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A Case Study of the Radiative Forcing of Persistent Contrails Evolving Into Contrail-Induced Cirrus

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A Case Study of the Radiative Forcing of Persistent Contrails Evolving Into Contrail-Induced Cirrus This is a repository copy of A case study of the radiative forcing of persistent contrails evolving into contrail-induced cirrus. White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/43211/ Article: Haywood, JM, Allan, RP, Bornemann, J et al. (7 more authors) (2009) A case study of the radiative forcing of persistent contrails evolving into contrail-induced cirrus. Journal of Geophysical Research - Atmospheres, 114 (D24201). ISSN 0148-0227 https://doi.org/10.1029/2009JD012650 Reuse See Attached Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing [email protected] including the URL of the record and the reason for the withdrawal request. [email protected] https://eprints.whiterose.ac.uk/ JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, D24201, doi:10.1029/2009JD012650, 2009 A case study of the radiative forcing of persistent contrails evolving into contrail-induced cirrus James M. Haywood,1 Richard P. Allan,2 Jorge Bornemann,1 Piers M. Forster,3 Peter N. Francis,1 Sean Milton,1 Gaby Ra¨del,4 Alexandru Rap,3 Keith P. Shine,4 and Robert Thorpe1 Received 11 June 2009; revised 17 September 2009; accepted 24 September 2009; published 19 December 2009. [1] The radiative forcing due to a distinct pattern of persistent contrails that form into contrail-induced cirrus near and over the UK is investigated in detail for a single case study during March 2009. The development of the contrail-induced cirrus is tracked using a number of high-resolution polar orbiting and lower-resolution geostationary satellite instruments and is found to persist for a period of around 18 h, and at its peak, it covers over 50,000 km2. The shortwave (SW) and longwave (LW) radiative forcing of the contrail-induced cirrus is estimated using a combination of geostationary satellite instruments, numerical weather prediction models, and surface observation sites. As expected, the net radiative effect is a relatively small residual of the much stronger but opposing SW and LW effects, locally totaling around 10 W mÀ2 during daylight hours and 30 W mÀ2 during nighttime. A simple estimate indicates that this single localized event may have generated a global-mean radiative forcing of around 7% of recent estimates of the persistent contrail radiative forcing due to the entire global aircraft fleet on a diurnally averaged basis. A single aircraft operating in conditions favorable for persistent contrail formation appears to exert a contrail-induced radiative forcing some 5000 times greater (in W mÀ2 kmÀ1) than recent estimates of the average persistent contrail radiative forcing from the entire civil aviation fleet. This study emphasizes the need to establish whether similar events are common or highly unusual for a confident assessment of the total climate effect of aviation to be made. Citation: Haywood, J. M., R. P. Allan, J. Bornemann, P. M. Forster, P. N. Francis, S. Milton, G. Ra¨del, A. Rap, K. P. Shine, and R. Thorpe (2009), A case study of the radiative forcing of persistent contrails evolving into contrail-induced cirrus, J. Geophys. Res., 114, D24201, doi:10.1029/2009JD012650. 1. Introduction study is restricted to the first of these, and we therefore refer to contrail-induced cirrus throughout this work. Contrails [2] The rapid growth and the forecast future expansion of may form when emissions of hot, warm engine exhaust in the aviation industry mean that the potential climatic effects the upper troposphere mix with the cool moist ambient have received considerable attention over the past decade atmosphere. Under certain atmospheric conditions (super- [e.g., Intergovernmental Panel on Climate Change (IPCC), saturated with respect to ice), contrails can persist for 1999; Sausen et al., 2005; Lee et al., 2009]. The civil several hours. If the atmospheric conditions are favorable aviation industry currently emits around 2–3% of all carbon for ice crystal growth these persistent contrails may grow dioxide emissions on a global basis, but the total impact and spread out to form contrail-induced cirrus clouds [e.g., upon the Earth radiation budget is thought to be higher Fahey et al., 1999]. Persistent contrails and contrail-induced primarily because of the radiative impact of persistent cirrus exert a radiative forcing in both the shortwave (SW) condensation trails (contrails) and aviation-induced cirrus. solar spectrum and longwave (LW) terrestrial spectrum Aviation-induced cirrus can occur through two different [e.g., Stuber et al., 2006; Ka¨rcher and Spichtinger, 2009]. pathways: via contrails spreading out and by injection of They reflect incident sunlight back to space thereby bright- aerosols into the upper troposphere to provide ice nuclei that ening the planet and leading to a negative SW radiative may subsequently form cirrus clouds [Lee et al., 2009]. Our forcing that is associated with a cooling. They also trap LW radiation within the Earth atmosphere system leading to a 1 Forecasting Research and Development, Met Office, Exeter, UK. positive LW radiative forcing that is associated with a 2ESSC, University of Reading, Reading, UK. 3Department of Environmental Science, University of Leeds, Leeds, warming. The net radiative effect of persistent contrails UK. and contrail-induced cirrus is the sum of the negative SW 4Department of Meteorology, University of Reading, Reading, UK. radiative forcing and positive LW radiative forcing, result- ing in a net forcing that is believed to be positive but rather Copyright 2009 by the American Geophysical Union. 0148-0227/09/2009JD012650 D24201 1of17 D24201 HAYWOOD ET AL.: CONTRAIL-INDUCED CIRRUS D24201 small in magnitude [e.g., Myhre and Stordal, 2001, Stuber during March 2009 (Figure 1). Figure 1 (left) shows that the et al., 2006, Ra¨del and Shine, 2008]. CCC is difficult to detect in visible imagery owing to the [3] The earliest comprehensive estimate of the impact of presence of extensive stratocumulus cloud at lower levels. aviation emissions for aircraft operations in 1992 [Prather Indeed the shadow of the contrail on the lower stratocumu- et al., 1999; IPCC, 1999] suggested a radiative forcing of lus clouds, which causes a reduction in reflectance, is more 20 mW mÀ2 for the formation of persistent contrails with readily detectable than any increase in reflectance. Figure 1 considerable uncertainty. The estimate of the radiative (right) shows that the CCC is, however, very readily forcing from contrail-induced cirrus was thought to be so discernable in the infrared wavelengths because of the large uncertain that it could not even be quantified. The radiative difference in the emission temperature between the CCC forcing of persistent contrails and contrail-induced cirrus and the low-level stratocumulus. Both images show that has been estimated in more recent studies at 10 mW mÀ2 by while the CCC is by far the most distinguishable contrail Sausen et al. [2005] and 30 mW mÀ2 (range 10–80 mW occurring over the North Sea, several other contrails are also mÀ2)byStordal et al. [2005], respectively. This assessment visible off the coast of Scotland and England and encroach of persistent contrails was adopted by Forster et al. [2007] over southeast England. and IPCC [2007] who assigned a 90% confidence interval [6] In this study we analyze the meteorological condi- of 6–30 mW mÀ2. Forster et al. [2007] also point out the tions which are shown to favor persistent contrail formation inherent ambiguity in trying to determine and separate between approximately 25,000–35,000 ft (1 ft = 0.3048 m) aviation-induced cloudiness from persistent line-shaped (7.5 to 10.5 km) (section 2). Polar orbiting satellite instru- contrails: the line-shaped contrails typically shear and ments that detect cloud at infrared (10.8 mm) wavelengths spread and lose their characteristic shape while evolving are used to record the evolution of the CCC over a 10 h into contrail-induced cirrus [e.g., Minnis et al., 1998]. Thus, period and an atmospheric dispersion model is used to estimates of the ratio of the RF from contrail-induced cirrus verify the position of the coil-shaped contrail as it shears to persistent contrails are highly uncertain and range from and spreads (section 3). The study also uses independent about 1 to 8 [e.g., Lee et al., 2009]. These estimates of the measurements from the geostationary Meteosat-9 satellite radiative forcing of contrail-induced cirrus typically are Spinning Enhanced Visible and InfraRed Imager (SEVIRI) derived from satellite retrievals by considering the spatial instrument to track the CCC and other persistent contrails as correlation of the radiances in water vapor, infrared, and/or they advect and evolve. Although analysis of the evolution solar channels with aviation traffic routes and by applying of ice crystal effective radius and cirrus optical depth are suitable threshold criteria [e.g., Minnis et al., 1998, 2004; hampered by the presence of low-level stratocumulus and Mannstein and Schumann, 2005], but the difficulties in the advection over variable land surfaces, retrievals are distinguishing contrail-induced cirrus from natural cirrus are possible over ocean surfaces free from low-level cloud severe [Mannstein and Schumann, 2007]. (section 4). The advection of the contrail-induced cirrus [4] The uncertainty in estimates of the radiative forcing of over land means that instrumented meteorological observa- persistent linear contrails and the evolution into contrail- tion sites may also be used to determine the SW radiative induced cirrus means that observational case studies are forcing at the surface (section 5). The SW and the LW top of necessary to better understand their physical and radiative the atmosphere radiative forcings are estimated by compar- properties. Minnis et al. [1998] used geostationary satellite ing the SW and LW irradiances derived from the Met Office instruments to track distinctly shaped contrails evolving into high-resolution (4 km) UK4 operational NWP model (sec- cirrus in three separate events during April–May 1996 and tion 6).
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