||On average during summer and fall season, there are three to four typhoons affecting Taiwan per year. With their violent wind and heavy rainfall, these typhoons often cause severe disaster. The data analysis from the monitor stations of EPA (Environmental Protectional Administration) shows, high ozone episodes may occur in the western central Taiwan when the center of typhoon locates 1000-2000 km away from Taiwan. This study analyzed the relationship between typhoons or tropical depressions and the variation of ozone concentration in the central region during 1994-2003 (June to November). And further more, 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 region.|
The statistical result shows that when high ozone concentration episodes occur (O3 > 120 ppb); the center of typhoons or tropical depressions is mainly located in the South China Sea and the region between Taiwan and the Philippines. Most tracks of such typhoons are classified to the track of 5, 6 and 9 follow the Central Weather Bureau’s classification, and the episodes affected by typhoons with tracks 5 and 6 seem to last longer. Besides, we also find a high correlation (66%) between typhoons with track 9 and the occurrence of high ozone episodes, which is the highest correlation in all tracks.
In addition, this study also used MM5 model to simulate the high ozone episode during 16-18 September 2005. The simulation results exhibit that the atmospheric condition in Taiwan is stable and the synoptic wind is southeastly, as a result of the anti-cyclonic flow of the subtropical high pressure and Typhoon Vicente. 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 simulation results also show that the middle prevailing flow pass over the CMR and then move downward to the lower levels, causing the height of mixing layer lower. According to the vertical wind velocity, skew-T diagram and backward trajectory analysis, there is sinking motion from surface to 700 hPa, the top of inversion layer is about 900 hPa and the airflows are from the northwest, which means that the sea-breeze may transport the pollutants from coastal area to inland. The inversion layer and sinking motion trapped the pollutants under the height of about 900 hPa, which lead to the occurrence to high ozone episode in the western central region on 17th of September.
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