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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