AirWare User Manual

AERMOD: A Dispersion Model
for Industrial Source Applications

This documentation is extracted from Perry et al. (1998).

Meteorological Parameters

The primary meteorological variables used by the ISC2 model are hourly surface-level observations of wind speed, wind direction, and temperature. The major derived parameter used by ISC2 is PG stability class. Stability class is used to characterize dispersion, select the appropriate plume rise equation, and set the temperature gradient. With the exception of wind speed, the approach taken by ISC2 assumes that these meteorological parameters do not vary with height. However, it is well known that significant height variations can occur in all of these variables.

One of the major improvements which AERMOD brings is its ability to characterize the PBL through both surface and mixed layer scaling. AERMOD constructs vertical profiles of needed meteorological variables based on measurements and extrapolations of those measurements using similarity (scaling) relationships. Therefore, vertical profiles of wind speed, wind direction, s w, s v, temperature, and temperature gradient can be estimated by the model without requiring significant input data. AERMOD is designed to operate as a refined model, without the need to collect extensive data. Furthermore, it has been AERMIC's goal to have the model applied, where appropriate, using readily available (e.g. National Weather Service data) representative meteorological data. Specific meteorological requirements for regulatory applications of AERMOD will be established in the future.

Estimation and use of the mixing height is another area in which AERMOD represents a major change. In ISC2, the mixing height is used only as an elevated reflecting surface. In addition to a reflecting surface, AERMOD uses zi as the primary height scale in the CBL. As such, the vertical profiles of wind direction and s w explicitly depend on zi. In the SBL, ISC2 does not account for the surface-based mechanical mixing layer. In AERMOD, the mechanical mixed-layer height (h) is calculated and used both as a reflecting surface and as the height scale for calculating the vertical profiles of s v and s w within the surface layer. To estimate mixing height, ISC2 uses a simple linear interpolation scheme to describe both the growth and collapse of the mixing layer. During the transition between day and night, this technique can produce unrealistic mixing heights. The approach taken by AERMOD, which is similar to the method used in CTDMPLUS, is to calculate zi and h from knowledge of the heat flux and momentum flux properties of the boundary layer.

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