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On the Life Cycle of Individual Contrails and Contrail Cirrus

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  • 1 Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
  • | 2 National Center for Atmospheric Research, Boulder, Colorado
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Abstract

The life cycle of individual (initially line shaped) contrails behind aircraft and of contrail cirrus (aged contrails mixed with other ice clouds) is described. The full contrail life cycle is covered, from ice formation for given water, heat, and particulate emissions; to changes in the jet, wake, and dispersion phases; through final sublimation or sedimentation. Contrail properties are deduced from various in situ, remote sensing, and model studies. Aerodynamically induced contrails and distrails are explained briefly. Contrails form both in clear air and inside cirrus. Young contrails consume most of the ambient ice supersaturation. Optical properties of contrails are age and humidity dependent. Contrail occurrence and radiative forcing depends on the ambient Earth–atmosphere conditions. Contrail cirrus seems to be optically thicker than assessed previously and may not only increase cirrus coverage but also thicken existing cirrus. Some observational constraints for contrail cirrus occurrence and radiative forcing are derived. Key parameters controlling contrail properties—besides aircraft and fuel properties, ambient pressure, temperature, and humidity—are the number of ice particles per flight distance surviving the wake vortex phase, the contrail depth, and particle sedimentation, wind shear, turbulence, and vertical motions controlling contrail dispersion. The climate impact of contrails depends among other things on the ratio of shortwave to longwave radiative forcing (RF) and on the efficacy with which contrail RF contributes to surface warming. Several open issues are identified, including renucleation from residuals of sublimated contrail ice particles.

Denotes content that is immediately available upon publication as open access.

© 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author e-mail: Ulrich Schumann, ulrich.schumann@dlr.de

Abstract

The life cycle of individual (initially line shaped) contrails behind aircraft and of contrail cirrus (aged contrails mixed with other ice clouds) is described. The full contrail life cycle is covered, from ice formation for given water, heat, and particulate emissions; to changes in the jet, wake, and dispersion phases; through final sublimation or sedimentation. Contrail properties are deduced from various in situ, remote sensing, and model studies. Aerodynamically induced contrails and distrails are explained briefly. Contrails form both in clear air and inside cirrus. Young contrails consume most of the ambient ice supersaturation. Optical properties of contrails are age and humidity dependent. Contrail occurrence and radiative forcing depends on the ambient Earth–atmosphere conditions. Contrail cirrus seems to be optically thicker than assessed previously and may not only increase cirrus coverage but also thicken existing cirrus. Some observational constraints for contrail cirrus occurrence and radiative forcing are derived. Key parameters controlling contrail properties—besides aircraft and fuel properties, ambient pressure, temperature, and humidity—are the number of ice particles per flight distance surviving the wake vortex phase, the contrail depth, and particle sedimentation, wind shear, turbulence, and vertical motions controlling contrail dispersion. The climate impact of contrails depends among other things on the ratio of shortwave to longwave radiative forcing (RF) and on the efficacy with which contrail RF contributes to surface warming. Several open issues are identified, including renucleation from residuals of sublimated contrail ice particles.

Denotes content that is immediately available upon publication as open access.

© 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author e-mail: Ulrich Schumann, ulrich.schumann@dlr.de

1. Introduction

Contrails (condensation trails) form behind aircraft as line-shaped cirrus clouds, with high concentrations of small ice particles compared to other cirrus (see Fig. 3-1). Contrails may form as “exhaust contrails” from water and particles emitted by the aircraft engines (Schumann 1996) or as “aerodynamic contrails” forming because of adiabatic cooling near curved surfaces of the aircraft (Gierens et al. 2009; Kärcher et al. 2009). Distrails (dissipation trails) and aircraft-induced cloud holes may also form (Heymsfield et al. 2011). Contrails are mostly short-lived but may persist for many hours when forming in ice-supersaturated air (Minnis et al. 1998). Individual contrails deform with time, often merge with other contrails and cirrus, and eventually form “contrail cirrus” (Schumann 2002). Contrail cirrus can be distinguished from other cirrus only when traced back to the formation process (Graf et al. 2012). Early studies discussed the visibility and detection aspects of contrails (aufm Kampe 1943; Brewer 1946; Appleman 1953; Ryan et al. 2011), and these studies contributed to the detection of ice supersaturation, contrail persistence, the dryness of the stratosphere, the Brewer–Dobson circulation (Brewer 2000), and hollow ice particles (Weickmann 1945). The climate impact got more attention later (Penner et al. 1999). The mean radiative forcing (RF) from contrails is likely positive, possibly contributing to global warming (Boucher et al. 2013). In contrast to many other climate effects discussed for aviation, contrail cirrus is observable (see Fig. 3-1f). This review summarizes the present understanding of contrail cirrus and identifies some open questions. The insight gained may be of relevance for cirrus research, contrail climate assessment, and mitigation (including optimized aircraft and engines, changed routing, and alternative fuels), not discussed here (Fuglestvedt et al. 2010; Lee et al. 2010; Grewe et al. 2014a; Brasseur et al. 2016). The paper describes first the formation and properties of “individual contrails.” This term is used instead of “line-shaped contrails” because it is not clear when a line-shaped contrail ends. The second part describes contrail cirrus.

Fig. 3-1.
Fig. 3-1.

Contrail types. (a) Exhaust contrail (photo by Josef P. Williams; Unterstrasser et al. 2012). (b) Aerodynamic contrail (photo by Dieter Klatt; Gierens et al. 2011). (c) Aircraft-induced lines and holes in supercooled liquid clouds (cloud-top temperatures −35° to −25°C); section of image with blue border lines, near northwest corner of Texas (29 Jan 2007, NASA, Jeff Schmaltz, MODIS Rapid Response Team). (d) Contrail visible shortly behind B747-400 engines, 38 000 ft, −61°C, 28 May 2004; photo by Robert Falk. (e) “Soot cirrus” observed at DLR, Oberpfaffenhofen, 0905 UTC 3 Nov 2013. (f) Persistent contrails west of lake Ammersee, Germany, photo by C. Koenig, DLR, 23 Jun 2002. (g) Persistent contrails and contrail cirrus, a false-color NOAA-12 AVHRR image, 5 Apr 1995, processed by DLR.

Citation: Meteorological Monographs 58, 1; 10.1175/AMSMONOGRAPHS-D-16-0005.1

2. Individual contrails

a. Mean contrail properties

Since contrails start as line-shaped clouds, unlike most other cirrus clouds, special integral properties are used. Just as other cirrus particles, each contrail particle has an individual mass mi, projected particle cross-section area Ai, habit, and orientation (see the appendix for a list of symbols). These properties plus the phase, composition, temperature, and wavelength determine the optical extinction efficiency Qext,i and other microphysical and optical properties of each particle (Hansen and Travis 1974). Integral properties peculiar to contrails are the geometrical cross-section area Ac, and the total crystal number Nice, total extinction EA, and total ice mass I per flight distance:
e3.1
e3.2
e3.3

Here, the local and the volume-mean ice particle concentrations ni and nice, extinction ε, and ice water content IWC occur, with mean extinction efficiency Qext and ice-bulk density ρice, besides optical depth τ and contrail width W. The summation sums over all ice particles in the contrail cross section. The above equations also define the volume and area-mean radius of the contrail particles, rvol and rarea. By tradition, the definitions are set up so they equal the geometrical radius in the idealized case of monodisperse spherical particles (Hansen and Travis 1974). The effective radius reff is defined by the ratio of mean particle volume to cross-section area, reff = 3IQext/(4ρiceEA). Again the factor (¾) enters for consistency with a hypothetical sphere radius. By definition, rvol/reff = . The ratio C = rvol/reff depends on ice particles sizes and habits and needs to be determined empirically; C ≤ 1 if the ice particles were spherical. Measured data collected for contrails show that C ≈ 0.7 ± 0.3 varies during the lifetime of the contrails and depends on ambient humidity (Schumann et al. 2011).

The aircraft wake evolution is traditionally divided into the jet, vortex, and dispersion regimes (Hoshizaki et al. 1975). After a roll-up phase (Misaka et al. 2015) overlapping with the jet regime, the vortex regime exhibits two phases: a first phase of a coherent counterrotating vortex pair and a second phase with rapid hydrodynamic vortex breakup and subsequent turbulent dissipation (Gerz and Holzäpfel 1999; Paoli and Shariff 2016).

b. Formation of exhaust contrails

1) Formation conditions

Exhaust contrails form during the jet phase because of engine emissions of water and particles [mainly soot, i.e., impure carbon particles resulting from incomplete combustion of hydrocarbon fuels (Bond et al. 2013; Petzold et al. 2013)] acting as cloud condensation nuclei (CCN). The emissions depend on fuel mass flow mF per flight distance (all-engine contribution) and on emission indices (EI) for exhaust species [gaseous or particulate (PEI); mass and number of emitted species per mass of fuel burned]. A fraction η = F/(QmF) of the specific combustion energy Q is used to propel the aircraft (depending on mF and thrust F), so that the fraction (1 − η) of Q appears in the young exhaust jet plume, partly in form of kinetic energy of the jets (Schumann 1996). The remainder heats the aircraft wake later, when all motions induced by the aircraft are dissipated. The term η is known as “overall propulsion efficiency” in engine technology (Penner et al. 1999). Contrails form when the exhaust gases exceed liquid saturation at least for a short time. Humidity (measured by partial vapor pressure) and heat (measured by temperature) in the exhaust plume decrease during mixing of the warm and humid exhaust gases with cool ambient air at about the same rate, along a straight mixing line in Fig. 3-2a. Relative humidity is the ratio of the partial pressure of water vapor in the exhaust relative to the saturation vapor pressure for given temperature. From the Clausius–Clapeyron equation, the saturation pressure follows a curve in this figure. Therefore, relative humidity is higher inside the plume than at engine exit and in ambient air. Maximum potential supersaturation is reached when the plume temperature reaches liquid maximum (LM) TLM. The plume humidity exceeds liquid saturation at least briefly when the ambient air temperature T is below a threshold temperature [liquid critical (LC)] TLC. Temperature TLC depends on ambient relative humidity RH (for liquid saturation) and a parameter G,
e3.4
with saturation pressure psat(T) over liquid water. From these equations, TLM(G) and TLC(G, RH) can be determined by Newton iteration or from approximate solutions (Schumann 2012). The parameter G covers the dependency of the threshold on ambient pressure p, and EIH2O, Q, and η, defined above, and the specific heat capacity cp of air, and molar masses MH2O and Mair of water and air. This Schmidt–Appleman criterion (SAC; Schumann 1996) does not consider phase changes and the process of ice production. It assumes quick conversion of kinetic energy from the engine jet into internal energy and simultaneous mixing of heat and water vapor between the exhaust jet and cloud-free ambient air. Within the accuracy of temperature and humidity measurements, the SAC has been verified experimentally (Busen and Schumann 1995; Schumann et al. 1996; Jensen et al. 1998b; Schumann et al. 2000). The measurements confirmed earlier findings that ice saturation is insufficient for formation of visible contrails. Figures 3-2b–f show probabilities of contrail formation properties for the global air traffic of 2006 and meteorology from the European Centre for Medium-Range Weather Forecasts (ECMWF). The plots show the frequency of flight conditions (p, T, η, RHi), in relation to threshold conditions. Potentially, without phase changes, 90% of the contrails experience liquid supersaturation (RHLM − 1) higher than 5% (up to 230% for low T), mainly near plume temperatures of 230 K for ambient temperatures near 220 K.
Fig. 3-2.