| 摘要: | 本研究使用2017/18到2021/22，共5年冬季(12~2月)的剖風儀資料，研究冬季西北臺灣和東沙島低層噴流(Low-Level Jets, LLJs)的統計特徵。LLJs通常分為兩類：(1)邊界層急流(Boundary Layer Jet, BLJ)和(2)綜觀系統低層噴流(Synoptic system–related Low-Level Jet, SLLJ)。若BLJ和SLLJ同時發生則被稱為double LLJs(DLLJs)。定義LLJ-dominated日為一天中LLJ發生超過6小時的日子。本研究亦以再分析資料對LLJ-dominated日進行綜觀環境場分析，並探討其底層風場日變化及垂直變化之機制。統計結果顯示，冬季西北臺灣與東沙島皆有較多的日子被BLJ主導。西北臺灣BLJ在高度約500公尺處出現頻率最高，伴隨較強風速(約16 m/s)。發生頻率呈現夜間和清晨高、下午較低的明顯日變化。自BLJ日的角度，大氣底層風速同具明顯日變化，可能與太陽輻射加熱造成的近地面亂流混合及夜晚近地表輻射冷卻相關。東沙島BLJ發生高度較低，發生頻率之日變化趨勢與西北臺灣相似，但變化幅度稍小，差異可能源自東沙島具較小的表面摩擦及白天地表加熱效應較小。綜觀環境場分析中，西北臺灣BLJ日有較深的西風槽和較強的風速。在850 hPa和950 hPa層有勢力較強的大陸冷高壓和阿留申低壓使東亞沿海風速增加。臺灣北部海面底層風場分析則顯示，實際風於主要貢獻項的地轉風再疊加非地轉風的共同作用下，並無顯著日變化。另外在BLJ日，垂直剖面分析發現，邊界層內，975 hPa以下(上)，屬於次(超)地轉風。比較梅雨季和冬季的低層噴流特徵，發現西北臺灣冬季BLJ發生高度較低，但發生頻率的日變化趨勢相似，兩時期均在夜間至清晨有最大風速。東沙島冬季BLJ多出現在夜間與清晨且高度較低，而梅雨季BLJ則在中午最為明顯。此外，冬季與梅雨季BLJ的風速日變化幅度、亂流動能和垂直風切皆顯示出太陽輻射加熱在梅雨季比冬季更加顯著。;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. |
