The Wind Field and Diffusion of Air Pollutant in a Small Complex Terrain

• Main Authors: Wei-Chieh Chiang, 江偉傑
• Other Authors: Hsu-Cherng Chiang
• Format: Others
• Language: zh-TW
• Published: 2000
• Online Access: http://ndltd.ncl.edu.tw/handle/16910137168452238789

碩士 === 淡江大學 === 水資源及環境工程學系 === 88 === Taiwan is an island with very complicated topography. Complex wind fields generated by irregular terrain and atmospheric circulation such as land/sea breeze and mountain-valley wind which affect the transport and the diffusion of air pollutants in this island. In this study, the dispersion of accidentally released chemicals in northern Taiwan is studied numerically. A three dimensional, nonhydrostatic, mesoscale meteorological model, RAMS, is used to determine the wind and turbulent fields. A four-layers nested grid system is employed for all simulations. The finest mesh has 1-km spacing in the horizontal. HYPACT diffusion model is used to simulate the diffusion of air pollutant in a small complex terrain. Two periods are selected for meteorological simulations. In general, the computational results are pretty well. The spatial and temporal variations of thermal-induced circulation as well complex flow generated by irregular terrain are found to be reasonable. When the numerical results were compared with surface observations, the following conclusions are obtained: 1.The model can accurately predict the diurnal variations of surface temperature. 2.The predictions of surface specific humidity are accurate for all stations except Tsutzuhu. The values in Tsutzuhu are under-predicted. 3.The predictions of surface wind speed are reasonable but not so well as the other variables. Six simulations are carried out to investigate the atmospheric dispersion in different ambient conditions. It is noted that when the synoptic wind speed is weak, the diffusion is mainly controlled by thermal- induced atmospheric circulation. For daytime, elevated releases, the pollutant will be blown offshore by upper wind initially. Then, it may flow back to coastal zone because of sea breeze. In the evening, offshore flow blows the pollutant to sea surface directly. When the synoptic wind speed is strong, the transport of pollutant is dominated by the prevailing wind. In this case, the release time has minor effect on the transport of pollutant. According to numerical simulations, the pollutants can surpass Datun mountain easily and move at southwest direction to Linkou, Taoyuan area when northeast wind is strong.