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 contrailslocated 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 humiditywith respect to ice within the prefrontal mixing zone allowed the contrails to persist, whilethe strictly regulated flight paths over the region account for their remarkable shapes. Thepositioning of the NAT flight paths to take advantage of the jet stream likely maximizedregional contrail formation. An estimation of the outgoing top of the atmosphere longwave(LW) flux from the CERES instrument shows that the contrails reduced the localinstantaneous LW emissivity by 20.96 (�0.26) W/m2. This example demonstrates that formiddle latitude regions, prefrontal mixing is an important factor governing the radiativeinfluence 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 thecontrails into daytime; moreover, several hours after formation the line-shaped contrailsspread 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 frontalinfluences of the North Atlantic, J. Geophys. Res., 117, D11201, doi:10.1029/2011JD017019.

1. Introduction

[2] Condensation trails (contrails) are aviation-inducedice-phase clouds, usually formed at pressures below 300 mb,at air temperatures less than �30�C. They can persist in airthat is saturated with respect to ice [Jensen et al., 1998].They primarily form due to the mixing of the warm, watervapor (and aerosol) rich air of the aircraft exhaust plumes,with the cold and less humid ambient air [Appleman, 1953;Scorer, 1955]. Like all clouds, contrails both cool and warmthe climate by influencing Earth’s radiative balance: theyreflect incident shortwave (SW) radiation back to space,resulting in a cooling effect, while they simultaneously trapoutgoing terrestrial longwave (LW) radiation, resulting in awarming effect. The net influence of contrails is believed tobe a small magnitude warming effect [Forster et al., 2007].[3] The lifetime of contrails depends on the ambient

atmospheric conditions, specifically the relative humiditywith respect to ice (RHI). Contrails formed under RHI

conditions below the ice saturation level are short-lived, astheir constituent ice crystals sublimate rapidly after forma-tion. Alternatively, if the RHI is greater than ice saturationlevels they may persist as nearly linear structures for anextended period of time (termed persistent contrails), or mayeven develop into a cirrostratus cloud layer (referred to asspreading contrails) [Jensen et al., 1998; Haywood et al.,2009]. Additionally, the injection of aerosols into the uppertroposphere from aircraft exhaust plumes also provides asource of ice forming nuclei which may subsequently influ-ence cirrus properties via heterogeneous nucleationmechanisms (the aerosol indirect effect) [Lee et al., 2009].[4] Contrails lead to an increase in cloud coverage due to

the fact that during the mixing of aircraft exhaust plume airwith the surrounding air, ice crystals are formed even thoughthe conditions for the formation of natural clouds may notyet have been reached. Observational studies suggest thatthe total coverage of line-shaped contrails over regions withheavy air traffic may be around several percent [Bakan et al.,1994; Meyer et al., 2002]. Observational estimates of theglobal coverage of aircraft-related cirrus clouds have beenmade by Stordal et al. [2005] from extrapolations of regionalcorrelations between cirrus detected by satellite data andmodeled air traffic simulations. These results suggest that inthe year 2000 global aircraft-related cirrus cover was approx-imately 0.1–0.38%; for a detailed comparison of regionaland global contrail coverage estimates, see Rap et al. [2010]and references therein. In addition to increasing cloud cover,it has also been noted that contrail cirrus may reduce naturalcirrus cloud formation. Model studies suggest that thesereductions may be around 1–2% regionally and may partially

1Instituto de Astrofísica de Canarias, San Cristóbal de La Laguna,Spain.

2Department of Astrophysics, Faculty of Physics, Universidad de LaLaguna, San Cristóbal de La Laguna, Spain.

3Department of Geography, University of Sussex, Falmer, UK.4NCAS-Climate, University of Reading, Reading, UK.5Asgard Consulting, Basingstoke, UK.

Corresponding author: B. A. Laken, Instituto de Astrofísica deCanarias, Via Lactea s/n, E-38205 La Laguna, Tenerife, Spain.(blaken@iac.es)

Copyright 2012 by the American Geophysical Union.0148-0227/12/2011JD017019

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, D11201, doi:10.1029/2011JD017019, 2012

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offset the impact of contrails on climate change [Burkhardtand Kärcher, 2011].[5] The radiative forcing (RF) resulting from aviation-

induced clouds may alter Earth’s climate, and the magnitudeof this effect is expected to increase over time as the volumeof air-traffic increases [Marquart et al., 2003]. The IPCCfourth assessment report rates the level of understandingof the global radiative forcing of contrails to be low (duelargely to the difficulties in identifying aircraft-inducedcirrus) [Forster et al., 2007]. Estimates suggest that forpersistent line-shaped contrails the RF in the year 2000 mayhave been approximately +0.01 W/m2 [Sausen et al., 2005].However, considering the additional effects of contrailscirrus, model results suggest that the global RF (of line-shaped and contrail cirrus combined) may be much higher,namely �0.0375 W/m2 for the year 2002 [Burkhardt andKärcher, 2011].

[6] The radiative impacts of contrails are primarily con-trolled by their extent, optical depth and lifetime. PersistentRHI supersaturations are often observed in the upper tro-posphere of the tropics and the lower troposphere of polarregions, while middle latitude (storm track) regions fre-quently show large variability in saturation levels [Kahnet al., 2009]. Cirrus clouds may form as a result of hetero-geneous or homogenous nucleation processes: homogenousice nucleation requires supersaturations of >45% [Koopet al., 2000] and may be the dominant process of naturalcirrus formation, whereas heterogeneous ice nucleationmechanisms operate at lower supersaturations [DeMott et al.,2003; Möhler et al., 2006], and are an important nucle-ation process influencing contrail formation and properties[Kärcher and Yu, 2009]. Consequently, there are manycloud free regions where supersaturation conditions existat levels too low to allow for homogenous nucleation to

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 frontalcloud system and development of the contrails over an area of the North Atlantic.

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occur, but which are susceptible to contrail formation viacondensation freezing of either aircraft or pre-existing ambientaerosols [Gierens et al., 1999; Kärcher et al., 2007].[7] The heterogeneous freezing of aging soot particles

from the aircraft exhaust plume may cause an additionalincrease in cloud cover. However, the magnitude of thiseffect is not yet known, as it is currently unclear if soot par-ticles are efficient ice forming nuclei in atmospheric condi-tions. Furthermore, an increase in cloud cover resulting fromthis process may not happen within the plume phase, makingit problematic to observe resulting cloud cover changes[Jensen and Toon, 1997].[8] Once formed, the radiative impacts of persistent and

spreading contrails may be locally intense. Using a casestudy of the radiative effects of a spreading contrail devel-oping in to a cirrostratus cloud over England, Haywood et al.[2009] estimated a local net radiative forcing of +10 to+30 W/m2 for daytime and nighttime respectively. This casestudy showed a single localized event generating a global-mean RF equivalent to 7% of the estimated diurnally aver-aged linear contrail RF globally. This result is in agreement

with the suggestion of Burkhardt and Kärcher [2009] thatcontrail-induced cirrus coverage globally may be dominatedby a few major events, and scale with supersaturationrather than the frequency of contrail formation. Conse-quently, the uncertainty regarding future changes in airtemperature and RHI at upper tropospheric levels may havesignificant consequences for the RF of aviation inducedcloud [Forster et al., 2007]. In this work, we present a casestudy of a persistent contrail event formed under the influ-ence of frontal mixing in the North Atlantic region, includingan estimate of the line-shaped contrails LW radiative impact,with the intention of increasing the sparse number of obser-vational studies available for evaluating model estimates.

2. Satellite Observations of North AtlanticContrails on 1 September 2007

[9] Satellite telemetry reveals the presence of severalstriking high level cloud formations to the northwest ofEngland on 1 September 2007 prior to sunrise. Figure 1shows the development of these features as recorded by the

Figure 2. Pressure and wind conditions. (a) Synoptic surface level pressure conditions (mb) from the UKMet Office at 00:00 UT 01/09/2007, and (b) vector winds at 225 mb over a polar stereographic projectionof the Northern hemisphere, from the NCEP Climate Forecast System Reanalysis (CFSR), as a mean of00:00–06:00 UT on 01/09/2007.

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Advanced Very High Resolution Radiometer (AVHRR)instrument in channel 5 (11.5–12.5 mm) over a 14-h period.During this time an extensive, high-level cloud system ispresent to the northwest of the British Isles, extendingroughly from the southern tip of Greenland to the west ofNorway, moving in a southeasterly direction with the typicalcharacteristics of a frontal cloud system.[10] Figure 1 shows that a series of persistent line-shaped

contrails clustered in groups are visible within the cloudsystem, extending beyond its southern edge at 03:32 UT.At 05:11 UT the density and extent of the contrail clustershas greatly increased, and they have developed into aremarkable distribution pattern. By the mid-day satellite pass

the persistent line-shaped contrails are no longer visible,although less apparent contributions to the high-level cloudcoverage may still remain (Figure 1).[11] The remarkable shapes and high densities of the line-

shaped contrail clusters observed are attributed to the east-bound (nighttime) North Atlantic Track (NAT) flight pathsof transatlantic routes. The North Atlantic region has ahigh volume of heavily regulated air traffic. On the date of01/09/2007 between the hours of 00:00 – 05:00 UT approx-imately 600 aircraft were flying along 5 NAT paths. Thelocations of the NATs are primarily determined by themeteorological conditions: the NATs are planned so thatthe eastbound aircraft fly with the aid of the polar jet stream.On 01/09/2007 the jet stream was concentrated over a smallregion, indicated by the frontal cloud system (the jet streamconditions are described in detail in the following section),as a result the NATs are located closely together (the mini-mum legal distance between individual tracks is 1�). Aninteresting feature of the contrail groups is the apparentconvergence of each track to a single point. This reflects aconvergence of air traffic along the transatlantic flightcorridor, which appears relatively dispersed toward itsAmerican side but gets narrower as aircraft approach theheavily trafficked and tightly regulated West European air-space. As a result of the aircraft converging on the NAT pathsand traveling in great circles the observed pattern of inter-secting points is produced (clearly seen at 05:11 UT inFigure 1).

3. Analysis of Atmospheric Conditions

[12] Synoptic surface level pressure conditions on1 September 2007 at 00:00 UT from the UK Met Office arepresented in Figure 2a. The figure shows cold fronts advectingacross the North Atlantic toward a region of high pressure,while occluded fronts advance northeastward across the UK.The position of the Polar jet stream is apparent from the225 mb vector winds, shown in Figure 2b with data fromthe NCEP Climate Forecast Reanalysis System [Saha et al.,2010]; from a consideration of the synoptic conditions it isclear that the cloud system is formed under the influence ofa polar low pressure front.[13] The form and location of the contrails themselves

give vital clues regarding the local atmospheric conditions.Specifically, the contrails all terminate abruptly at roughlythe same distance from the frontal cloud system. This indi-cates a region of ice supersaturation, which allows the line-shaped contrails to persist. This notion is confirmed from anexamination of the RHI and water vapor sensitive satellitechannels presented in Figure 3. Figure 3a shows the high-level cloud retrieved from the 05:11 UT AVHRR satellitepass overlaid on to a 6-h average of RHI data (between00:00–06:00 UT) at 230 mb from the NCEP CFSR project,while panel 3B shows data from the Meteosat SpinningEnhanced Visible and Infrared Imager (SEVIRI) IR/watervapor channel (5.35–7.15 mm) taken at 06:00 UT.[14] Although the cloud and RHI data are not simulta-

neous, panel A does provide a way to approximately assessthe relationship between the contrails and RHI. As a result ofthe southeasterly motion of the cloud system, the 6-h aver-aging means that the cloud/contrail mask is likely offset fromthe RHI. However, from the figure we see that the contrails

Figure 3. Relative humidity and contrails. (a) Contrailand high level cloud features from AVHRR (channel 5)05:11 UT overlaid on to NCEP CFSR relative humidity withrespect to ice (%) data at 230 mb, which has been interpolatedfrom a 0.5� � 0.5� grid. Reanalysis data is a six-hour averagebetween 00:00 to 06:00 UT. The edge of the AVHRR sensorswath is denoted by the dotted line, and intersects the twomost northerly contrail clusters. (b) Meteosat SEVIRI IR/water vapor channel (5.35–7.15 mm) retrieved at 06:00 UT.

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appear to terminate at RHI saturations of approximately 85%.To compliment this data, panel B shows that the position of thecontrails is strongly constrained by the presence of high watervapor concentrations, resulting from pre-frontal mixing.[15] The contrails do not resume after the reduction in RHI

south of the frontal cloud system despite the presence ofadditional regions of high RHI (Figure 3). This discontinuitymay be explained by the flight paths of the air traffic, which,rather than crossing through the additional regions of highRHI instead pass through a corridor of lower RHI extendingtoward England. Alternatively, the contrails may not be seenin additional areas of supersaturation as the air temperaturesmay be too high for formation to occur.

4. Estimated Radiative Changes dueto the Presence of Persistent Contrails

[16] Figure 4 shows an estimate of the change in outgoingLW radiation (5–100 mm) from the top of the atmosphere asmeasured by the Clouds and Earth Radiant Energy System(CERES) instrument, on board the AQUA satellite platform[Wielicki et al., 1996]. The figure shows the field of view(FOV) of the CERES FM3 (cross track scan mode) sensor(available from the CERES CES, Cloud and RadiationSwath data set), between 04:00–05:00 UT on 01/09/2007presented on a 0.25 � 0.25� resolution stereographic plot.[17] The frontal cloud is evident as the blue (low value)

pixels, as a result of the cloud absorbing large parts of theoutgoing surface LW radiation, while regions of clear sky/low cloud are indicated by red (high value) pixels. Thecontrails can be clearly seen extending from the cloud asseveral distinct light colored protrusions. It is important tonote that the intensity of these regions has likely beendiminished by the interpolation that we applied to the data.[18] To estimate the local instantaneous outgoing top of

the atmosphere (TOA) LW radiative forcing of the contrailswe objectively identify contrail pixels, and compare them to

pixels located between the contrails (hereafter referred to asintracontrail pixels). For that we use the raw (uninterpolated)CERES FM3 retrievals. From these data we found theintracontrail pixels to have a mean (error given is the 1sstandard error of the mean) of 247.08(�0.14) W/m2, whilethe contrail pixels had a value of 226.12 (�0.26) W/m2,thus the contrails are estimated to have a local instantaneousLW impact of 20.96 (�0.26) W/m2. A paired (two-tailed)Student’s T-test performed on ensembles of the uninterpolatedcontrail/intracontrail pixels shows the difference between thecontrail and non-contrail sites to be statistically significantabove the 0.99 probability level, confirming that the contrailshave significantly reduced the local outgoing instantaneousLW radiation flux.[19] During daylight hours, a large portion of this LW

forcing would be offset as a result of an opposite sign SWforcing. It should also be noted that our estimation ofLW forcing might be overly simplistic, as the radiativeforcing of a contrail is the flux change due to the presence ofthe contrail with all other parameters held constant. However,we make no attempt to account for any variations in tem-perature or humidity between the contrail/intracontrail pixelsin our calculation. Although we have estimated the impactof the line-shaped contrails on the LW radiation balance,the full extent of the contrails impact on the local netradiation balance is difficult to determine, as the contrailspersist into the daytime and undergo spreading, developingin to cirrostratus clouding complicating the consideration oftheir forcing considerably.[20] Figure 5 shows a series of 10.8 micron infrared

retrievals taken at hourly intervals between 08:00 and11:00 UT from the Meteosat Second Generation (MSG) 2Spinning Enhanced Visible and InfraRed Imager (SEVIRI)instrument. These images show the spreading of the line-shaped contrails into the high level cloud of the frontalsystem, gradually becoming indistinguishable for both cloudfree areas and pre-existing frontal clouds. The ambiguity

Figure 4. CERES instantaneous LW flux. Top of the atmosphere upward LW flux (5–100 mm) from theCERES instrument (FM3) cross-track scanner on board NASA’s AQUA satellite from the CERES-CESdata set for 01/09/2007 (04:00–05:00 UT). The data have been converted to a 0.25 � 0.25� grid.

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between the aviation-induced cloud and natural cloud isclear; this example highlights the difficulties with assessingthe full impacts of aviation-induced cloud on the climatesystem.[21] The contrail development identified in this study

illustrates the importance of frontal processes for the gener-ation of persistent contrails, by the creation of areas of clearsky, with high relative humidity levels. These results areconsistent with the findings that contrail development (andoutbreaks) are found to frequently occur over regions asso-ciated with frontal activity, cyclonic waves, and jet streamactivity, where the upper tropospheric areas are associatedwith baroclinic ridging, and the conditions become higherand colder than average [Kästner et al., 1999; DeGrandet al., 2000; Carleton et al., 2008].[22] For regions such as the North Atlantic, where there is

both a high volume of air traffic and where the favorableconditions for contrail formation and persistence are pro-vided by mixing processes along frontal zones, the radiativeimpact of contrails in the future may vary depending on howclimate change influences frontal activity over the region.Furthermore, it is significant to note that eastbound (night-time) NAT paths are specifically designed to maximize thevolume of air traffic flying within the polar jet stream, inefforts to minimize fuel consumption and journey time.

While such a strategy reduces CO2 emissions, it may alsomaximize the potential regional contrail radiative impact.

5. Concluding Remarks

[23] We have presented a case study of persistent con-trails, developing in an area of prefrontal mixing over theNorth Atlantic region. These contrails are remarkable due totheir unusual appearance; this is attributed to a clustering ofindividual contrails around NAT flight paths, resulting in theappearance of five distinct contrail groups with intersectingpoints. We have analyzed data regarding the evolution andproperties of these contrails from a variety of satelliteinstruments including a CERES sensor pass. We estimatethat the contrail groups significantly reduced the instanta-neous outgoing LW radiation flux by 20.96 (�0.26) W/m2.This value is comparable in magnitude to the (nighttime)value obtained by Haywood et al. [2009] of 30 W/m2.Although, the aforementioned study included the effects ofcontrail cirrus in addition to line-shaped contrails, while thisstudy only estimates the radiative impact of line-shapedcontrails. A chief limitation of estimating the effects of con-trails on climate from observations lies in the ambiguity ofdistinguishing aircraft initiated cloud from natural cirrusformations. While for persistent line-shaped contrails this is

Figure 5. Development of spreading contrails. Meteosat Second Generation (MSG) 2 SpinningEnhanced Visible and InfraRed Imager (SEVIRI) 10.8 micron infrared retrievals (1 km2 resolution), athourly intervals from 08:00–11:00 UT 01/09/2007. The spreading of the line-shaped contrails in to thefrontal cloud system is clearly visible, indicating that the aviation-induced cloud has enhanced the naturalcloud coverage, making the true extent of the radiative impacts of the contrails difficult to determine.

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relatively straightforward due to their unnatural line-shapedstructure, when considering spreading contrails developingin to cirrostratus cloud it becomes far more difficult [Forsteret al., 2007]. From Figure 5, it is clear that the contrailsstudied here underwent spreading. As a result, the contrailspresence certainly lead to further radiative effects in additionto secondary impacts on the dynamics of the frontal system,which are unaccounted for in our study. Further, it is unclearif natural clouds would have formed in the absence of thecontrail-induced cirrus.

[24] Acknowledgments. We kindly thank Steve Ghan (PacificNorthwest National Laboratory), and three anonymous reviewers for theircomments. The CFSR data were developed by NOAA’s National Centersfor Environmental Prediction (NCEP). Reanalysis data for this study arefrom NOAA’s National Operational Model Archive and Distribution System(NOMADS), maintained at NOAA’s National Climate Data Center (NCDC).The CERES and CALIPSO data were obtained from the NASA LangleyResearch Center Atmospheric Sciences Data Center (ASDC). Meteosatand AVHRR imagery were provided by NEODAAS/University of Dundee.Synoptic surface pressure plot was created by the UK Met Office. Specialthanks go to Neil Lonie (University of Dundee), Gill Compo (Universityof Colorado), Keith Butt (ICAO supervisor, Gander ACC) and Gabriel Pérez(Instituto de Astrofísica de Canarias). E. Pallé and B. Laken acknowledgesupport from the Spanish MICIIN grant CGL2009–10641.

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