Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
1992 2050 (Scenario Fa1)
Optical Depth at 0.55 µm SW LW Net SW LW Net
0.1 -0.0030 0.0111 0.0081 -0.018 0.067 0.049
0.3 -0.0081 0.0246 0.0165 -0.049 0.148 0.099
0.5 -0.0124 0.0327 0.0203 -0.075 0.197 0.122
Variable (from temperature-dependent IWC) -0.0038 0.0135 0.0097 -0.024 0.084 0.060
a) Cloud amount, cloud distribution, and cloud optical depth are 1986 ISCCP 3-h data interpolated to 1 h, for 4 months (January, April, June,
October), with mean cloud cover of 68% of the Earth surface.Ice particles are modeled as hexagons with mean diameters of 20 µm. Water clouds
consist of droplets with 60-µm mean diameter. Surface skin temperature and surface albedo are taken from ISCCP and Staylor and Wilber (1990).
Winter and standard temperature and moisture profiles are assumed for pressures < 50 hPa, and numerical weather analysis monthly mean profiles
of National Meteorological Center (NMC; now National Center for Environmental Protection, NCEP) are used for pressures > 50 hPa. Continental and
marine aerosols were also included in the model. Contrails are assigned at a pressure
of 200 hPa and assumed tobe 220-m thick with aspherical ice particles of 24-µm volume mean diameter.IWC in contrails was adjusted to result in
anoptical depth of 0.1, 0.3, or 0.5. A case with IWC set to half the amount of water available for ice formation from vapor at 100% humidity relative to
liquid saturation (temperature-dependent IWC) is also considered (variable optical depth).Net TOA forcing is computed with model FL as difference of
results with and without contrails.
3.6.4. Radiative Forcing of Line-Shaped Contrail Cirrus
Model studies indicate the importance of contrails in changing the Earth's radiation budget. One-dimensional (1-D) models represent contrails as plane-parallel clouds
in a homogeneously layered atmosphere and use area-weighted sums for fractional contrail cover (Fortuin et al., 1995; Strauss et al., 1997; Meerkötter et al., 1999).
As a consequence, contrail forcing grows linearly with contrail cover in these models. Inhomogeneity effects may be large in natural cirrus (Kinne et al., 1997) and
small for vertically thin contrail clouds (Schulz, 1998) but may be important for thick and narrow contrails. Computations of radiative forcing by contrails have been
done for fixed atmospheric temperatures in the North Atlantic flight corridor (Fortuin et al., 1995). Normalized to 100% contrail cover (as provided by 1-D models),
these computations found a net forcing in the range of -30 to +60 W m -2 in summer and 10 to 60 W m -2 in winter. The negative forcing values apply to clouds that are
much thicker than typical contrails. A radiative convective model used to simulate the climatic conditions of a mid-European region (Strauss et al., 1997) found a
radiative flux change of almost 30 W m -2 at the tropopause for 100% contrail cover with 0.55-µm optical depth of 0.28, and a surface temperature increase on the order
of 0.05 K for a 0.5% increase in current contrail cloud cover. With a 2-D radiative convective model, a 1 K increase was found in surface temperature over most of the
Northern Hemisphere for an additional cirrus cover of 5% (Liou et al., 1990). The potential effects of contrails on global climate were simulated with a GCM that
introduced additional cirrus cover with the same optical properties as natural cirrus in air traffic regions with large fuel consumption (Ponater et al., 1996). The induced
temperature change was more than 1 K at the Earth's surface in Northern mid-latitudes for 5% additional cirrus cloud cover in the main traffic regions. Meerkötter et al.
(1998) applied three established radiation transfer models to compute the static radiative forcing due to a prescribed additional cloud cover by contrails. These models,
which assume plane parallel cloud cover, are the two- and four-stream models of Fu and Liou (1993) (FL); the matrix operator method of Plass et al. (1973), also used
by Strauss et al. (1997) (M); and the four-stream model of Nakajima and Tanaka (1986, 1988) (N). The FL and M models have participated in model comparison
exercises (Ellingson and Fouquart, 1990). Table 3-6 on the previous page shows the results of a parameter study that was carried out for various regions and seasons
using models M and FL for spherical and hexagonal particles. The contrails cause a net forcing that is positive in all cases after summing over negative SW and
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