15.4 How Should the Vertical Layers Be Selected?
As described in Section 16, the preferred approach for generating meteorological data
fields for input to air quality simulation models is to use a prognostic meteorological model with
four dimensional data assimilation (FDDA). Such models normally use more than 30 vertical
layers. To minimize a number of assumptions needed to interface meteorological and air quality
models, it is better to use identical vertical resolution in the air quality and meteorological
models. However, application of air quality models with as many as 30 vertical layers may not
be feasible or cost effective. In this section we identify factors to consider in choosing the
number of vertical layers chosen for the air quality model applications.
In the past, short ozone episodes of only several days usually encompassed periods of
mostly clear skies with very little precipitation. As such, ozone models often did not explicitly
model clouds or precipitation. However, we are recommending modeling longer episodes (or
even a full ozone season) for ozone and a full year for PM2.5 and regional haze applications.
Therefore the photochemical model needs to account for cloud processes and a full range of
precipitation types. In order to adequately parameterize these processes, the top of the modeling
domain should typically be set at the100 millibar level (~16,000 meters). In turn, this means that
many more vertical layers will be needed to capture the meteorological processes both below and
above the boundary layer, up to the top of the model.
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