CNRS-NSF Workshop
Transport and Mixing of reactive and non reactive flows in the Atmospheric Boundary Layer
Paris, July 4, 2001



    H.J.S. Fernando
    Department of Mechanical and Aerospace Engineering, Environmental Fluid Dynamics Program, Arizona State University, Tempe, USA

    Vertical Mixing and Dispersion in Urban Airsheds

    Most of the world's cities are located in areas replete with mountains, valleys, escarpments etc. (the so-called complex terrain), and airflow in these regions are characterized by up-valley and up-slope flow during the day and down-valley and down-slope winds at night. The down slope/valley winds strongly interact with stably stratified nocturnal boundary layers of cities, leading to important flow phenomena signified by instabilities, waves and turbulence that profoundly influence the dispersion of contaminants in urban areas. The transition from up to down slope/valley winds in the evening is also complex, and involves localized wave and mixing phenomena. The available atmospheric boundary layer models do not predict stably stratified periods with a reasonable accuracy nor do they adequately describe flow during the evening transition period, pointing to the inadequacy of the mixing parameterizations used in these models to describe stable periods. The results of field observational, laboratory, theoretical and numerical modeling programs to understand the nocturnal micrometeorology of urban airsheds will be presented, and the role of waves and turbulence in determining the air quality of complex terrain cities will be described.



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