| dc.description.abstract | 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. | en_US |