Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
prevents further ice nucleation on the remaining particles (Jensen and Toon, 1994). Once
ice crystals form and take up available water vapor, supersaturation declines and further
nucleation of ice ceases. This selectivity causes ice crystals in cirrus to be larger relative to
droplets in liquid water-containing clouds-apart from differences in saturation vapor
pressures over ice compared to water, which causes more water vapor to be available for
deposition on ice particles than on water droplets. Large ice particles may precipitate
rapidly. Many cirrus clouds have a "fuzzy" appearance because rapid precipitation causes
optically thin edges of clouds to be diffuse, and precipitation allows particles to spread in the
wind, forming long tails of cloud. Ice crystal nucleation also depends on available aerosol in
the upper troposphere, the properties of which are only poorly known (Ström and
Heintzenberg, 1994; Podzimek et al., 1995; Sassen et al., 1995; Schröder and Ström,
1997). In some locations, upper tropospheric particles are dominated by sulfates (Yamato
and Ono, 1989; Sheridan et al., 1994). However, more recent data show that minerals,
organic compounds, metals, and other substances may often be present in significant
quantities (Chen et al., 1998; Talbot et al., 1998).
Cirrus clouds occur mainly in the upper troposphere. The mean tropopause altitude is about
16 km in the tropics and 10 km (250 hPa) north of 45°N latitude (Hoinka, 1998). The
tropopause temperature at northern mid-latitudes varies typically between -40 and -65°C; it
may reach below -80°C in the tropics. At mid-latitudes, the upper troposphere often is humid
enough for cirrus and persistent contrails to form. The stratosphere is commonly so dry that
cirrus (PSCs) and persistent contrails form only in polar winter (for T > -60°C and p < 250
hPa, a 10-ppmv H 2 O mixing ratio corresponds to less than 15% relative humidity). Ground-
based observers report a mean cirrus cover of 13% over oceans and 23% over land
(Warren et al., 1986, 1988). Satellite data (Wang et al., 1996; Wylie and Menzel, 1999)
identify larger cloud cover (~40%) by subvisible (optical depth t at 0.55 mm below ~0.03)
and semi-transparent (0.1 < t < 0.6) cirrus clouds. Cirrus clouds are typically 1.5 (0.1 to 4)
km thick; are centered at 9 (4 to 18) km altitude; have a crystal concentration of 30 (10 -4 to
10 4 ) L-1, with crystal lengths of 250 (1 to 8000) µm (see Dowling and Radke, 1990); and
have an optical depth at 0.55 µm of about 0.3 (0.01 to 30) (Wylie and Menzel, 1999). These
typical values of cirrus crystal concentration and mean particle size may be biased by early
studies that failed to make adequate measurements of ice crystals smaller than about 50
µm. More recent studies suggest that crystal concentrations in cirrus are often on the order
of 1 cm -3 (Ström et al., 1997; Schröder et al., 1998b), although crystal concentrations in
wave clouds can exceed 10 cm -3 . PSCs (between the tropopause and ~25-km altitude) and
Figure 3-12: Contrails over central Europe on 0943 UTC 4 May
1995, based on NOAA-12 AVHRR satellite data (from Mannstein,
1997).
noctilucent clouds (~80-km altitude) contain ice and might therefore be included as very high altitude forms of cirrus clouds. Systems of cirrus clouds have a lifetime
that may reach several hours or even days (Ludlam, 1980), but the lifetimes of particles within clouds is much shorter.
Persistent contrail formation requires air that is ice-supersaturated (Brewer, 1946). Ice-supersaturated air is
often free of visible clouds (Sassen, 1997) because the supersaturation is too small for ice particle nucleation
to occur (Heymsfield et al., 1998b). Supersaturated regions are expected to be quite common in the upper
troposphere (Ludlam, 1980). The presence of persistent contrails demonstrates that the upper troposphere
contains air that is ice-supersaturated but will not form clouds unless initiated by aircraft exhaust (Jensen et
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