Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
transport schemes in CTMs. Owing to the lack of extensive observations, evaluation efforts to date have been restricted mainly to model-model intercomparisons.
The 222Rn model intercomparison concluded that tropospheric CTMs based on 2-D models and monthly averaged 3-D models have a fundamental flaw in
transporting tracers predominantly by diffusion; thus, these models cannot be viewed as reliable in simulating the global transport of tracers. Synoptic 3-D models need
significantly improved representations of boundary layer processes, clouds, and convection. Large differences are found among established 3-D CTMs in the rates of
global-scale meridional transport in the UT-particularly, interhemispheric transport. These latter differences are particularly relevant to the current issue of subsonic
aircraft impacts.
2.3.1.2.2. Fast photochemistry
More than 20 model groups participated in the tropospheric photochemical model intercomparison exercise, PhotoComp-a tightly controlled experiment in which
consistency was determined among models used to predict tropospheric ozone changes (IPCC, 1996; Olson et al., 1997). A similar study, involving fewer models, was
carried out as part of a U.S. National Aeronautics and Space Administration (NASA) assessment (Friedl, 1997). As with the radon case, there are no easy
observational tests of model fast photochemistry, so model-model intercomparison exercises were carried out in both cases.
Over the intercomparison tests for fast photochemistry of the sunlit troposphere, modeled OH concentrations fell within a ±20% band, and ozone changes fell within a
±30% band. These obvious variations between model results did not correlate with other model differences, and no single model input parameter appeared to account
for all of the spread in the results. Nevertheless, ozone photolysis rates used in the models accounted for about half of the root mean square (RMS) differences; further
investigation of these parameters and their comparison with observations is called for. The results also became more uncertain in model experiments involving
NMHCs. Further CTM development is required so that models have the required grid and time resolutions to simulate accurately the scales of chemistry required to
describe the removal of NO x and NMHCs while producing and destroying ozone, quantitatively.
2.3.1.2.3. Transport of NO x
Passive transport of subsonic aircraft NO x emissions has been studied with a hierachy of global CTMs (Friedl, 1997; van Velthoven et al., 1997). The 3-D CTMs
showed that the monthly mean NO x concentrations varied by a factor of three longitudinally and that the temporal variability of background NO x in the air traffic corridor
was about ±30% on synoptic time scales. Vertical redistribution by convection strongly affected the maximum NO x concentrations at subsonic aircraft cruise altitudes.
A number of model deficiencies and biases were found, including the oscillatory nature of NO x distributions obtained with a spectral advection scheme, the strong
diffusion of GCMs into polar regions, and the too-intense interhemispheric exchange found in some 2-D CTMs. The intercomparisons concluded that assessment of
the tropospheric impacts of subsonic aircraft NO x emissions could be performed better with 3-D CTMs.
2.3.1.2.4. Comparison of model data and observations of tropospheric ozone
An increasing number of activities are aimed at evaluating global model results in relation to ozone observations (Wang et al., 1998b; Wauben et al., 1998). However,
there are too few ozone data, especially in the tropics, to allow for comprehensive evaluations. Comparisons are showing that model simulations are reproducing the
broad features of monthly mean measured ozone concentrations. Some models do not produce the observed seasonality in the northern mid-latitude troposphere.
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