AERMOD Tech Guide

Gaussian Plume Air Dispersion Model

3. Meteorological Preprocessor (AERMET)

The basic purpose of AERMET is to use meteorological measurements, representative of the modeling domain, to compute certain boundary layer parameters used to estimate profiles of wind, turbulence and temperature. These profiles are estimated by the AERMOD interface (described in section 4).

The structure of AERMET is based upon an existing regulatory model preprocessor, the Meteorological Processor for Regulatory Models (MPRM) (Irwin, et al., 1988). However, AERMET’s processing of meteorological data is similar to that done for the CTDMPLUS (Perry, 1992) and HPDM (Hanna and Paine, 1989: Hanna and Chang, 1993) models.

The surface parameters provided by AERMET are the Monin-Obukhov Length, L, surface friction velocity, u* , surface roughness length, z0 , surface heat flux, H, and the convective scaling velocity, w * . AERMET also provides estimates of the convective and mechanical mixed layer heights, zic and zim , respectively. Although AERMOD is capable of estimating meteorological profiles with data im from as little as one measurement height, it will use as much data as the user can provide for defining the vertical structure of the boundary layer. In addition to PBL parameters, AERMET passes all measurements of wind, temperature, and turbulence in a form AERMOD needs.

The growth and structure of the atmospheric boundary layer is driven by the fluxes of heat and momentum which in turn depend upon surface effects. The depth of this layer and the dispersion of pollutants within it are influenced on a local scale by surface characteristics such as the roughness of the underlying surface, the reflectivity (albedo), and the availability of surface moisture. Unlike ISC3, which is based on an assumed open-country vegetation cover for all sites, the state of the PBL computed by AERMET is a function of the underlying surface characteristics. Therefore, meteorological profiles and ambient concentrations may change from site to site (all other things being equal) or as the up-wind fetch changes with wind direction.