季內振盪發展與熱帶波動、濕化、對流輻射過程之診斷分析

Abstract

本研究利用ERA-Interim再分析與衛星觀測資料診斷低層水氣與整層濕靜能收支，以探討濕化過程與對流輻射反饋過程在季內振盪不同相位所扮演角色，並著重比較印度洋與海洋大陸之區域差異。為進一步探討前一個季內振盪的存在對下一個季內振盪生成與東移之影響，本研究進一步將季內振盪區分為successive與primary兩種類型，在1982-2011年北半球冬季期間分別挑選出22與5個東移訊號明顯的強季內振盪。研究結果顯示前一個季內振盪對流抑制區所引發之水氣平流與增強之輻射冷卻使環境之不穩定度增加，提供有利於下一個季內振盪對流發展之條件。 更進一步將successive類型的季內振盪生命期分成四個階段：對流抑制相位、對流成長相位、對流相位、對流消散相位。在對流抑制相位，大尺度沉降運動抑制對流發展高度，此時非降水性淺對流扮演水氣垂直傳送的角色，透過再蒸發將邊界層水氣往中低對流層傳送，提供有利對流發展之環境。另外，前一個季內振盪對流抑制區所產生之季內尺度東風將暖池區較濕的空氣帶往印度洋，有利於對流在印度洋生成。在對流成長相位，主要之濕化過程除上述之東風水氣濕平流外，還有對流中心東側因邊界層磨擦產生之水氣輻合，兩者在對流區與其東側提供不穩定度，有利對流成長與東移。在對流相位，對流區潛熱釋放所產生之季內尺度西風與Rossby環流將西印度洋與熱帶外較乾之空氣帶入熱帶印度洋，使大氣變乾不利對流發展；對流區強降水為另一有效將水氣從大氣移除之機制。此時，前一個季內振盪對流抑制區東移至海洋大陸，其西側(對流區之東側)往兩極之Rossby環流將赤道地區較濕之水氣往南北兩側傳送，濕化對流區東側大氣，有利季內振盪由印度洋傳至海洋大陸。在對流消散相位，季內尺度西風增強，其水氣乾平流使對流逐漸消散。整體而言，海洋大陸地區主要之濕化過程與印度洋一致，惟在海洋大陸南北向水氣平流之重要性提升。 整層濕靜能收支診斷顯示，季內尺度輻射變化主要由長波輻射隨雲量與雲高之變化主導，對流相位減弱之輻射冷卻增加整層濕靜能，有利於季內尺度濕靜能之維持。對流消散相位增強之西風與熱帶印度洋背景西風同向，增強海表蒸發，此為另一維持季內尺度濕靜能之過程。

The moistening processes for the Madden-Julian Oscillation (MJO) over Indian Ocean and Maritime Continent are investigated through a diagnosis of ECMWF Re-analysis (ERA-Interim) data in November-April, 1982-2011. During this period, 27 MJO events with strong magnitude and clear propagation are identified and further classified as either successive or primary, according to the existence of preceding events. The successive events are analyzed in the current study, whereas the primary events will be explored individually next. A composite of scale-separated lower-tropospheric (1000-700 hPa) moisture (qL) budget is analyzed in four stages: suppressed, cloud developing, convective, and decaying, each corresponding to the MJO Bimodal index phase 567, 81, 2, and 34 for Indian Ocean and 781, 23, 4, 56 for Maritime Continent, respectively. In the suppressed stage, the dominant moisture source over both region is surface evaporation/shallow convection (-Q2). Nonlinear zonal (meridional) advection by synoptic disturbances also has non-negligible contribution over IO (MC). In the cloud developing stage, qL approaches maximum with moistening tendency to its east. This moistening is contributed by the advection of mean moisture by anomalous easterlies associated with downstream Rossby wave response of the dry anomaly and boundary layer frictional moisture convergence. In the convective stage, while the zonal advection of anomalous westerlies and intense precipitation dries the atmosphere, the moistening of meridional advection by downstream Rossby anti-cyclonic gyres leads to the eastward propagation of deep convection. In the decaying stage, the strong westerlies bring in dry air from the west causing widespread drying. Overall, the moisture evolution of MC is consistent with IO except meridional component is more essential in suppressed stage. A column-integrated moist static energy (MSE) budget is also analyzed to further identify the role of radiation and surface turbulent fluxes. The result shows that longwave heating is the dominant term in convective stage and latent heat flux is more prominent in decaying stage when the westerly is strong. The in-phase relation of longwave heating with column-integrated MSE suggests that longwave heating acts to maintain MSE and retard the propagation. Latent heat flux also slows down the propagation due to the phase lag.