台灣梅雨季(5月中~6月中, Chen and Chen 2003)過後的6月下旬，因適合降雨的大尺度條件較梅雨季變得較差，降雨量銳減。於2008年的六月下旬，在台灣西南部地區降雨量遠大於13年(1997-2009)6月下旬的平均值。其中以6月27日及6月28日兩日的降雨最為顯著。其局部豪雨事件(>130mm/day) 主要分別發生在山區以及沿海低窪地區。因前人對台灣西南部在6月下旬局部豪雨的研究較少，為了探討地形效應如何影響降雨發生的區域(在平地或山區)，本研究採取下列步驟探討。
首先藉由European Centre for Medium-Range Weather Forecasts (ECMWF)/Tropical Ocean Global Atmosphere (TOGA)及中央氣象局觀測資料，分析個案發生前綜觀天氣現象及台灣降水分佈，得知局部豪雨發生前綜觀環境。在2008年6月27日、28日，在高層有從蒙古地區向西南延伸的中國槽線向東移近中國西南部，並且太平洋高壓東退，但台灣附近高層並無顯著輻散場，中層有上升運動。在低層台灣西南部上游地區有高位溫(>345K)及高水汽混合比(>17g/kg)向台灣傳送。
利用ECMWF/TOGA的綜觀環境經由理想橢圓地形的WRF (Weather Research and Forecasting Model)數值實驗。在盛行西南風影響下，山脈夫如數(Mountain Froude number,Fr=U/NH)約為0.26。由模擬得知在地形西南部低層沿岸地區會產生輻合帶、上升運動、高相對濕度、穩定度下降及較低的自由對流面高度，有利於降水之環境。
最後利用WRF模擬27日及28日天氣現象，檢討在理想地形實驗所得到的結論是否有利於實際大氣。在27日及28日兩日的山脈夫如數約為0.26，所以低層氣流容易在西南部產生繞山運動，使得沿岸地區產生低層輻合帶。當上游低層有較高水汽量移入時(如28日)，因為沿岸輻合帶及上升運動容易在西南部平地產生較大的降雨。當水汽量相對較少時(如27日)，降水較容易發生在山區(平地降雨量比28日少)。另外設計移除台灣地形實驗，驗證地形效應對平地及山區降雨加強之影響。也測設不同降雨微物理參數對日降量雨的影響。

After the cessation of Mei-Yu season (mid-May to mid June), which in one of the major rainfall periods in Taiwan, rainfall declines quite significantly in late June (16 to 30). Rain showers are the main sources of rainfall over southwestern Taiwan. The averaged annual heavy rainfall occurrence is about 8.8 rainfall stations. However, in 2008, there are 55 heavy rainfall occurrences during late June (Shu 2010). On 27 and 28 2008, the heavy rainfall occurrences are 10 and 4, respectively. During these two day period, the occurrences of heavy rainfall are significantly larger then annual average. On 27, continuous rainfall appeared over southwestern Taiwan with peak rainfall rate over sloped areas during the period from 0900 to 1600 LST and produced 204 mm maximum accumulated rainfall over sloped areas. On 28 June, most of rainfall appeared over lowland areas near coasts from 0900 to 1600 LST and the maximum accumulated rainfall was 160 mm over lowland areas. The occurrences of localized heavy rainfall over either sloped areas or lowland areas present a difficult, yet important, challenge to skilful weather forecasting.
The hypothesis for causing localized heavy rainfall over sloped areas on 27 June is that the inland movement of the existing convection from nearby ocean embedded in the moist southwesterly winds is enhanced by orographic lifting over sloped areas in southern Taiwan. On 28 June, the lifting on the existing convection from nearby ocean embedded in the moist southwesterly winds over low level convergence areas in lowland due to prevailing southwesterly flow deflection resulting from orographic effects may facilitate heavy rainfall over lowland areas. In addition, the cool air from sloped areas may enhance the interaction between off shore flow and the prevailing wind to strengthen low level convergence.
The objective of this study is to perform observational analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF) data, satellite imagery, radar reflectivity, and rainfall data for the synoptic and mesoscale processes to support the importance of the above two mechanisms. The mesoscale processes associated with the development of the heavy rainfall event and the orographic effects on the enhancement of accumulated rainfall are examined by the Weather Research and Forecast (WRF) model.
The WRF model was employed to help understand the details of the dynamics and physical processes responsible for the localized high rainfall on 27 and 28 June 2008. In order to better resolve the orographic effect on the production of rainfall over southwestern Taiwan, nested grids were used at horizontal resolutions of 27km, 9km, and 3 km. All domain were comprised of 31 vertical levels from the surface to 50 hPa. The moisture processes included the subgrid-scale convective parementerization of Grell in Domains 1 and 2 and the grid-resolvable WSM 6-class microphysics scheme. Planetary boundary layer processes were represented by the YSU PBL paramenterization (Hong et al. 2006). The model was initialized and lateral boundary conditions derived from the ECMWF/TOGA analyses at 0800LST (0000 UTC) 26 June 2008 and integrated for 66h. In order to examine the Taiwan orographic effects on the occurrence of the heavy rainfall over southwestern Taiwan, we performed two sensitivity test without Taiwan’s topography (here after abbreviated to NT) and with the decrease of height of topography to half of those in the control run (here after abbreviated to HC) on 9 and 3-km grid spacing simulations, while keeping everything else identical to the control run. The procedure of the setup of topography in the NT is similar to that in Chiao et al. (2004). In order to investigate the effect of the cool air on the enhancement of rainfall over lowlands over southwestern Taiwan, a sensitivity experiment without the evaporation of raindrops (here after abbreviated to NEV) is performed.
Simulation results confirm our hypothesis that orographic effects offer an important role on generating localized heavy rainfall episode in southwestern Taiwan in late June during post-Mei-Yu period.

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