Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
(Mannstein et al., 1999).
Contrails often occur in clusters within regions that are cold and humid enough to allow
persistent contrails to form. Contrail clusters observed in satellite data indicate that these air
masses cover 10 to 20% of the area over mid-Europe (Mannstein et al., 1999) and parts of
the United States of America (Carleton and Lamb, 1986; Travis and Changnon, 1997),
consistent with the fraction of air masses expected to be ice-supersaturated at cruise
altitudes. Hence, as air traffic increases in these regions, persistent contrail coverage is also
expected to increase, possibly up to a limit of 10-20%.
Estimates of global coverage by air masses that are sufficiently cold and humid for
persistent contrails can be obtained using meteorological analysis data of temperature and
humidity, a model to estimate the frequency of ice-saturation in each grid cell as a function
of the analyzed relative humidity, and the Schmidt-Appleman criterion (depending on h; see
Section 3.2.4.1) for contrail formation conditions. Using 11 years of meteorological data from
the European Center for Medium Range Weather Forecast (ECMWF), the cover of suitable
air masses is found to be largest in the upper troposphere, especially in the tropics (global
mean value of 16%) (Sausen et al., 1998). This coverage is similar in size to the area
covered by clusters of contrails in satellite data and the frequency of ice-supersaturated air
masses observed along commercial aircraft routes (Gierens et al., 1999). The expected
contrail cover is then computed from the product of the air mass coverage and the fuel
consumption rate in the same region, with the latter chosen as one of several possible
measures of air traffic. The product is scaled to give a 0.5% mean contrail cover in the
European/Atlantic region (30°W to 30°E, 35 to 75°N) as considered by Bakan et al. (1994).
The resulting contrail cover (see Figure 3-16) for 1992 fuel emissions and h = 0.3 is 0.087%
Figure 3-16: Persistent contrail coverage (in % area cover) for the
1992 aviation fleet, assuming linear dependence on fuel consumption
and overall efficiency of propulsion h of 0.3. The global mean cover is
0.1% (from Sausen et al., 1998).
Figure 3-17: Time series of ice crystal concentration (Ncvi, thick line)
and concentration of light-absorbing material contained in ice crystals
(d, shaded area) for a flight through a cirrus layer in a region with
heavy air traffic (adapted from Ström and Ohlsson, 1998).
(about 0.1%) in the global mean, with a local maximum of 5% over the eastern United States of America. The computed contrail coverage over southeastern Asia is
only slightly smaller than that over Europe and North America. Although air traffic is much less extensive over southeastern Asia, this high contrail coverage may result
because this region more often has ice-supersaturated air. However, the predicted contrail coverage over North America and southeastern Asia has not yet been
verified by observations. Predicted regional differences are sensitive to the representation of the number of aircraft in operation. For example, in some regions, a large
fuel consumption density is caused by a few large aircraft (Gierens et al., 1998). Studies have not been performed to evaluate the accuracy of humidity values provided
by meteorological data. The results depend linearly on scaling by the cover observed in the reference region, as provided here by Bakan et al. (1994). Other data imply
global cover values between 0.02 and 0.1% (Gierens et al., 1998). A value larger than 0.1% (possibly 0.2%) cannot be excluded because the analysis uses only
thermodynamic contrail formation conditions and the scaling is based solely on observed line-shaped contrail cover.
3.4.4. Contrail Properties
The relatively small particles present in newly formed contrails serve to distinguish contrail radiative properties from those of most natural cirrus (Grassl, 1970;
Ackerman et al., 1998; see also Section 3.6). Measurements of contrail particles with impactors and optical probes (see also Section 3.2.4 and Table 3-1) reveal a
wide variety of size, shape, and spectral size distributions. The results depend on plume age, ambient humidity, ambient aerosol, and other parameters. At a plume
age of 30 to 70 s, ice particles have been found to form a single-mode log-normal size distribution with a volume-equivalent radius in the range of 0.02 to 10 µm, a
mean radius of about 2 µm, and maximum dimension of 22 µm. Particle shapes are mainly hexagonal plates, along with columns and triangles. The axial ratios of the
columns were found to be less than 2, and the shapes of the crystals were already established for particles of about 1-µm radius (Goodman et al., 1998). Contrail
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