| dc.description.abstract | During autumn and winter season, with the eastward movement of the Asia continental anticyclone, the synoptic wind in Taiwan changes into easterly direction. The western Taiwan due to situated in lee-side of the mountain, exhibits the stagnant wind conditions. Sometimes the leeside vortex can form when the airflow bypasses the mountain of Taiwan, which in turn influence the transport of air pollutants and cause subsequent high PM2.5 event.
In order to understand the structural characteristics of the leeside vortex and its influence on the transport of air pollutants, we using the observation data from Central Weather Bureau (CWB), air quality monitoring station of Environmental Protection Administration (EPA), the wind profiler and the lidar in Taiwan to analyze the day of the high pollution event that occurred on December 19, 2018. It was found that the central area on the leeside was not only affected by the local circulation (ex. the land sea breeze), but also existed the southerly wind in the morning, indicating that the leeside vortex could change the structure of the ground wind field.
In addition, because the generation of the leeward vortex is affected by the wind field and Taiwan′s complex topography, coupled with the lack of marine observation data, the simulation uncertainty of the leeward vortex is quite high, this study also uses 21 ensemble members (WRF-LETKF) to analyze the characteristics of the leeside vortex. From the simulation results, it is found that the vortex southerly wind structure close to the land within 200 meters of the bottom layer is easily affected by the sea breeze. The result coincides with the observation that the wind field has gradually changed from southerly to westerly (sea breeze) over time. The vortex circulation structure is clear at the height of 400 meters than on the ground due to the reducing thermal heating effect. Further analysis of the cross section of the vortex shows that the thickness of the vortex is about 800 meters and gradually tilt to the west as the height increases.
CMAQ simulation results show that the southern area is easy to accumulate air pollutants under northeast wind weather condition. Until the development of the boundary layer enhances the vertical mixing effect, bringing near-surface pollutants to the upper air in the morning. Then the southerly wind blows the air pollutants north to the Taoyuan area. | en_US |