副熱帶西北太平洋春季冷空氣爆發之能量收支與渦動通量

Abstract

東亞春季是冬季斜壓系統活躍期與東亞夏季季風建立前的轉換季節。在氣候背景環流轉換的影響下，鋒後海氣能量交換及海洋邊界層（MBL）內的能量收支可能與冬季寒潮事件有所不同。本研究探討2025年4月12–15日發生於西北太平洋的春季冷空氣爆發事件，著重分析MBL內的能量收支與渦動通量。 本研究利用新海研一號研究船（R/V New Ocean Researcher 1, NOR1）、日本石垣島及南大東島的探空觀測建立三角形分析區域，並透過WRF模擬探討能量收支及渦動通量的時間演變。探空估算結果顯示，在2025年4月14日0000 UTC，鋒後濕靜能（moist static energy, MSE）收支主要受水平平流與渦動通量輻合所控制。地表渦動通量約為50 W m⁻²，而地表至850 hPa層的渦動通量輻合垂直積分約為170 W m⁻²，顯示春季個案的邊界層能量交換較先前冬季寒潮研究為弱。 WRF模擬結果顯示，鋒面通過後近地面水平平流迅速減弱，由4月13日1200 UTC的 −270 W m⁻²，在一天內減弱至 −15 W m⁻²。同時，近地面渦動通量輻合由 +210 W m⁻² 減弱至4月14日0000 UTC的 +120 W m⁻²，之後維持在約 +120 W m⁻² 附近波動。隨著水平平流逐漸減弱，中緯度斜壓系統所伴隨的下沉運動使垂直平流的重要性逐漸增加。在分析時間後期，MBL上方的乾空氣與MBL內相對濕潤的空氣於邊界層頂附近形成強烈的垂直水氣梯度，進而造成約 −120 W m⁻² 的顯著負向垂直水氣平流。 WRF模擬進一步顯示，邊界層頂由4月14日0000 UTC的925–850 hPa層逐漸下降至4月14日1200 UTC的地表–925 hPa層，與石垣島及南大東島的探空觀測結果一致。此現象與先前冬季寒潮研究形成鮮明對比；在冬季個案中，較強的海氣能量交換通常伴隨較深厚的海洋邊界層及較高的邊界層頂。 研究結果顯示，春季冷空氣爆發期間的能量收支及邊界層演變，主要受到鋒後北風較弱且持續時間較短的影響。隨著水平MSE平流快速減弱，垂直MSE平流與渦動通量輻合於鋒面通過約24小時後成為MSE收支的主要貢獻項。此外，較弱的渦動通量輻合與較弱的亂流混合可能限制了邊界層的加深，使鋒後下沉運動得以壓低邊界層頂。 未來研究若能將相同的能量收支架構應用於鋒前環境，並進一步分析不同區域與季節的冷空氣爆發個案，將有助於判斷本研究所提出的機制是否具有代表性。後續研究也需要進一步釐清鋒後下沉運動與海表渦動通量的相對重要性，如何共同影響邊界層深度及MBL內低雲的形成。這些分析將可透過耦合海氣模式模擬、直接亂流觀測，以及更多探空觀測資料獲得進一步驗證。

Spring in East Asia is a transition season between active wintertime baroclinic systems and the onset of the East Asian summer monsoon. Under the changing seasonal background circulation, postfrontal air–sea energy exchange and the energy budget within the marine boundary layer (MBL) may differ from those during winter cold-surge events. This study investigates a springtime cold-air outbreak over the western North Pacific during 12–15 April 2025, focusing on the energy budget and eddy flux within the MBL. Radiosonde observations from the R/V New Ocean Researcher 1 (NOR1), Ishigakijima, and Minamidaitojima were used to construct a triangular analysis domain, and a WRF simulation was conducted to examine the temporal evolution of the energy budget and eddy flux. The radiosonde-derived budget showed that, at 0000 UTC 14 April, the postfrontal moist static energy (MSE) balance was mainly controlled by horizontal advection and the convergence of eddy flux. The surface flux was approximately 50 W m⁻², and the layer-integrated convergence of eddy flux within the surface–850 hPa layer was approximately 170 W m⁻², indicating weaker energy exchange in MBL than in previous winter cold-surge studies. WRF results showed that near-surface horizontal advection weakened rapidly after frontal passage, changing from −270 W m⁻² at 1200 UTC 13 April to −15 W m⁻² within one day. Meanwhile, near-surface convergence of eddy flux weakened from +210 W m⁻² to +120 W m⁻² by 0000 UTC 14 April and subsequently fluctuated around +120 W m⁻². As horizontal advection weakened, subsidence associated with the midlatitude baroclinic system made vertical advection increasingly important. During the later stage, dry air above the MBL and relatively moist air within it produced a strong vertical moisture gradient near the MBL top, resulting in pronounced negative vertical moisture advection of approximately −120 W m⁻². The WRF simulation further showed that the MBL top gradually descended from the 925–850 hPa layer at 0000 UTC 14 April to the surface–925 hPa layer by 1200 UTC 14 April, consistent with the soundings from Ishigakijima and Minamidaitojima. This behavior contrasts with previous wintertime cold-surge studies, in which stronger air–sea energy exchange was associated with a deeper MBL. These results suggest that the energy budget and MBL evolution during springtime cold-air outbreaks are regulated by the weaker and shorter-lived postfrontal northerly flow. As horizontal MSE advection weakened rapidly, vertical MSE advection and eddy flux convergence became major contributors to the MSE budget approximately 24 hours after frontal passage. In addition, weaker eddy flux convergence and weaker turbulent mixing may have limited MBL deepening, allowing postfrontal subsidence to lower the MBL top. Future studies applying the same energy-budget framework to prefrontal conditions and to additional cold-air outbreak cases from different regions and seasons would help determine whether the mechanism identified in this study is representative of similar environments. Further work is also needed to clarify how the relative importance of postfrontal subsidence and surface flux controls MBL depth and low-cloud formation within the MBL. Such analyses would benefit from coupled ocean–atmosphere simulations, direct turbulence observations, and additional radiosonde measurements.