Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
al., 1998a). Aircraft initiate contrail formation by increasing the humidity within their exhaust trails, whereas
local atmospheric conditions govern the subsequent evolution of contrail cirrus clouds. Indeed, the ice mass
in long-lasting contrails originates almost completely from ambient water vapor (Knollenberg, 1972).
Ice-supersaturated air masses are often formed when ice-saturated air masses are lifted by ambient air
motions. While the air lifted, it may remain cloud-free until it is cooled adiabatically to near-liquid saturation
(Ludlam, 1980). Other evidence for large supersaturation occurring in the upper troposphere is provided by
cirrus fallstreaks that grow while falling through supersaturated air layers (Ludlam, 1980) and by a few
localized humidity measurements (Brewer, 1946; Murphy et al., 1990; Ovarlez et al., 1997; Heymsfield et al.,
1998b). Recent humidity measurements by commercial aircraft show that-in flights between Europe, North
and South America, Africa, and Asia-14% of flight time was in air masses that were ice-supersaturated with a
mean value of 15% (Helten et al., 1998; Gierens et al., 1999).
3.4.2 Cirrus and Contrail Models
Small-scale, regional, and global models have been used to study cirrus clouds and contrails. Small-scale
models simulate the details of cloud formation in a single parcel of air along some defined trajectory (e.g.,
Jensen et al., 1994b). High-resolution 2-D models have simulated the dynamics and microphysics of cirrus
(Starr and Cox, 1985) and contrails (Gierens, 1996; Chlond, 1998). Regional and global models describe the
large-scale dynamics of clouds with simpler representations of microphysical cloud processes (Sundqvist,
1993; Lohmann and Roeckner, 1995; Fowler et al., 1996; Westphal et al., 1996). Regional-scale (typically 10km
horizontal grid-scale) and global-scale models are not able to resolve the vertical motions and small-scale
temperature and humidity fluctuations (Gierens et al., 1997) that drive supersaturations involved in cirrus
nucleation. However, global models with prescribed or parameterized contrail cover have been useful for
understanding the radiative impact of a change in cloud cover as a result of aircraft emissions (Ponater et al.,
1996).
3.4.3. Contrail Occurrence
At plume ages between 1 min and
1 h, contrails grow much faster
horizontally (to several km width)
than vertically (200 to 400 m),
especially in highly sheared
environments (Freudenthaler et al.,
1995, 1996; Sassen, 1997). Young
contrails spread as a result of
turbulence created by aircraft
vortices (Lewellen and Lewellen,
1996; Gerz et al., 1998; Jensen et
al., 1998a,b,c), shear in the
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Figure 3-13: Time series of GOES-8 satellite
images showing the evolution of a contrail from an
initial oval shape to extensive cirrus clouds (from
Minnis et al., 1998a).