| 摘要: | 研究期間：10108~10207;The major precipitation stems in summer over Taiwan are caused by tropical disturbances and afternoon rainfall due to terrain effects (Chen and Chen 2003). According to Chen et al. (2004), about 50% of rainfall over southwestern Taiwan during mid-July to August in the period of 1993 to 2005 is not caused by typhoons after Chen et al. (2004) analyzed rainfall data from Automatic Rainfall and Meteorological System (ARMTS). Wang and Chen (2008) pointed out that 51% of rainfall amount during mid-June to mid-July in the period of 1993 to 2005 is due to local circulations. Meanwhile, 41% of rainfall days are caused by local circulations. During this period, most rainfall is over west-central slope of Taiwan. However, during mid-July to September in the period of 1993 to 2005, about 43% of rainfall is due to local circulations. Meanwhile, 45% of rainfall days are caused by local circulations. During this period, most rainfall is over western slope and southwestern Taiwan. The diurnal variations of convective activity are very significantly during summer. In the afternoon and evening, convective activity is most pronounced over slope and mountains as seen in the ARMTS analysis (Kerns et al. 2010) and radar echoes analysis (Lin et al. 2011). Kern et al. (2010) and Lin et al. (2011) also noted that convective systems moved westward toward Taiwan Strait during evening. The movement of convective systems from land to ocean is common in the tropics (Ohsawa et al. 2001; Zudema 2003; Kikuclui and Wang 2008). The convective systems moved out of land to ocean in the evening and late night. In the late morning or around noon, convective activity is strongest over ocean. The studies of afternoon convective activity of Taiwan island is well established ( Kerns et al. 2010 and Lin et al. 2011). However, the studies about the maintenance and movement mechanisms about some long-lived afternoon convective systems (more than 10 hours life span) are not many. This project collected the afternoon convective system with accumulated daily rainfall amount exceeding than 50 mm under weak synoptic-scale forcing over Taiwan. There are 130 days during July and August in the period of 2005- 2010. However, we find only six events satisfies our long-lived criterion. Because of the missing of radar data, we focus on three events. The dates of these events are 9 July 2008, 18 August 2006 and 6 August 2008. The objective of this study is to perform observational data analyses, including NCEP data, satellite imagery, radar reflectivity, and rainfall data, and numerical simulations with the Weather Research Forecast (WRF) model to investigate mechanisms for the maintenance and movement of the long-lived afternoon convective systems. |
