Atmospheric Turbulence over Islands – a Theoretical Study of the Air Pollutant Transport over Coastal Cities

(Prevailing) Winds over coastal cities are characterized by the diurnal cycle of sea-land breeze and the sharp change in wind profile across the sea-land interface of different surface roughness. The micro-scale wind environment is further complicated in islands with hilly terrain because of the flow stagnation (windward side), separation, and re-attachment (leeward side). These local flow scales, which are often tremendously different from their prevailing counterparts, could substantially modify the transport of atmospheric constituents, e.g. pollutant entrapment on leeward slope. While the prevailing flows have been studied in meso-scale meteorology for decades, our understanding of the micro-scale flows is rather limited, e.g. how does the slope modify the pollutant retention. This study is thus conceived to examine the correlation among terrain configuration, sea breeze and pollutant retention.

In the proposed study, the terrain configuration, sea breeze, and pollutant retention are measured by the length scale H, velocity scale U, and time scale T (Figure 1). As such, the core scientific question to be addressed is:

How to formulate the dimensionless group(s) that could characterize the pollutant transport over coastal cities such as Hong Kong?

The fundamental scientific merit of the proposed study is a thorough understanding of atmospheric flows and pollutant transport over (hilly) islands and across sea-land interface. Moreover, the findings could bridge the aforementioned knowledge gap in micro-scale wind environment which directly affects the pollutant retention and air quality of coastal cities. A solid background of atmospheric flows over islands with hilly terrain will be developed which will then be used to elucidate the turbulent transport processes of air pollutants and the associated air quality problems in coastal urban areas.

While the micro-scale processes are often overlooked using the current meso-scale meteorological approaches in engineering practice, the significance of the proposed study is to arouse practitioners the importance of micro-scale wind environment in modern urban planning. Unlike the conventional meso-scale meteorological analysis, the original contribution of the proposed dimensional analysis is to set up a framework for quick (and reliable) estimate of the pollutant removal over coastal cities such as Hong Kong, Shanghai, or Los Angeles. This handy model could serve the community for better urban environment.