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Including dispersive shear stress in urban environments for single column dispersion models.
Jonathan Retter1,2, David Heist2, Michael Pirhalla3
1ORAU ORISE Research Participation Program Hosted at EPA, Research Triangle Park, NC, USA.
Boundary-Layer Meteorology
|March 12, 2026
Summary
This study reveals that incorporating dispersive stresses into friction velocity calculations significantly impacts air pollution dispersion modeling in urban areas. Including these stresses improves predictions, especially near buildings, by accounting for complex wind patterns.
Area of Science:
- Environmental science
- Atmospheric science
- Computational fluid dynamics
Background:
- Gaussian dispersion models often neglect dispersive stresses in friction velocity calculations.
- This oversight limits the accuracy of models like AERMOD in complex urban environments.
- Accurate friction velocity is crucial for parameterizing pollutant dispersion.
Purpose of the Study:
- To investigate the impact of including dispersive stresses on friction velocity calculations.
- To compare these new calculations with existing methods in urban dispersion modeling.
- To assess the effect on predicted pollutant concentrations in idealized urban settings.
Main Methods:
- Utilized wind tunnel-validated, neutrally-stratified Large Eddy Simulations (LES).
- Simulated idealized uniform and nonuniform urban areas with varying wind angles (0°, 10°, 30°, 50°).
- Calculated friction velocity using both Reynolds and dispersive stress components above the urban canopy.
Main Results:
- Nonuniform urban models showed significant dispersive stresses across most blocks and flow angles.
- A simple power law expression accurately predicted urban friction velocity within 9.6%.
- Including dispersive shear stress increased predicted pollutant concentrations in the first row (+20.6% to +21.2%) but had minor effects elsewhere.
Conclusions:
- Dispersive stresses are critical for accurate friction velocity estimation in urban dispersion modeling.
- Existing AERMOD methods show poor agreement with LES results.
- Future Gaussian models should incorporate dispersive stresses for improved urban air quality predictions.
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