Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Clearly, an important uncertainty in the evaluation of the effects of aircraft is the representation of transport in the 2-D and 3-D models used to simulate the effect of
supersonic aircraft emissions. The research community recognizes this general limitation, and an ongoing exercise (Models and Measurements II, Park et al., 1999)
should help to quantify some of the transport differences among the models. Models and Measurements II is an activity supported by NASA to test models through
detailed comparisons with measurements. It is a follow-on of the Models and Measurements workshop completed several years ago (Prather and Remsberg, 1993).
The results of Models and Measurements II should be available by the end of 1999.
New analysis techniques of models relying on "age of air" (Hall and Plumb, 1994), sulfur hexafluoride (SF6 ) (Elkins et al., 1996), and CO2 (Boering et al., 1996)
measurements allow more detailed comparisons to test the transport of models. Most models tend to underestimate the observed "age of air;" therefore, the derived
delta NOy and H2O for these models is probably a lower limit. As a result, the models will tend to underestimate O3 depletion from supersonic aircraft.
Another measure of transport uncertainty is illustrated by significant differences in mixing ratios calculated for NOy accumulation in the stratosphere from supersonic
aircraft emissions (Figure 4-6a). This uncertainty reflects differences in lower stratospheric residence times for the different models.
The atmosphere undergoes large variations from one year to the next. These interannual variations will drive a different transport of constituents, which will lead to
significant differences in the impact of supersonic emissions on O 3 . Jackman et al. (1991) simulated very different aircraft-induced O 3 changes using separate
dynamical fields; a slow (rapid) circulation simulation caused larger (smaller) O 3 decreases. The SLIMCAT model has been used with a yearly varying transport
recently and has computed total O 3 decreases that vary significantly in the same months of different years.
4.4.2.2. Model Resolution/Dimensionality
As noted above, the formidable computational resources required to conduct long-term multiyear 3-D stratospheric simulations for assessment has made it necessary
to use 2-D models. Although the 2-D models have very sophisticated formulations for gas phase chemical processes, their meridianal resolution is typically 5° (550 km)
or coarser in latitude and 1 km or more in altitude throughout the stratosphere. Intrinsically, this resolution limits their ability to treat LS transport processes. In addition,
because atmospheric species are treated in a zonally averaged manner, effects such as PSC formation and chemical processing must be parameterized more crudely
than in 3-D models.
Issues of spatial resolution and dimensionality of models are very pertinent to the treatment of PSCs and the processing of polar air. 2-D models do not adequately
parameterize the zonal variation of temperature, thus the formation of PSCs. Even 3-D models do not capture the different horizontal temperature scales within the
Arctic stratosphere. For example, even though synoptic conditions would suggest otherwise, low temperatures and PSCs may be induced by wave flow over
mountains (Carslaw et al., 1998); this mesoscale phenomenon is not directly addressed by most models to date. This uncertainty in assessing impacts from aircraft
could be an even more important issue in trying to assess future impacts when the stratosphere may be cooler because of enhanced CO 2 mixing ratios (Shindell et al.,
1998).
Yet another, quite different, resolution problem that may limit our ability to model chemical effects is that of filamentation processes, whereby winter polar air consists of
long but narrow (< 5 km) filaments of gas in which species densities are different. O 3 depletion studies performed at very high horizontal resolution for the winter Arctic
(Edouard et al., 1996) have underlined the uncertainty in calculating chemical loss of O 3 from differing resolutions.
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