Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
positive
LW flux changes. The magnitude of SW forcing is larger over dark ocean than over bright
snow surfaces and larger for hexagonal ice particles than for spheres. LW forcing is larger in
the tropics than in polar regions. Despite the small ocean albedo, net forcing is largest in the
tropics because it has the warmest lower atmosphere. These results are consistent with the
expectation that net forcing is smallest over cool and dark surfaces. Net forcing over the midlatitude
continent is larger in summer than in winter. Hexagonal particles cause a larger
albedo than spherical ones, therefore less net forcing. These results show that contrail
heating generally prevails over cooling in the atmosphere-surface system. However, this
finding does not preclude situations in which contrails cause a net cooling-for example, very
cold surface, high atmospheric humidity, low surface albedo, very small particles (< 10 µm;
the limit depends on the ice water path in the cloud), or large optical contrail depth (> 10).
TOA radiative forcing depends mainly on the cover and on parameters that determine the
solar optical depth of contrails and to a minor degree on other parameters. Forcing is very
weakly sensitive to the methods used for radiative transfer calculations. This sensitivity can
be seen from Table 3-7, which lists the results of a parameter sensitivity study with models
FL, M, and N (Meerkötter et al., 1999). Except for the first two rows of Table 3-7, results are
from model N only because the results of the three models agree with each other to within
3%. The largest model differences are found for aspherical particles, which are represented
by different shapes in various spectral regions in the models, but all models show smaller
net forcing for aspherical particles than for spherical ones, as expected (Kinne and Liou,
1989). The results also depend rather strongly on solar zenith angle, ice water content,
particle diameter, surface albedo, and relative humidity. Low-level clouds with large cover
and optical depth reduce SW and LW forcing of the cloud-free case below a contrail,
causing a small net increase of forcing. Variations in surface temperature (here ± 5 K) cause
Figure 3-21: Global distribution of net instantaneous radiative forcing
at the top of atmosphere in daily and annual average for present
(1992) climatic conditions and analyzed contrail cover (see Figure 3-
16) and 0.55-µm optical depth of 0.3 (Minnis et al., 1999).
small LW flux changes. Lowering the altitude of a contrail for fixed IWC reduces the LW effect slightly. Lowering the altitude of the contrail and using the IWC that is
expected for the higher temperature at lower levels makes the lower contrail optically thicker and radiatively more effective than the higher contrail. Hence, TOA
radiative forcing by contrails grows with increasing surface temperature, surface albedo, and IWC. For the same IWC, contrails with small ice particles are more
effective in radiative forcing than contrails with larger particles. Representative forcing values are approximately 25 to 40 W m -2 for 100% contrail cover and 0.55-µm
optical depth of 0.5. Radiative forcing by contrails depends strongly on the optical depth of the contrails and is different at the surface than at the TOA. Figure 3-20
shows computed SW, LW, and net change in radiative fluxes at the TOA (actually 50 km) and at the surface for 100% contrail cloud cover for the mid-latitude summer
continental reference case with spherical ice particles. The ice water content was varied to yield different values of the 0.55-µm optical depth t. The trends are
consistent with those found in cirrus studies (Fu and Liou, 1993). At the TOA, LW forcing is larger than SW forcing, giving a net heating of the atmosphere that is
maximum near t = 3. The flux increases slightly less than linearly with optical depth for small values of t. The net forcing changes sign and becomes negative for t > 10
(not shown), but contrails are probably never that optically thick. This analysis indicates that contrails heat the atmosphere below them.
Table 3-9: Global radiative forcing by contrails and indirect cloud effects in 1992 and 2050 (scenario Fa1). No entry indicates insufficient information for bestestimate
value.
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