Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
m 2 g -1 in the solar range depending on relative humidity (Boucher and Anderson, 1995; Lacis and Mishchenko, 1995). Soot is a strong absorber of solar energy, with
mass extinction coefficients of about 10 m 2 g -1 (Pueschel et al., 1992; Penner, 1995; Petzold and Schröder, 1998). As a consequence of the product of given column
load and extinction efficiency, the maximum change in zonal-mean optical depth from aircraft soot and sulfate aerosol is less than 4 x 10 -3 . For comparison, the solar
optical depth of stratospheric aerosol varies typically between 0.005 and 0.15, depending on volcanic aerosol loading (Sato et al., 1993). Regionally, within the main
flight corridors, a particle concentration change of approximately 30 cm -3 at 0.1-µm mean diameter over a vertical layer of 2 km cannot be ruled out (Schumann et al.,
1996; Friedl, 1997; Schlager et al., 1997), implying a mass load on the order of 10 ng cm -2 and a solar optical depth of about 0.001. This regional change is small
compared with other regional variations. Tropospheric aerosol layers with optical depth of 0.1 to 0.5 occur frequently off the coasts of North America and Europe
(Russell et al., 1997).
3.6.2. Radiative Properties of Cirrus Clouds
Contrails are ice clouds with radiative effects similar to thin cirrus cloud layers (Liou, 1986; Raschke et al., 1998). At the TOA, thin layers of cirrus clouds or contrails
tend to enhance radiative forcing and hence the greenhouse effect because they cause only a small reduction of the downward solar flux but have relatively larger
impact on the upward terrestrial radiative flux. An increase in cloud cover by thin cirrus clouds may therefore cause an increase in the net energy gain of the planet
(Stephens and Webster, 1981; Fu and Liou, 1993). Contrails and cirrus clouds also have an impact on the radiative energy budget at the Earth's surface. Although
radiative forcing at the TOA is most important for long-term and global climate changes, forcing at the surface may have short-term regional consequences.
Longwave (LW) radiative forcing by cirrus or contrails is greatest when clear-sky radiative
flux to space is large (i.e., larger over warm than over cool surfaces, larger in a dry than in a
humid atmosphere) and cloud emissivity is large (Ebert and Curry, 1992; Fu and Liou,
1993). For thin clouds, emissivity increases with ice water path, which is the product of the
ice water content (IWC) of the cloud and its geometrical depth. The emissivity of ice
particles in a cirrus layer is much larger, in particular at 8 to 12 µm (King et al., 1992; Minnis
et al., 1998c), than that of the same amount of water in gaseous form. Hence, absorption
and emission from an atmospheric layer increase when ice particles form at the expense of
ambient water vapor (Detwiler, 1983; Meerkötter et al.,1999).
Shortwave (SW) radiative forcing of cirrus clouds is determined mainly by solar zenith angle,
surface albedo, and cloud optical depth (which increases with ice water path) (Ebert and
Curry, 1992; Platt, 1997). For fixed ice water path, clouds containing smaller particles have
larger optical depth and exhibit larger solar albedo (Twomey, 1977; Betancor-Gothe and
Grassl, 1993). Aspherical particles cause a larger albedo than spherical ones (Kinne and
Liou, 1989; Gayet et al., 1998). SW forcing is negative when the cloud causes an increase
of system albedo. In general, SW forcing has a greater magnitude over dark surfaces than
over bright surfaces.
The net radiative forcing of clouds is the sum of SW and LW flux changes and may be
positive or negative. Thin cirrus clouds cause a small but positive radiative forcing at the
TOA; thick cirrus clouds may cause cooling (Stephens and Webster, 1981; Fu and Liou,
1993). In the global mean, an increase in cirrus cloud cover warms the Earth's surface
Figure 3-20: Shortwave (SW), longwave (LW), and net instantaneous
radiative flux change from contrails with 100% cover under midlatitude
summer conditions averaged over a day as a function of
optical depth (0.55 µm): Near the top of the atmosphere (50 km) and
surface (0 km) (from Meerkötter et al., 1999).
(Hansen et al., 1997). Maximum heating is reached at intermediate ice water path, corresponding to an optical depth of about 2 to 3, and the effect shifts to small
cooling for optical depths greater than about 10 to 20 (Platt, 1981; Jensen et al., 1994a). Net forcing also varies with particle size but less than its two spectral
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