Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Radiative Forcing
1992
Best Estimate
or Range
Uncertainty Range
with 2/3 Probability
Line-shaped contrail cirrus 0.02 W m -2 0.005-0.06 W m -2
Additional aviation-induced cirrus clouds
0-0.04 W m -2
Status of
Understanding
fair
- very poor
Other indirect cloud effects - either sign, unknown magnitude very poor
2050
Line-shaped contrail cirrus 0.10 W m -2 0.03-0.4 W m -2
Additional aviation-induced cirrus clouds
0-0.16 W m -2
fair
- very poor
Other indirect cloud effects - either sign, unknown range very poor
In all cases, TOA and top of the troposphere radiative flux changes from contrails differ by only about 10%; therefore, the instantaneous or static flux change gives a
reasonable approximation for the adjusted radiative forcing as considered in IPCC (1996).
At the Earth's surface, LW flux changes are much smaller because water vapor closes much of the infrared radiation window in the lower atmosphere. In contrast, SW
flux changes are only a little smaller than at TOA. Hence, the daytime SW contribution dominates and cools the surface in the daily mean. A reduction of solar radiation
by 40 W m -2 has been measured locally in the shadow of contrails, although the simultaneous change in infrared flux in the shadow of contrails was very small
(Sassen, 1997). Hence, the Earth's surface is locally cooled in the shadow of contrails. This analysis does not exclude warming of the entire atmosphere-surface
system driven by the net flux change at TOA. As radiation-convection models show, for example, vertical heat exchange in the atmosphere may cause a warming of
the surface even when it receives less energy by radiation (Strauss et al., 1997). Cooling of the atmosphere below contrails is also suggested by measurements of
solar and infrared upward and downward fluxes above and below a few persistent contrails (Kuhn, 1970). These measurements show a strong (10-20%) reduction of
net downward radiation just below approximately 500-m-thick contrails with little change in LW fluxes. Further investigation is required to demonstrate how these
results depend on the geometry and age of contrails.
Contrail cirrus induce a heat source by the change in divergence of solar and infrared radiation fluxes mainly within but also below the contrail in the upper troposphere
(Liou et al., 1990; Strauss et al., 1997; Meerkötter et al., 1999). In the atmosphere below a contrail, the change in heat source is on the order of 0.3 K/day for 100%
cover. When the contrail is located above a thick lower level cloud, the atmosphere is heated only above the lower cloud; the heat source is essentially zero below the
lower cloud. In the diurnal cycle, radiative forcing by contrails is positive and strongest during the night because of the absence of negative SW forcing (see Table 3-6).
For small optical depth, net forcing at TOA is also positive during the day, hence always positive regardless of the diurnal cycle of contrail cover. Negative SW forcing
is maximum not at noon but during morning or afternoon hours, when the solar zenith angle is near 70°. Other than at TOA, the net flux change is negative at the
surface even for thin contrails during the day, in particular at intermediate zenith angles. The maximum day-night difference in net radiative forcing at the surface for
cloudless summer mid-latitude conditions is about 20 W m -2 for constant 100% contrail cover with optical depth of 1 and constant surface temperature, and slightly less
when accounting for the daily temperature cycle.
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