Abstract-
Most of the world's cities are located in areas replete with mountains, valleys
and escarpments (the so-called complex terrain airsheds), and airflow in these
regions is 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. The results of a combined field observational, laboratory, theoretical
and numerical modeling program to understand the slope and valley circulation 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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