Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
components. For thin cirrus, smaller particles (but > 3-mm radius) tend to cause stronger heating by increasing cloud albedo less strongly than emissivity (Fu and Liou,
1993).
3.6.3. Radiative Properties of Contrail Clouds
Contrails are radiatively important only if formed in ice-supersaturated air, where they may persist and spread to several-kilometer lateral widths and a few hundred
meters vertical depth (Detwiler and Pratt, 1984; Jäger et al., 1998) (see also Section 3.4.4). In contrast, shorter lived contrails have much smaller spatial and temporal
scales, hence contribute much less to the radiation budget (Ponater et al., 1996). The impact of contrails on transmission of radiation depends on their optical depth. In
the solar range (near a wavelength of 0.55 µm), the optical depth of observed persistent contrails varies typically between 0.1 and 0.5 (Kästner et al., 1993; Sassen,
1997; Jäger et al., 1998; Minnis et al., 1998a). Occasionally, very thick contrails (on the order of 700 m) with optical depths greater than 1.0 are found at higher
temperatures (up to -30°C) (Schumann and Wendling, 1990; Gayet et al., 1996). The optical depth in the 10-µm range is about half that near 0.55 µm (Duda and
Spinhirne, 1996). Particles in young persistent contrails are typically smaller (mean diameter 10 to 30 µm) than in other cirrus cloud types (greater than 30 µm)
(Brogniez et al., 1995; Gayet et al., 1996) but grow as the contrail ages and may approach the size of natural cirrus particles within a time scale on the order of 1 h
(see Section 3.4) (Minnis et al., 1998a). The number density of ice crystals in contrails (on the order of 10 to 200 cm-3 ) is much larger than in cirrus clouds (Sassen,
1997; Schröder et al., 1998b). Reported IWC values in aged contrails vary between 0.7 and 18 mg m-3 (Gayet et al., 1996; Schröder et al., 1998b), consistent with
results from numerical studies (Gierens, 1996). As for cirrus clouds (Heymsfield, 1993; Heymsfield et al., 1998b), the IWC of contrails is expected to depend on
ambient temperature (Meerkötter et al., 1999) because the amount of water mass available between liquid and ice saturation (for temperatures < -12°C) increases with
temperature (Ludlam, 1980). Hence, contrails may be optically thicker at lower altitudes and higher temperatures. Contrail particles have been found to contain soot
(see Section 3.2.3). Soot in or on ice particles may increase absorption of solar radiation by the ice particles, hence reduce the albedo of the contrails. The importance
of soot depends on the type of internal or external mixing and the volume fraction of soot enclosures. Because soot particles are typically less than 1 mm in diameter,
their impact on the optical properties of ice particles in aged contrails is likely to be small. Contrail particles often deviate from a spherical shape (see Section 3.4.4).
The magnitude and possibly even the sign of the mean net radiative forcing of contrails depends on the diurnal cycle of contrail cover. For the same contrail cover, the
net radiative forcing is larger at night. Satellite data reveal a day/night contrail cover ratio of about 2 to 3 (Bakan et al., 1994; Mannstein, 1997). Aviation fuel
consumption inventories suggest a longitude-dependent noon/midnight contrail cover ratio of 2.8 as a global mean value (Schmitt and Brunner, 1997).
Table 3-8: Shortwave (SW), longwave (LW), and net radiative flux changes (W m -2 ) at top of atmospherein global mean as caused by contrails in
1992 and 2050 scenario Fa1 (see Section 3.7.2 and Chapter 9), for contrail cover as shown in Figure 3-16 and for solar optical depths of 0.1, 0.3,
and 0.5 for contrails (Minnis et al., 1999). The last line gives the results for temperature-dependent ice water content (IWC) with variable optical
depth.
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