emissions-driven more than meteorologically-driven and the costs do not warrant the generation

of additional resolution in the meteorological data. In these specific situations it is

recommended that the air quality model application use available results from meteorological

models on the next coarser scale (i.e., 36 km for a desired 12 km estimate, 12 km for a desired 4

km estimate). The coarse grid meteorological fields can be mapped to the more finely resolved

air quality modeling domain.

Selecting Physics Options: Most meteorological models have a suite of "physics

options" that allow users to select how a given feature will be simulated. For example, there may

be several options for specifying the planetary boundary layer scheme or the cumulus

parameterization. In many situations, the "optimal" configuration cannot be determined without

performing an initial series of sensitivity tests which consider various combinations of physics

options over specific time periods and regions. While these tests may not ultimately conclude

that any one configuration is clearly superior at all times and in all areas, it is recommended that

these sensitivity tests be completed, as they should lead to a modeling analysis that is best-suited

for the domain and period being simulated. Examples of sensitivity analyses can be found in

McNally (2002), Olerud, (2003), and Huang, (2005). Typically, the model configuration which

yields predictions that provide the best statistical match with observed data over the most cases

(episodes, regions, etc.) is the one that should be chosen, although other more qualitative

information can also be considered. Additionally, model configurations should be designed to

account for the pollutants and time periods that are of most interest. As an example, a

wintertime PM simulation in the Midwest (with high measured nitrate concentrations) may need

a meteorological model configuration that employs a land-surface model that properly handles

snow cover fields and their effects on boundary layer humidities and temperatures.


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