dc.description.abstract | In this study the synoptic patterns conductive to the occurrence of O3 episodes in central Taiwan are categorized by an inspection of the weather charts over the period of 1994-2005. Since the high ozone episodes of the central part of Taiwan are closed related to typhoon and tropical cyclones activities, focus the discussion on the typhoon-related ozone episodes. Furthermore, we used the mesoscale model (Fifth-generation Penn. State/NCAR Mesoscale Model, MM5) to do a case simulation to investigate the influence of typhoon activities on the air quality of the central Taiwan.
The statistical result shows that about 31% of total ozone episodes is caused by typhoon or trpical cyclone. When the typhoon-related ozone episodes occur, the center of tropical cyclones is mainly located between Taiwan and the Philippines. Most tracks of such cyclones are classified to the track of 5th and 6th follow the Central Weather Bureau’s classification. The compose weather map of high O3 episodes occurred in the central Taiwan shows that a low-pressure circulation is located in the Philippines. The surface mean wind is 3.27 m/s, larger than others and the synoptic wind direction is ENE in 850hpa. High wind speed is favorable for pollutant transportation. In the further more, we used MM5 model to simulate the typhoon-related ozone episode during 20-22 September 2005 (track 5th).
When the center of typhoon Damrey locates 600-1000 km away from central Taiwan, the subsidence effect of its periphery made air very stable. These conditions favor the development of both local thermal circulations and mechanically-induced circulations in the complex terrain area of the Central Taiwan. At this stage, due to the transportation effect of local circulation, the high ozone concentration comes up at the downward city, Nantou. After the typhoon move north-westward, the typhoon circulation covered the whole Taiwan. The low-level prevailing flow is subject to splitting due to the blocking effect of the Central Mountain Range (CMR), a weak wind speed zone is notable in the western central region, which is located in the leeward side of the CMR. The prevailing easterly wind accompany with typhoon Damrey, lead to the lee-side subsidence. The adiabatic warming associated with this leeside subsidence resulted to the appearance of stable atmosphere condition in the Central and western area of Taiwan. These meteorological conditions are unfavorable for pollutant dispersion and cause high ozone day in the city, Taichung. The high ozone episodes just occurred in the vicinity of high emission urban area. | en_US |