Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
m 2 /sec) to simulate rapid mixing. Studies indicate that stratosphere-troposphere exchange may be dominated by transport from the mid- and high-latitude lower
stratosphere to the troposphere (Eluszkiewicz, 1996). In a 2-D model, this transport will manifest itself as eddy flux along isentropic surfaces across the tropopause
boundary (Shia et al., 1993). If this premise is true, calculated residence time of aircraft emissions in the LS would be sensitive to the horizontal and vertical resolutions
of the models because resolution constrains the location and seasonal variation of the tropopause.
4.3.1.2. Temperature, Transport Parameters, and Solvers
The THINAIR model is the only model that calculates temperature and transport circulation consistent with calculated O 3 . The rest of the models generated results in
CTM mode; in other words, results are obtained using pre-calculated temperature and transport fields. Different methods are used to compute residual mean
circulation and eddy diffusion coefficients. Transport circulation for the UNIVAQ 2-D model is taken from a low-resolution spectral GCM (Pitari et al., 1992). Winds and
temperature for the LARC and SCTM1 models are from off-line GCM simulations, and those for SLIMCAT are from the UKMO analysis. There is no accepted method
to validate computed transport parameters. Temperature is used to compute temperature-dependent reaction rate constants and, in some models, to predict the
surface areas of PSCs. Different numerical schemes are used to solve the mass-continuity equations. Given the different methods used in deriving the transport
parameters in the models, it is not surprising that there are large differences in calculated distributions of trace gases in the models. Large differences in modelsimulated
distributions of chemically inert tracers such as sulfur hexafluoride point to transport differences as a major contributor. The research community is trying to
identify a climatological database for zonal-mean distributions of trace gases through an ongoing exercise (Models and Measurements II, Park et al., 1999) that can be
used to diagnose transport parameters.
4.3.1.3. Chemistry
Previous model intercomparison exercises have shown that chemistry solvers in most models calculate the same partitioning of radicals under the same constraints
(solar zenith angle, overhead O 3 , local temperature, local sulfate surface area, and local concentrations of the long-lived species) when they are used as box models in
photochemical steady-state. Because the 2-D models transport zonal-mean concentrations, zonal-mean production and loss rates are needed in the mass-continuity
equations for long-lived species. Different techniques are used to obtain zonal mean production and loss rates, including integrating diurnally varying concentrations of
radicals obtained by explicit time marching to compute the zonal-mean rates or using diurnally averaged radical concentrations calculated from average solar zenith
angles corrected by pre-calculated correction factors.
Table 4-9: Sulfate surface area density (SAD) fields used in this assessment. SAD distributions
derived in the coupled AER 2-D/ Sulfate Microphysical models (i.e., SA1) are obtained by calculating the difference between perturbed and reference SADs in
the model.
For these calculations, an EI(S)=0.2 was assumed. This difference or aircraft-produced SAD is then added to the volcanically clean reference distribution
(SA0).
The cruise altitude for most HSCT scenarios is at a standard (Std) height.
http://www.ipcc.ch/ipccreports/sres/aviation/049.htm (3 von 13)08.05.2008 02:42:26