| dc.description.abstract | This study conducted an analysis of wind roses in the Yilan region during the autumn and winter seasons from 2011 to 2021, covering a total of 44 months. The investigation focused on the variations in land and sea breezes in the southern and western areas of the Lan-Yang Plain (LYP) during both daytime and nighttime. The results of the wind rose analysis indicate that the northern part of the LYP is primarily influenced by the northeast monsoon, while the southern part exhibits a noticeable westerly wind component even during the daytime. This highlights that the wind field distribution in Yilan is influenced by the unique topography of the region.
To understand the three-dimensional circulation in the Yilan region, this study analyzed observational data from the 2020 Yilan Experiment for Severe Rainfall (YESR2020). Based on daily accumulated rainfall, the study categorized three cases: the first intense rainfall event on November 21, a weak rainfall event on November 22, and the second intense rainfall event on November 23. The research explored variations in local circulation under the influence of topography, characteristics of rainfall distribution, and the mechanisms leading to heavy rainfall. In the synoptic environmental context, the weak rainfall event is primarily influenced by a weak east wind. The upstream atmospheric environment is relatively stable with lower moisture flux, resulting in no daytime rainfall in the Yilan. Additionally, there is a noticeable sea-land breeze circulation. In contrast, the two intense rainfall events exhibit unstable and significantly high moisture flux. The northeast monsoon transports moisture to the Yilan, leading to rainfall in the southern mountainous region of the LYP. In addition, there is also a convective system off the eastern coast of Taiwan that moves into Yilan with the northeast monsoon, causing Yilan to receive heavy rainfall. According to the observation data of YESR2020, it is found that the surface wind fields of these two heavy rainfall are different in Yilan. Especially in the second heavy rainfall cases, there is a westerly component in front of the mountainous area in the south, and the surface wind field has a wind direction discontinuity zone. This phenomenon was not found in the first heavy rainfall cases. In addition, the rainfall intensity between the two is also different. The rainfall intensity and extreme rainfall values of the first heavy rainfall cases are greater than those of the second heavy rainfall cases.
This study uses the Weather Research and Forecast Model (WRF) to simulate the first heavy rainfall case on November 21 to understand the generation and development process of heavy rainfall in Yilan. Through the simulation results, we found that the reasons for the heavy rainfall intensity in Yilan that day can be summarized as the following points. First, the northeast monsoon produces rainfall due to orographic lifting on the windward side of the southern mountainous areas. At the same time, the southerly winds of the upper atmosphere stationary the precipitation system in this area, causing continuous rainfall in the southern mountainous areas. Secondly, the rainbands from the eastern sea of Taiwan move into the Yilan with the northeast monsoon, intensifying the rainfall along the Suaou coast. Finally, as the overall environmental wind field changed, a convergent rainband of cyclonic circulation appeared in the eastern sea of Taiwan. This circulation interacted with Yilan′s topography, again causing precipitation in the southern mountainous and coastal areas. It can be seen that the topography of Yilan has a significant impact on the formation and maintenance of this heavy rainfall case. | en_US |