Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
usually indistinguishable from natural cirrus; hence, satellite detection algorithms based on
the linear structures of young contrails will not detect these dispersed contrails.
At present, observations of actual frequency or areal coverage of Earth by contrails are
limited to a few selected regions. Contrail frequency refers to the probability that a contrail
will be observed somewhere within the scene being viewed; area of coverage refers to the
fraction of the area of the scene in which contrails are observed. Estimates of contrail
coverage or occurrence have been made directly or indirectly from surface (Detwiler and
Pratt, 1984; Minnis et al., 1997) and satellite observations (Joseph et al., 1975; Carleton and
Lamb, 1986; Lee, 1989; DeGrand et al., 1990; Schumann and Wendling, 1990; Betancor-
Gothe and Grassl, 1993; Bakan et al., 1994; Mannstein et al., 1999).
Data for the frequency of contrails are available from surface-based observations at 19
locations across the continental United States of America for every hour during 1993-94
(Minnis et al., 1997). The data indicate that contrail frequency peaks around February/March
and is at a minimum during July. Annual mean persistent contrail frequency (not the cover)
for the 19 sites was 12%. When related to fuel use and extended to the remainder of the
country, mean annual contrail frequency for the United States of America is estimated at 9%
(Minnis et al., 1997). The relationship between fuel consumption and contrail frequency from
this data set is shown in Figure 3-14. The correlation implies that contrail coverage is limited
mainly by the number of aircraft flights, not by atmospheric conditions at cruise altitudes.
Pilots flying over the former Soviet Union have reported that contrails occur most frequently
in winter and spring and less often during summer (Mazin, 1996). Sky photographs taken
from 1986 to 1996 over Salt Lake City, Utah, reveal a seasonal cycle in contrail frequency,
Figure 3-15: Annual mean corrected contrail coverage at noon over
mid-Europe in 1996, as derived from AVHRR data from the NOAA-14
satellite (from Mannstein et al., 1999).
with a maximum in fall and winter and a minimum in July (Sassen, 1997). These data are similar to observations from a site 64 km north (Minnis et al., 1997) of Salt
Lake City. Sassen (1997) and Minnis et al. (1997) found that contrails occur with cirrus in approximately 80% of observations. The coincidence of cirrus and contrails
suggests that cloud-free supersaturated regions are usually interspersed with areas in which natural clouds have actually formed.
From an analysis of AVHRR infrared images taken over the northeast Atlantic and Europe,
a mean contrail cover of 0.5% was derived for 1979-81 and 1989-92 (Bakan et al., 1994). A
distinct seasonal cycle was found, with a southward displacement of the maximum cover
during winter. Maximum coverage (~2%) occurred during summer, centered along the North
Atlantic air routes. Using AVHRR channel 4 and 5 differences and a pattern-recognition
algorithm to differentiate line-shaped clouds from fuzzy cirrus clouds, the contrail coverdefined
as line-shaped clouds over mid-Europe-was systematically evaluated for all of 1996
using nearly all noon passages of the NOAA-14 satellite (see Figure 3-15) (Mannstein et al.,
1999). Contrails are not uniformly distributed; instead, they lie along air traffic corridors and
accumulate near upper air route crossings. As with observations over the United States and
the former Soviet Union, maximum coverage occurred during winter and spring. At noon on
average over the year, line-shaped contrails cover about 0.5% of the area of the mid-
European region shown in this figure. This coverage represents the lower bound for the
actual contrail cover because the algorithm cannot identify non-line-shaped contrails.
Contrail coverage at night over Europe is one-third of the noon-time contrail cover
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