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1.5.2 Downwind, Crosswind and Radial Distances

The calculation of downwind and crosswind distances is described in Section 1.1.2.  Since the complex terrain algorithms in ISC are based on a sector average, the radial distance is used in calculating the plume rise (see Section 1.5.4) and dispersion parameters (see Section 1.5.5).  The radial distance is calculated as R = [x2 + y2]1/2, where x is the downwind distance and y is the crosswind distance described in Section 1.1.2.

1.5.3 Wind Speed Profile

See the discussion given in Section 1.1.3.

1.5.4 Plume Rise Formulas

The complex terrain algorithm in ISC uses the Briggs plume rise equations described in Section 1.1.4.  For distances less than the distance to final rise, the complex terrain algorithm uses the distance-dependent plume height (based on the radial distance) as described in Section 1.1.4.10.  Since the complex terrain algorithm does not incorporate the effects of building downwash, the Schulman-Scire plume rise described in Section 1.1.4.11 is not used for complex terrain modeling.  The plume height is used in the calculation of the Vertical Term described in Section 1.5.6.

1.5.5 The Dispersion Parameters

The dispersion parameters used in the complex terrain algorithms of ISC are the same as the point source dispersion parameters for the simple terrain algorithms described in Section 1.1.5.1, except that the radial distance is used instead of the downwind distance.  Since the lateral distribution of the plume in complex terrain is determined by the sector average approach, the complex terrain algorithm does not use the lateral dispersion parameter, σy.  The procedure to account for buoyancy-induced dispersion in the complex terrain algorithm only affects the vertical dispersion term (see Equation 1-48).  Since the complex terrain algorithm does not incorporate the effects of building downwash, the enhanced dispersion parameters and virtual distances do not apply.

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