| Abstract: | 使用499 MHz的剖風儀資料，來研究2018~2020年梅雨期間(5/16~6/15)低層噴流(low-level jet，LLJ)在北台灣與東沙島的特徵。北台灣發生LLJ的頻率在垂直上有明顯峰值，出現於500~1000 m，其平均風速為17 m/s。此外，也發現400 m以上的LLJ主要是吹西南風；而400m以下的LLJ大多是吹東北風，這與梅雨鋒面通過台灣有關。從晝夜變化看，夜間北台灣底層發生LLJ頻率較高，這是因白天太陽加熱垂直混合變強，導致低層發生LLJ數減少，而夜間近地表輻射冷卻及西海岸的氣壓差增大，導致低層發生LLJ數增多。東沙島發生LLJ的頻率在垂直上有兩個明顯的峰值，分別於600 m和900 m，其平均風速皆為15m/s，並且風向多為盛行季風所引起的西南風。分析東沙島LLJ發生頻率的兩個峰值之晝夜變化，發現上層峰值最大發生頻率在中午和午夜，其日夜變化與綜觀氣壓梯度及非地轉風的大小及方向有關；而下層峰值最大發生頻率在傍晚，與綜觀氣壓梯度及日落後形成的穩定邊界層有關。
定義LLJ日為一天中發生LLJ的小時數超過6小時的日子。北台灣發生LLJ日時，北台灣位於鋒面系統前緣，底層風速極值出現於台灣西北測。東沙島發生LLJ日時，鋒面系統位於台灣上空，底層風速極值出現於台灣東南側及南海。定義邊界層噴流(boundary layer jet, BLJ)為LLJ發生在1000 m以下者，而與綜觀天氣系統有關的低層噴流(synoptic system-related low-level jet, SLLJ)為LLJ發生在1000 m以上者。北台灣及東沙島發生SLLJ日時，其台灣上游水氣來源皆主要來自中國華南沿海至南海的邊界層內；北台灣及東沙島發生BLJ日時，其台灣上游水氣分別主要源自中國華南沿海的邊界層內和南海的邊界層內。北台灣發生強BLJ事件的當下，台灣平均時雨量極值發生於新竹至台中的西北沿海和南部山區；而北台灣發生強SLLJ事件的同一段時間，台灣平均時雨量極值發生於桃園至新竹的西北沿海。東沙島發生強BLJ事件的當下，台灣平均時雨量極值發生於苗栗的西北沿海至山區和西南沿海至山區；東沙島發生強SLLJ事件的時候，台灣則沒有明顯的降水。
;The 499 MHz wind profiler is used to study the characteristics of the low-level jet (LLJ) over northern Taiwan and Dongsha island during the mei-yu season (5/16~6/15) of 2018~2020. On the vertical structure, the LLJ frequency over northern Taiwan has an apparent peak, appearing at 500-1000 m, and the average wind speed is 17 m/s. In addition, the LLJ above 400m is mainly southwesterly wind caused by the prevailing monsoon. The LLJ below 400m is mostly northerly winds, which are caused by the mei-yu front passing through Taiwan. From the diurnal structure, the LLJ frequency over northern Taiwan is higher at night because of solar heating on the surface during the daytime, radiation cooling on the surface at night, the air pressure difference on the west coast. On the vertical structure, the LLJ frequency over Dongsha island has two peaks, which appear at 600 m and 900 m respectively, and the average wind speed is 15 m/s. The LLJ is mainly southwesterly wind caused by the prevailing monsoon. During the study period, the LLJ frequency is 21 %. According to the two peaks appearing in the vertical structure, we try to analyze its diurnal structure. The upper peak of LLJ frequency maximum appears at noon and midnight, and the diurnal variation is related to the sub-synoptic scale pressure gradient and the ageostrophic wind. The bottom peak of LLJ frequency maximum appears in the evening, which is related to the sub-synoptic scale pressure gradient and the stable boundary layer formed just after sunset.
The LLJ day is defined as LLJ that occurs more than 6 hours in a day. On the LLJ day of northern Taiwan, the bottom wind speed extreme appears on the northwest side of Taiwan, and Taiwan locates at the front edge of the frontal system. On the LLJ day of Dongsha island, the extreme bottom wind speed appears on the southeast side of Taiwan and the South China Sea, and the frontal system locates over Taiwan. The boundary layer jet (BLJ) is defined as LLJ occurs below 1000 m, and the synoptic system-related low-level jet (SLLJ) is defined as LLJ occurs above 1000 m. On the SLLJ day of northern Taiwan and SLLJ day of Dongsha island, water vapor transportation mainly comes from the coastal South China to the South China Sea in the boundary layer. On the BLJ day of northern Taiwan and BLJ day of Dongsha island, water vapor transportation mainly comes from the coastal South China and the South China Sea in the boundary layer, respectively. When the strong BLJ events occurred in northern Taiwan, the average hourly extreme rainfall happened in the southern mountains and the northwest coast from Hsinchu to Taichung. When the strong SLLJ events occurred in northern Taiwan, the average hourly extreme rainfall happened on the northwest coast from Taoyuan to Hsinchu. When the strong BLJ events occurred on Dongsha island, the average hourly extreme rainfall happened on the southwest coast to the mountains and the northwest coast of Miaoli to the mountains. When the strong SLLJ events occurred on Dongsha island, there was no significant precipitation in Taiwan. |
