Retter et al. (2025) Revisiting the Convective Like Boundary Layer Assumption in the Urban Option of AERMOD

Identification

• Journal: Atmosphere
• Year: 2025
• Date: 2025-11-27
• Authors: Jonathan E. Retter, Robert Christopher Owen, Annamarie Leske, Michelle Snyder, R.W.H. Sargent, David Heist
• DOI: 10.3390/atmos16121342

Research Groups

• Oak Ridge Institute for Science and Education Research, Research Triangle Park, NC, USA
• U.S. EPA Office of Research and Development, Center for Environmental Measurement and Modeling, Research Triangle Park, NC, USA
• Johns Hopkins Applied Physics Labs, Laurel, MD, USA
• WSP Global Inc., Durham, NC, USA
• Department of Statistics, Virginia Tech University, Blacksburg, VA, USA

Short Summary

This study re-examines the AERMOD urban option's convective-like boundary layer assumption, which significantly overestimates nighttime sensible heat flux, and proposes replacing its population-based temperature difference parameterization with remotely sensed land surface temperature data to provide more realistic city-specific advection corrections. The proposed methodology yields sensible heat flux values consistent with observations and reveals that AERMOD's original urban option inadvertently addressed a low-level jet rather than an urban heat island effect.

Objective

• To re-examine the formulation, assumptions, and original evaluation of the AERMOD urban option, specifically its "convective like boundary layer" assumption.
• To investigate replacing the population-based parameterizations of urban-surrounding temperature differences (∆T) in AERMOD with observations of remotely sensed land surface temperature (LST) data to provide city-specific horizontal advection corrections to sensible heat flux estimates.

Study Configuration

• Spatial Scale: 480 continental United States (CONUS) urban areas (2010 US Census definitions); GOES-16 Advanced Baseline Imager (ABI) data at ~2–4 km spatial resolution; 30 km buffer from urban boundaries for surrounding sectors.
• Temporal Scale: Monthly averaged, hourly, wind direction-dependent, clear sky land surface urban heat island ∆T database; LST data analyzed from 2018 to 2022, with 2021 data highlighted.

Methodology and Data

• Models used: AERMOD (v22112), AERMET (meteorological pre-processor for AERMOD).
• Data sources:
  • Remotely sensed land surface temperature (LST) and sea surface temperature (SST) from the Advanced Baseline Imager (ABI) on the GOES-16/R/East geostationary satellite.
  • 2010 US Census urban area definitions (shape files).
  • United States Geological Survey (USGS) National Land Cover Database (NLCD) from 2019.
  • Indianapolis Urban Power Plant Study (1985) dataset (ground-level SF6 concentrations, urban/suburban/rural meteorological data, turbulent kinematic heat flux).
  • Surface meteorological input files from AERMET for 2021 (from EPA’s Human Exposure Model website).

Main Results

• The existing AERMOD urban option's predicted nocturnal ∆T values were 794% (Cleveland), 416% (Amarillo), 1048% (Atlanta), and 758% (Baltimore) higher than GOES-16 observations.
• This led to AERMOD predicting unphysical nighttime sensible heat flux values > 100 W/m², often rivaling daytime values.
• Using GOES-16 observations for horizontal advection corrections, with a modified energy balance, resulted in realistic annual nighttime average sensible heat flux values for 2021: −0.8 W/m² (Cleveland), 8.6 W/m² (Amarillo), 3.0 W/m² (Atlanta), and 3.1 W/m² (Baltimore).
• The proposed approach classified nocturnal stability as unstable 53% (Cleveland), 92% (Amarillo), 62% (Atlanta), and 55% (Baltimore) of the time, with heat flux values of −10 to 20 W/m², consistent with literature.
• Re-evaluation of the Indianapolis Urban Power Plant Study showed that AERMOD's urban option, despite its unphysical heat flux inputs, improved concentration predictions. However, an alternative modification (setting heat flux to 0 W/m² and increasing vertical mechanical turbulence by 4.67 times) yielded even better performance, suggesting the original urban option inadvertently accounted for a low-level jet rather than an urban heat island.

Contributions

• Developed a novel, city-specific, directional, monthly averaged, hourly, clear-sky land and sea surface ∆T database for 480 CONUS urban areas using GOES-16 ABI data.
• Proposed and demonstrated an improved urban energy balance formulation for AERMOD that incorporates remotely sensed ∆T observations as horizontal advection corrections, leading to more realistic nighttime sensible heat flux values and stability classifications.
• Critically re-evaluated the original validation of the AERMOD urban option with the Indianapolis dataset, revealing that its "convective like boundary layer" assumption was likely an inadvertent attempt to account for a low-level jet (mechanical turbulence) rather than an urban heat island (convective turbulence).
• Provided a more physically representative approach for urban sensible heat flux modeling in dispersion models like AERMOD.

Funding

• U.S. Environmental Protection Agency (EPA) Research Participation Program administered by the Oak Ridge Institute for Science and Education (ORISE).
• ORISE is managed by ORAU under DOE contract number DE-SC0014664.

Citation

@article{Retter2025Revisiting,
  author = {Retter, Jonathan E. and Owen, Robert Christopher and Leske, Annamarie and Snyder, Michelle and Sargent, R.W.H. and Heist, David},
  title = {Revisiting the Convective Like Boundary Layer Assumption in the Urban Option of AERMOD},
  journal = {Atmosphere},
  year = {2025},
  doi = {10.3390/atmos16121342},
  url = {https://doi.org/10.3390/atmos16121342}
}