Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
uncertain. Transport in the lowermost stratosphere is considerably different, and in many
ways even more difficult to represent in 2-D models, than transport at higher altitudes. This
fact certainly does not bode well for the ability of current 2-D models to describe accurately
the dynamic context within which the current subsonic fleet is operating.
2.3.1.4.3. Comparison of model data and observations of stratospheric ozone
and aerosol emissions in the stratosphere from present subsonic
aviation.
As part of the M&M II effort, results from a group of stratospheric models were compared with a recently developed ozone climatology (WMO, 1998). The data used for
the climatology are from sonde stations and from SAGE II, the latter data set having been evaluated by comparison with other satellite, lidar, sonde, and Umkehr data.
Although agreement between models and between models and observations is relatively good above 25 km, differences between modeled and observed ozone are
found to increase rapidly below 25 km and are largest between 20 km and the tropopause. The modeled ozone tends to be larger than observed ozone by up to a
factor of 2 at these altitudes.
Overestimation of LS ozone in some models may be ascribed partly to the fact that they have tropopauses at mid-latitudes that are either invariant or do not vary
correctly with season. However, based on chemistry and dynamics tests described in the preceding subsections, it is likely that differences between models and
observations are caused in large part by deficencies in model transport representation.
2.3.1.5. Implications for Modeling Aviation Impacts
Global tropospheric 3-D CTMs are now the main modeling tools for climate-chemistry studies, including the role of subsonic aircraft NO x emissions. Although 3-D
models with high temporal and spatial resolution have performed significantly better than 2-D or monthly averaged 3-D CTMs in the 222Rn, PhotoComp, NO x , and
Ozone/GIM intercomparison exercises, key fundamental problems have been identified that are crucial to the representation of the impacts of subsonic NO x emissions
from aircraft.
3-D CTM studies have provided only preliminary estimates of subsonic impacts, which exhibit significant scatter, as Table 2-1 shows. At present, we are unable to
rationalize these real differences in results between studies because there is no one aspect of input data or process parameterization that can account for the spread
in model results. Furthermore, the extent of model evaluation is highly variable, and no models have been evaluated comprehensively against all of the key issues
detailed in Section 2.3.1.1.
These same difficulties apply to the subset of models adopted in Chapter 4 to examine the future impact of subsonic aircraft. There is no suggestion that these models
have any distinguishing features that identify them as being inherently more or less reliable for assessment of the tropospheric impacts of subsonic aircraft NOx emissions. Furthermore, we have no concrete means of establishing a higher level of confidence in the models used in Chapter 4, compared with any of the similar 3-D
models listed in Table 2-1.
Although the effects of present aviation on ozone are calculated to be much smaller in the stratosphere than in the troposphere -primarily because of the smaller
fraction of exhaust released into the stratosphere-the performance of 2-D stratospheric models has not been extensively evaluated in the lowermost stratospheric
region. Consequently, the results reported in Section 2.3.1.3 represent only preliminary estimates of subsonic aviation impacts on the stratosphere. The modeling
situation is significantly better for evaluating the effects of future supersonic aircraft in that a number of intercomparisons have established the general quality of
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