| Summary: | 碩士 === 國立中央大學 === 大氣物理研究所 === 100 === Under weakly synoptic weather conditions, the occurring of convective lines are often observed over the eastern sea surface of Taiwan in the winter. These convective lines are oriented northeast – southwest or south – north direction and approximately parallel to the coastline. Additionally, these lines usually develop near the coast (<40 km) but they could develop off the coast (>40 km) occasionally. The main purpose of the study is to discuss the main formation mechanism of convective lines which developed far from the coast. Whether the orographic effects resulted in terrain returned airflow or barrier jet will be investigated in this study. Since, the lacking of observation data over the ocean area increases the difficulty of observational study, so this study aims to investigate these questions by high-resolution numerical model.
This study conducts simulation by WRFV 3.2.1 (Weather Research and Forecasting) numerical model and validates with different observation data. Four nested domains were used with horizontal resolutions of 40.5, 13.5, 4.5 and 1.5 km, respectively. Three different types of case are chosen, the first one is the case Jan. 3 in 2004, the second is the case Dec. 11 in 2006 and the third is the case Oct. 29 in 2008. The formation mechanism of the first case (200401) is that the airflow in the upstream was blocked by mountains so that barrier jet formed on the west side of convective lines, and resulted in stronger northly winds. Since, there are north-easterly winds prevailed in the east side of convective lines; these two different directions of flows collide and converge about 50 km off the coast, to stimulate initiation of the convective lines. The formation mechanism of the second case (200612) is blocking of prevailing winds by mountains so that prevailing winds couldn’t across mountains. Therefore, there are terrain returned airflows and terrain around flows (south-westerly winds) forming in front of mountains. These two different terrain induced flows could formation of converge with large scale prevailing wind in the low level off the coast, and lead to the convective lines. The formation mechanism of the third case (200810) is the local circulation during the night time and the large scale prevailing wind which are two reverse flows converge near the coastal area. Additionally, regional wind shear convergence enhance the development of mid-level convective line near the coast of Cheng Kung in the 850 hPa.
This study also designs the sensitivity test besides the control simulation with real terrain of Taiwan (referred to as CTRL), we conducted simulations with reduced terrain of 50% (TER50) and 0% (TER00) of the real terrain. The sensitive test shows that when the terrain height is reduced, the blocking of terrain is not obvious, and part of airflows will cross mountains. Therefore, the reduced orographic effects caused the offshore flow weaker than CTRL; the forming position of convective lines near the coast , and the life cycle is shorter. This sensitive test could confirm one of the key factors to the control formation and development of convective lines is terrain height.
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