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13th EMS Annual Meeting & 11th European Conference on Applications of Meteorology ( ECAM) September 9 to 13, 2013; Reading , United Kingdom. Parameterization of Pollutant Plume Dispersion in Neutral Stratification over Hypothetical Urban Areas. Chi-To Ng and Chun-Ho Liu*.

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13th EMS Annual Meeting & 11th European Conference on Applications of Meteorology (ECAM)

September 9 to 13, 2013; Reading, United Kingdom

Parameterization of Pollutant Plume Dispersion in Neutral Stratification over Hypothetical Urban Areas

Chi-To Ng and Chun-Ho Liu*

Department of Mechanical Engineering

The University of Hong Kong

*Corresponding Author: Chun-Ho LIU

Department of Mechanical Engineering, 7/F Haking Wong Building, The University of Hong Kong, Pokfulam Road, Hong Kong

Tel: (852) 2859 7901; Fax: (852) 2858 5415;;



  • Ground-level pollutants from human activities and vehicles are the primary pollutant sources, which adversely affect the health and living quality of urban inhabitants.
  • Large-scale roughness elements such as buildings and mountains can significantly modify the pollutant dispersion behaviour.
  • The Gaussian model of plume dispersion is commonly used to estimate the downwind pollutant concentration distribution. However, one of its major parameters, the dispersion coefficient (σz), often underestimates the importance of buildings in urban areas.


  • The aerodynamic effects of idealized urban roughness were parameterized with a single variable – friction factor (f), which is believed to be sufficient to measure the resistance of urban roughness at constant UBL height for comparison in our previous studies (Wong and Liu, 2013).
  • Parameterize pollutant dispersion behaviour (σz) with idealized urban roughness (f) to enrich the understanding of pollutant dispersion and removal mechanism in urban areas.


  • Renormalization Group (RNG) of kappa-epsilon (k-ϵ) turbulence model
  • 2-dimensional computational domains consisting of idealized street canyons were used to represent the hypothetical urban areas
  • A total of 144models with eightdifferent building shapes and eighteendifferent building-height-to-street-width (aspect) ratios (ARs)
  • Covering the characteristic skimming flow, wake interference and isolated roughness regimes

Friction Factor ( f )

  • A dimensionless number represents the surface roughness caused by different building morphology (building shapes and ARs)

, L



Friction Factor ( f ) vs ARs

  • Friction Factor decreases when the street is extremely wide, it is because the wind flows and street configurations reached isolated roughness regime

z0 vsFriction Factor ( f )

  • z0 is obtained from ensemble average of velocity profile in horizontal direction and log profile


  • The mean value of σz / x1/2 was found to be direct proportional to f1/4 with the coefficient of determination equal to 0.9407.
  • The mean value of σz / x1/2is close to the upper limit.
  • Friction factor (f) is believed to be able to estimate the range of dispersion coefficients (σz) for urban areas.