefrontal mixing zone allowed the contrails to persist, while the strictly regulated flight paths over the region account for their remarkable shapes. The positioning of the NAT flight paths to take advantage of the jet stream likely maximized regional contrail formation. An estimation of the outgoing top of the atmosphere longwave (LW) flux from the CERES instrument shows that the contrails reduced the local instantaneous LW emissivity by 20.96 (Æ0.26) W/m2. This example demonstrates that for middle latitude regions, prefrontal mixing is an important factor governing the radiative influence of contrails. However, a full estimation of the radiative impact is not possible, as we cannot specify the amount of shortwave forcing caused by the persistence of the contrails into daytime; moreover, several hours after formation the line-shaped contrails spread and overlapped with pre-existing cloud structures. Citation: Laken, B. A., E. Pallé, D. R. Kniveton, C. J. R. Williams, and D. A. Kilham (2012), Contrails developed under frontal influences of the North Atlantic, J. Geophys. Res., 117, D11201, doi:10.1029/2011JD017019. 1. Introduction conditions below the ice saturation level are short-lived, as their constituent ice crystals sublimate rapidly after forma- [2] Condensation trails (contrails) are aviation-induced tion. Alternatively, if the RHI is greater than ice saturation ice-phase clouds, usually formed at pressures below 300 mb, levels they may persist as nearly linear structures for an at air temperatures less than À30C. They can persist in air extended period of time (termed persistent contrails), or may that is saturated with respect to ice [Jensen et al., 1998]. even develop into a cirrostratus cloud layer (referred to as They primarily form due to the mixing of the warm, water spreading contrails) [Jensen et al., 1998; Haywood et al., vapor (and aerosol) rich air of the aircraft exhaust plumes, 2009]. Additionally, the injection of aerosols into the upper with the cold and less humid ambient air [Appleman, 1953; troposphere from aircraft exhaust plumes also provides a Scorer, 1955]. Like all clouds, contrails both cool and warm source of ice forming nuclei which may subsequently influ- the climate by influencing Earth’s radiative balance: they ence cirrus properties via heterogeneous nucleation reflect incident shortwave (SW) radiation back to space, mechanisms (the aerosol indirect effect) [Lee et al., 2009]. resulting in a cooling effect, while they simultaneously trap [4] Contrails lead to an increase in cloud coverage due to outgoing terrestrial longwave (LW) radiation, resulting in a the fact that during the mixing of aircraft exhaust plume air warming effect. The net influence of contrails is believed to with the surrounding air, ice crystals are formed even though be a small magnitude warming effect [Forster et al., 2007]. the conditions for the formation of natural clouds may not [3] The lifetime of contrails depends on the ambient yet have been reached. Observational studies suggest that atmospheric conditions, specifically the relative humidity the total coverage of line-shaped contrails over regions with with respect to ice (RHI). Contrails formed under RHI heavy air traffic may be around several percent [Bakan et al., 1994; Meyer et al., 2002]. Observational estimates of the global coverage of aircraft-related cirrus clouds have been 1 Instituto de Astrofísica de Canarias, San Cristóbal de La Laguna, made by Stordal et al. [2005] from extrapolations of regional Spain. 2Department of Astrophysics, Faculty of Physics, Universidad de La correlations between cirrus detected by satellite data and Laguna, San Cristóbal de La Laguna, Spain. modeled air traffic simulations. These results suggest that in 3Department of Geography, University of Sussex, Falmer, UK. the year 2000 global aircraft-related cirrus cover was approx- 4NCAS-Climate, University of Reading, Reading, UK. – 5 imately 0.1 0.38%; for a detailed comparison of regional Asgard Consulting, Basingstoke, UK. and global contrail coverage estimates, see Rap et al. [2010] Corresponding author: B. A. Laken, Instituto de Astrofísica de and references therein. In addition to increasing cloud cover, Canarias, Via Lactea s/n, E-38205 La Laguna, Tenerife, Spain. it has also been noted that contrail cirrus may reduce natural ([email protected]) cirrus cloud formation. Model studies suggest that these Copyright 2012 by the American Geophysical Union. reductions may be around 1–2% regionally and may partially 0148-0227/12/2011JD017019 D11201 1of7 D11201 LAKEN ET AL.: CONTRAILS UNDER FRONTAL INFLUENCES D11201 Figure 1. Contrail development from the AVHRR. A series of AVHRR Channel 5 (11.5–12.5 mm) images (1.1 km2 resolution) between 31/08/2007 and 01/09/2007 showing the precession of the frontal cloud system and development of the contrails over an area of the North Atlantic. offset the impact of contrails on climate change [Burkhardt [6] The radiative impacts of contrails are primarily con- and Kärcher, 2011]. trolled by their extent, optical depth and lifetime. Persistent [5] The radiative forcing (RF) resulting from aviation- RHI supersaturations are often observed in the upper tro- induced clouds may alter Earth’s climate, and the magnitude posphere of the tropics and the lower troposphere of polar of this effect is expected to increase over time as the volume regions, while middle latitude (storm track) regions fre- of air-traffic increases [Marquart et al., 2003]. The IPCC quently show large variability in saturation levels [Kahn fourth assessment report rates the level of understanding et al., 2009]. Cirrus clouds may form as a result of hetero- of the global radiative forcing of contrails to be low (due geneous or homogenous nucleation processes: homogenous largely to the difficulties in identifying aircraft-induced ice nucleation requires supersaturations of >45% [Koop cirrus) [Forster et al., 2007]. Estimates suggest that for et al., 2000] and may be the dominant process of natural persistent line-shaped contrails the RF in the year 2000 may cirrus formation, whereas heterogeneous ice nucleation have been approximately +0.01 W/m2 [Sausen et al., 2005]. mechanisms operate at lower supersaturations [DeMott et al., However, considering the additional effects of contrails 2003; Möhler et al., 2006], and are an important nucle- cirrus, model results suggest that the global RF (of line- ation process influencing contrail formation and properties shaped and contrail cirrus combined) may be much higher, [Kärcher and Yu, 2009]. Consequently, there are many namely 0.0375 W/m2 for the year 2002 [Burkhardt and cloud free regions where supersaturation conditions exist Kärcher, 2011]. at levels too low to allow for homogenous nucleation to 2of7 D11201 LAKEN ET AL.: CONTRAILS UNDER FRONTAL INFLUENCES D11201 Figure 2. Pressure and wind conditions. (a) Synoptic surface level pressure conditions (mb) from the UK Met Office at 00:00 UT 01/09/2007, and (b) vector winds at 225 mb over a polar stereographic projection of the Northern hemisphere, from the NCEP Climate Forecast System Reanalysis (CFSR), as a mean of 00:00–06:00 UT on 01/09/2007. occur, but which are susceptible to contrail formation via with the suggestion of Burkhardt and Kärcher [2009] that condensation freezing of either aircraft or pre-existing ambient contrail-induced cirrus coverage globally may be dominated aerosols [Gierens et al., 1999; Kärcher et al., 2007]. by a few major events, and scale with supersaturation [7] The heterogeneous freezing of aging soot particles rather than the frequency of contrail formation. Conse- from the aircraft exhaust plume may cause an additional quently, the uncertainty regarding future changes in air increase in cloud cover. However, the magnitude of this temperature and RHI at upper tropospheric levels may have effect is not yet known, as it is currently unclear if soot par- significant consequences for the RF of aviation induced ticles are efficient ice forming nuclei in atmospheric condi- cloud [Forster et al., 2007]. In this work, we present a case tions. Furthermore, an increase in cloud cover resulting from study of a persistent contrail event formed under the influ- this process may not happen within the plume phase, making ence of frontal mixing in the North Atlantic region, including it problematic to observe resulting cloud cover changes an estimate of the line-shaped contrails LW radiative impact, [Jensen and Toon, 1997]. with the intention of increasing the sparse number of obser- [8] Once formed, the radiative impacts of persistent and vational studies available for evaluating model estimates. spreading contrails may be locally intense. Using a case study of the radiative effects of a spreading contrail devel- 2. Satellite Observations of North Atlantic oping in to a cirrostratus cloud over England, Haywood et al. Contrails on 1 September 2007 [2009] estimated a local net radiative forcing of +10 to +30 W/m2 for daytime and nighttime respectively. This case [9] Satellite telemetry reveals the presence of several study showed a single localized event generating a global- striking high level cloud formations to the northwest of mean RF equivalent to 7% of the estimated diurnally aver- England on 1 September 2007 prior to sunrise. Figure 1 aged linear contrail RF globally.