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Buoyancy and turbulence-driven atmospheric circulation over urban areas 被引量:2

Buoyancy and turbulence-driven atmospheric circulation over urban areas
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摘要 In the buoyancy and turbulence-driven atmospheric circulations(BTDAC) that occur over urban areas where the approach means wind speeds are very low(less than turbulent fluctuations and typically 3 m/sec), the surface temperatures are significantly higher than those in the external rural areas, and the atmosphere above the mixing layer is stably stratified. In this paper, the mechanisms of BTDAC formation are studied through laboratory experiments and modelling, with additional low-level inflow from external rural areas and a divergent outflow in the opposite direction in the upper part of the mixed layer. Strong turbulent plumes in the central region mix the flow between lower and higher levels up to the inversion height. There are shear-driven turbulent eddies and weaker buoyant plumes around the periphery of the urban area. As the approach flow is very weak,the recirculating streamlines within the dome restrict the ventilation, and the dispersion of pollution emitted from sources below the inversion height leading to a rise in the mean concentration. Low-level air entrained from rural areas can, however, improve ventilation and lower this concentration. This trend can also be improved if the recirculating structure of the BTDAC flow pattern over urban areas breaks down as a result of the surface temperature distribution not being symmetrical, or as the approach wind speed increases to a level comparable with the mean velocity of circulation, or(except near the equator) the urban area is large enough that the Coriolis acceleration is significant. In the buoyancy and turbulence-driven atmospheric circulations(BTDAC) that occur over urban areas where the approach means wind speeds are very low(less than turbulent fluctuations and typically 3 m/sec), the surface temperatures are significantly higher than those in the external rural areas, and the atmosphere above the mixing layer is stably stratified. In this paper, the mechanisms of BTDAC formation are studied through laboratory experiments and modelling, with additional low-level inflow from external rural areas and a divergent outflow in the opposite direction in the upper part of the mixed layer. Strong turbulent plumes in the central region mix the flow between lower and higher levels up to the inversion height. There are shear-driven turbulent eddies and weaker buoyant plumes around the periphery of the urban area. As the approach flow is very weak,the recirculating streamlines within the dome restrict the ventilation, and the dispersion of pollution emitted from sources below the inversion height leading to a rise in the mean concentration. Low-level air entrained from rural areas can, however, improve ventilation and lower this concentration. This trend can also be improved if the recirculating structure of the BTDAC flow pattern over urban areas breaks down as a result of the surface temperature distribution not being symmetrical, or as the approach wind speed increases to a level comparable with the mean velocity of circulation, or(except near the equator) the urban area is large enough that the Coriolis acceleration is significant.
出处 《Journal of Environmental Sciences》 SCIE EI CAS CSCD 2017年第9期63-71,共9页 环境科学学报(英文版)
基金 supported financially by a RGC CRF project(HKU9/CRF/12G)of the Hong Kong SAR Government support from the Newton Ungku Omar fund administered by the British Council and the Malaysian International Group for High Technology
关键词 Urban heat island Buoyancy and turbulence-driven Eddy structures Atmospheric circulation Pollutant dispersion Urban heat island Buoyancy and turbulence-driven Eddy structures Atmospheric circulation Pollutant dispersion
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