Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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4.4. Uncertainties in Model Results
Other reports in this collection
It is not clear how we can properly represent the true uncertainty associated with atmospheric effects; such an assessment represents an unattainable goal for the
present. The model results presented in this chapter have been calculated using state-of-the-art atmospheric models from many groups. As a result of their different
treatment of physical, dynamic, and chemical processes, there is a considerable range in their computed impacts. We have attempted to bracket the results by using a
combination of the range of different results between models for the same scenarios and results from sensitivity tests of parameters judged to be important.
Many factors contribute to the uncertainty of model calculations for present and future descriptions of the atmosphere. These factors include deficiencies in the
representation of transport processes (e.g., stratosphere/troposphere exchange and tropical and polar region barriers); model resolution; the dimensionality of the
models (i.e., 2-D or 3-D); deficiencies in chemical and physiochemical processes; and the state of the future atmosphere. Many of these issues cannot be considered
in isolation. For example, the spatial resolution of a model may determine how well it is able to capture the meteorological details required for an accurate simulation of
the dispersion of plumes and the formation of PSCs. Furthermore, as noted earlier, the models we used for our calculations can be considered to be of two types:
Those that have been formulated for a detailed simulation of the troposphere, and those designed to simulate primarily stratospheric processes. This use of two model
types to attack a problem whose resolution requires the ability to model the troposphere and stratosphere adds further uncertainty to the results obtained.
It should also be noted that sensitivity studies from the 3-D tropospheric subsonic scenarios are often based on runs performed with a single model, thus should be
viewed with caution. In other cases, such as that of sensitivity to NMHC chemistry, the different models had different sensitivities.
As discussed in Chapter 2, time constants for transport in the stratosphere can be very long, and models must be run for many years to calculate the background state
and the perturbed atmosphere. In addition, the parameter space that must be probed for unbuilt HSCTs is much more extensive than for the characteristics of the
subsonic fleet. The use of 2-D models allowed for a more detailed analysis of the parameter space associated with supersonic aircraft. However, the results from these
runs are also limited by the 2-D nature of the models.
Some of the uncertainty issues noted above have been discussed from a somewhat different perspective in Section 2.3. In addition, several international assessment
reports (Stolarski et al., 1995; WMO, 1995; Fabian and Kärcher,1997; Friedl, 1997; Brasseur et al., 1998) have addressed some of these uncertainties. In the following
paragraphs, we describe some features related to model uncertainty that we have identified in this study.
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