One of the major improvements that AERMOD brings to applied dispersion modeling is its

ability to characterize the PBL through both surface and mixed layer scaling. AERMOD

constructs vertical profiles of required meteorological variables based on measurements and

extrapolations of those measurements using similarity (scaling) relationships. Vertical profiles of

wind speed, wind direction, turbulence, temperature, and temperature gradient are estimated

using all available meteorological observations. AERMOD is designed to run with a minimum

of observed meteorological parameters. As a replacement for the ISC3 model, AERMOD can

operate using data of a type that is readily available from National Weather Service (NWS)

stations. AERMOD requires only a single surface measurement of wind speed (measured

between 7 zo and 100m - where zo is the surface roughness height), wind direction and ambient

temperature. Like ISC3, AERMOD also needs observed cloud cover. However, if cloud cover

is not available (e.g. from an on-site monitoring program) two vertical measurements of

temperature (typically at 2 and 10 meters), and a measurement of solar radiation can be

substituted. A full morning upper air sounding (RAWINSONDE) is required in order to

calculate the convective mixing height throughout the day. Surface characteristics (surface

roughness, Bowen ratio, and albedo) are also needed in order to construct similarity profiles of

the relevant PBL parameters.

Unlike existing regulatory models, AERMOD accounts for the vertical inhomogeneity of

the PBL in its dispersion calculations. This is accomplished by "averaging" the parameters of

the actual PBL into "effective" parameters of an equivalent homogeneous PBL.

Figure 2 shows the flow and processing of information in AERMOD. The modeling

system consists of one main program (AERMOD) and two pre-processors (AERMET and

AERMAP). The major purpose of AERMET is to calculate boundary layer parameters for use

by AERMOD. The meteorological INTERFACE, internal to AERMOD, uses these parameters

to generate profiles of the needed meteorological variables. In addition, AERMET passes all

meteorological observations to AERMOD.





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