Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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3.4. Contrail Occurrence and Persistence and Impact of Aircraft Exhaust on Cirrus
Other reports in this collection
Aircraft cause visible changes in the atmosphere by forming contrails that represent artificially induced cirrus clouds. The conditions under which contrails form are
discussed in Section 3.2.4. This section describes the formation, occurrence, and properties of persistent contrails and how they compare with natural cirrus.
3.4.1. Cirrus and Contrails
Cirrus clouds (Liou, 1986; Pruppacher and Klett, 1997) contain mainly ice crystals (Weickmann, 1945). The distinctive properties of cirrus and contrails derive from the
physics of ice formation. Ice particle nucleation occurs either through homogeneous nucleation (when pure water droplets or liquid aerosol particles freeze) or through
heterogeneous nucleation, when freezing of the liquid is triggered by a solid particle or surface that is in contact with the liquid or suspended within the liquid. Both
processes depend strongly on temperature and relative humidity (Heymsfield and Miloshevich, 1995; see Section 3.2.4.2).
Comparisons of model simulations with observations suggest that we may understand homogeneous nucleation (Ström et al., 1997; Jensen et al., 1998b) and are
making headway in understanding the potential contribution of heterogeneous nucleation (DeMott et al., 1998; Rogers et al., 1998). Each of these freezing
mechanisms requires that the atmosphere be highly supersaturated with respect to the vapor pressure of ice before crystals can form. For instance, supersaturations
in excess of 40-50% with respect to ice are needed for sulfuric acid particles to freeze homogeneously (Tabazadeh et al., 1997) at temperatures above 200 K.
Observations of relative humidity with respect to ice at the leading edges of wave clouds are consistent with the requirement of large ice supersaturations for
nucleation of ice on the bulk of the atmospheric aerosol (Heymsfield et al., 1998a; Jensen et al., 1998c). Hence, there is a large supersaturation range in which
heterogeneous nuclei could lead to cirrus formation before the bulk of the atmospheric particles freeze (DeMott et al., 1997). This potential for heterogeneous nuclei to
cause ice formation at ice supersaturations that are relatively low compared to those needed to freeze sulfate particles leads to concern about the role of aircraft
exhaust in modifying ambient clouds (Jensen and Toon, 1997).
Ice crystal number densities are limited by competition between increasing saturation as a
result of cooling in vertical updrafts and decreasing saturation as a result of growth of ice
crystals. The depletion of vapor as a result of growth of the first few ice crystals nucleated
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