寒潮對南海邊界層結構、能量收支與紊流的影響

Abstract

東亞寒潮是東亞冬季風的常見綜觀天氣系統。寒潮發生時，低層北風增強進入南海並增強熱帶對流，低層的冷、乾平流增強南海的海表熱通量，混合作用增強、邊界層頂上升。但是，比起北大西洋、東太平洋區域，南海的邊界層相關研究較少，以致我們無法準確描述南海邊界層的紊流混合、往赤道的能量傳輸、經向環流。此研究中，我們使用2010年到2020年的南海北部的東沙島(116.69E, 20.70N)高解析度探空資料來判定邊界層頂、混合層頂、雲頂、雲底。我們亦使用再分析資料，如ERA-5的氣象參數和海表熱通量，以及MERRA-2雲輻射等，以及CloudSat-CALIPSO和ISCCP等衛星資料來判定雲種及雲輻射效應。再利用上述資料計算能量收支和診斷紊流垂直結構。結果顯示，寒潮發生時，為了平衡低層冷乾平流，南海北部邊界層內獲得更大的海表熱通量，混合作用增強，混合層平均提高到1公里，邊界層平均提高到2公里，邊界層內有淺對流發展，邊界層頂則有下沉運動抑制雲的垂直發展。簡言之，寒潮發生後的邊界層為條件性穩定、混合層為條件性不穩定，寒潮發生前的狀況則相反。南海邊界層分析對台灣和周邊國家天氣與氣候預報非常重要，我們希望能藉由這份研究，引起大家對南海邊界的注意與興趣，也希望未來能有大氣海洋聯合觀測。

Episodic cold surges associated with the East Asia (EA) winter monsoon can penetrate deep into the South China Sea (SCS), enhance consequent tropical rainfall, and further strengthen the EA meridional overturning circulation (MOC). These cold surges can promote strong surface fluxes and lead to a deeper marine planetary boundary layer (PBL). However, there is a lack of boundary layer studies over the SCS, unlike many other well-studied regions such as the north Atlantic Ocean and the central-eastern Pacific Ocean. The disparity can lead to unrealistic boundary layer turbulence and energy transport such that the tropical convection and the EA-MOC are incorrectly represented. In this study, we use high resolution radiosonde data of temperature and humidity profiles over Dongsha Island to identify the PBL height (PBLH), mixed layer height (MLH), cloud base, and cloud top for the period of December-January-February (DJF) from 2010 to 2020. We combined ERA-5 meteorological variables and surface fluxes, MERRA-2 cloud radiation data, and radiosonde-derived PBL parameters to perform an energy budget analysis and turbulent diagnostics based on a mixed-layer model. Here we show a strong turbulent flux convergence of both heat and moisture over the SCS during cold surges, which leads to a lifting of the MLH to ~1.0 km and PBLH to ~2.0 km and associated cloud development over Dongsha Island (116.69E, 20.70N). The cold and dry horizontal advection is balanced by this vertical turbulent flux convergence in the energy budget. Overall, at post-surge the PBL is stable but mixed layer is unstable, which contrasts with the pre-surge stage that features a stable mixed layer and a conditionally unstable PBL. We anticipate our study will motivate more atmosphere-ocean joint observation and PBL-related studies over the SCS region that is important for understanding and predicting weather and climate in Taiwan and SCS neighboring countries.