| dc.description.abstract | In this study, we utilized 5-year wind profiler radar data from the winters of 2017/18 to 2021/22 to investigate the characteristics of Low-level jets (LLJs) in northwestern Taiwan (NWT) and Dongsha island (DI) during winter seasons (December to February). The LLJ is typically classified into two types: (1) Boundary Layer Jets (BLJs) and (2) Synoptic system–related LLJs (SLLJs). The simultaneous occurrence of BLJ and SLLJ is termed double LLJs (DLLJs). An LLJ-dominated day (LLJ day) is defined as a day where LLJ that occurs for more than 6 hours. Reanalysis data were also used to analyze synoptic patterns on LLJ days and to investigate the mechanisms leading to the diurnal and vertical variations of the lower-level wind.
Statistical results showed that there are more BLJ-dominated days during the winter season over both NWT and DI. BLJs over NWT had the highest occurrence frequency at an altitude of ~ 500 m, accompanied by stronger wind speeds (~ 16 m/s), with a noticeable diurnal variation in occurrence frequency—higher at night and in the early morning, and lower in the afternoon. From the perspective of BLJ days, there was a clear diurnal variation in lower-level wind speeds, possibly related to the near-surface turbulent mixing induced by solar radiation heating and the nocturnal radiation cooling near-surface. BLJs over DI occurred at lower altitudes, with a diurnal variation trend similar to that over NWT, albeit with a smaller diurnal variation amplitude, possibly attributed to the smaller surface friction and less significant daytime surface heating effects over DI. In the synoptic patterns analysis, BLJ days over NWT exhibited deeper westerly trough and stronger wind speeds. The significant Siberian High and Aleutian Low at 850 hPa and 950 hPa levels contributed to increased wind speeds along the East Asian coast. Analysis of the lower-level wind field over the North Taiwan Offshore area indicated that the actual wind, influenced by the combined effects of geostrophic wind (primary) and ageostrophic wind (secondary), shows no significant diurnal variations. Additionally, on BLJ days, vertical profile analysis revealed that within the boundary layer, winds below (above) 975 hPa belonged to the subgeostrophic (supergeostrophic) wind. Comparing LLJs between the winter and the mei-yu season, it was found that winter BLJs over NWT occurred at lower heights, but their diurnal variation trends in occurrence frequency were similar, with peak wind speeds during the night and early morning. Winter BLJs over DI were more likely to occur at night and in the early morning, also at lower heights. In contrast, mei-yu season BLJs were most pronounced around noon. Furthermore, the diurnal variation amplitudes, turbulent kinetic energy, and vertical wind shear indicated that solar heating effect was more significant during the mei-yu season than in winter. | en_US |