碩士 / 國立中央大學 / 大氣物理研究所 / 101 / Under weakly and relatively stable synoptic weather conditions, convective lines can be observed by radar observation at east coast of Taiwan, which orientation are usually parallel with the coast line. In previous studies, the convective lines can be divided into nearshore and offshore type. The nearshore type mainly cause by land breeze convergence with prevail wind (Yu and Jou 2005). The prevail wind blocked by steep terrain, generate terrain return flow, terrain split flow and barrier jet. Which converge with prevail wind induced offshore convective lines(Yu and Hsieh 2009；Alpers et al 2010). The intensity and offshore distance of convective lines are related with terrain height and slope. However, the previous studies are focus in dynamic mechanism by topographic effect, but rarely discuss the thermodynamic effect like air-sea interaction by Kuroshio’s high SST (Yu and Hsieh 2009).
This study selected a dual convective lines case which forming off the coast of Yilan in 2012/02/01. This case can be separate with 4 stages and has many features rarely seen in the past case studies like: feathery like structure extending seaward in stage 2, nearshore and offshore type of convective line exist in same time in stage 3. Since insufficient observations over ocean, we use WRF 3.3.1 model to simulate this case. Model results show that prevail winds of convective line were northeasterly in formative stage, and northeasterly change to easterly respectively at southern part of convective line. These prevail wind interacted with the Central Mountain Range (CMR) and induced terrain split flow and barrier jet. Which converged with prevail wind induced offshore convective line. The locally horizontal wind direction changes and converged with prevail winds induced the feathery like structure. In stage 3, the northeasterly monsoon strengthened and split by northeast part of Taiwan terrain. A strong wind belt along eastern coast converged with northeasterly inflow to induce the nearshore convective line. The cross-section analysis indicated the land breeze making the northeasterly monsoon inaccessible to the Yilan plain, help the convective line forming at coast. With the northeasterly monsoon further enhanced, the nearshore convective line moved to the southeast offshore, and finally dissipated.
This study design three of sensitivity experiments: terrain height reduced, heat flux test and SST change. In terrain height reduced experiment, the intensity and offshore distance of convective line reduced by setting the terrain height half in stage 1 and stage 2. This shows the topography blocking the inflow. But in the stage 3 and stage 4, the intensity and offshore distance of convective line did not change so much in half terrain experiment. By set the terrain height to 0, we can see the northeasterly monsoon converged with easterly flow in stage 3 and stage 4, but the convective line did not form along coast. This shows that topography affects the northeasterly monsoon and resulted in flow splitting and changes the orientation of convective line. If we turn off the sensible heat flux, the land breeze disappeared and the nearshore convective line did not form in stage 3, then the intensity of convective line weakened because of potential instability had been reduced. When reduce SST, the convective lines weakened because of the convective instability reduced. The SST distribution will also change the formation and development of convective line. Summery these results, the sensible heat flux of sea surface associated with high SST of Kuroshio play an important role for the formation and development of convective lines.
| Identifer | oai:union.ndltd.org:TW/101NCU05021037 |
| Date | January 2013 |
| Creators | Chien-Chuan Chan, 詹前烜 |
| Contributors | Pay-Liam Lin, 林沛練 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 108 |
Page generated in 0.0122 seconds