西北太平洋熱帶氣旋生成之多尺度分析

An analysis of multi-scale nature of tropical cyclogenesis over the western North Pacific in June 2004

青麟(Lin Ching)

指導教授 ： 隋中興 ； 楊明仁

國立中央大學/地球科學學院/大氣物理研究所/博士(2011年)

摘要

2004年六月有破紀錄的五個熱帶氣旋(TC)生成在西北太平洋(WNP)，此異常的六月處在聖嬰發展階段，並且受到一個MJO對流相位所影響，聖嬰發展期的暖海溫距平和MJO的對流加熱導致赤道東風減弱，並在WNP產生大尺度的氣旋式環流距平，且伴隨MJO而有許多活耀的熱帶波動活動。我們對向外長波輻射(OLR)和低層(850hPa)風場做波數-頻率波譜分析方法，分解出MJO、Rossby波和MRG-TD波，其中MJO和氣候背景在WNP產生加熱並加強低層氣旋環流，會影響赤道Rossby波和MRG-TD波的發展、傳播與能量傳遞。因此，在多重尺度波動貢獻的有利環境下，使得2004年六月有異常多的TC生成。
為了進一步瞭解多重尺度波動跟熱帶氣旋生成的過程，我們將渦度場跟風場分解成低頻波動場和高頻波動場部分，低頻波動場中含五個部份即氣候尺度(Cli)、年際變化尺度(IAV)、季內震盪尺度(MJO)、赤道Rossby波(ER)、以及混合Rossby重力波和氣旋波動擾動(MRG-TD)，在此五個波段以外的部分即為高頻波動場。經過尺度分離後，低頻波動場能夠清楚表現出大尺度環境場的狀態，而高頻波動場則可保留高頻擾動的特性。接著，我們計算跟隨TC移動座標(TC-following coordinate)之渦度收支來探討高低頻波動跟TC間的交互作用。其中渦度收支中的餘差項(R)可以解釋成無法解析的高頻變化，主要包含了小尺度的對流。之後，將渦度收支中的貢獻項分成平衡動力和非線性動力兩部分：平衡動力項包含了渦度方程中低頻場的輻合項(D1)和傾斜項(T1)、高低頻場互乘的輻合項(D23)和傾斜項(T23)；非線性動力項則包含了高頻場的輻合項(DD)和傾斜項(TD)及表示對流過程貢獻的餘差項。
透過渦度收支計算，顯示TC A生成的主要貢獻為平衡動力過程，由一東風波移進季風槽，進而產生的高低頻波動間線性交互作用(D23項)。TC B生成的主要貢獻是透過非線性過程，由一幾乎靜止的局部擾動發展而成(R項)。TC C生成於MJO在WNP區域活耀相位時，其生成始於一東風波擾動在季風風切帶內傳播進而發展成熱帶氣旋，擾動的快速發展(R項)以及低頻波動在季風風切帶的貢獻都影響著TC C的生成過程。TC D生成的主要是由低頻波動所貢獻(D1項)，擾動隨著低頻波動所產生的季風渦旋往西北移動、集中並發展成熱帶氣旋。TC E生成時受到季風合流的影響，因此生成過程主要是透過平衡動力過程。綜合以上結果，在2004年生成的5個TC中有4個的生成過程都主要與平衡動力項有關，顯示低頻波動的貢獻和高低頻波動間的交互作用在2004年六月提供了有利TC生成的環境條件，且低頻波動的貢獻(D1)在TC C、D、E生成過程中都佔很大比重，即可對應六月中下旬的低頻波動特別活耀現象。而非線性動力項則在渦度快速增加時居最大項，表示對流尺度和次網格尺度在TC快速發展時的主導作用。總言之，我們透過尺度分離將高低頻波動場分離，可以清楚地辨識出大尺度背景場和擾動場個別的特性，而跟隨TC渦度中心的渦度收支可以量化高低頻波動跟熱帶氣旋生成間的交互作用，藉以了解不同尺度波動與TC生成的關係。

關鍵字

熱帶氣旋 ； 渦度收支 ； 熱帶波動

並列摘要

Record-breaking five tropical cyclones (TCs) formed in June 2004 in the western North Pacific (WNP) where June is normally a transition month to the typhoon season and therefore sensitive to climate oscillations. This special month (June 2004) was an unusual period in the developing stage of a warm (El Nino) episode and a strong convective phase of the Madden-Julian oscillation (MJO). Such climate background is shown to provide large-scale favorable circulations for TC formation: the warm sea surface temperature anomalies (SSTAs) associated with developing El Nino and convective heating of the MJO to jointly induce weaker easterly trade winds and a large-scale cyclonic circulation anomaly in the WNP. A space-time filtering of the outgoing longwave radiation (OLR) and 850-hPa wind fields is performed to identify the MJO, Rossby waves and mixed Rossby-gravity (MRG) waves (or tropical depression (TD)-type disturbances). From the evolution and structure of these high-frequency waves in relation to that of the MJO and the climate background, the heating and enhanced low-level cyclonic flow in the WNP associated with the MJO and climate background are attributed to the initiation, propagation and energy dispersion of tropical Rossby and MRG-TD waves, interacting with convection.
In order to examine the relationship between multi-scale waves and TC formation, the vorticity and wind fields were separated into low-frequency and high-frequency variations by space-time filtering. The low-frequency variation included five scale variables: climatology (Cli), interannual variability (IAV), intraseasonal variability (ISV), equatorial Rossby wave (ER), and mixed Rossby-gravity wave and TD-type disturbance (MRGTD). The remaining component was classified to the high-frequency variation. The scale separation help us to clarify the roles of low-frequency and high-frequency variations on TC formation. Then, the vorticity budget on TC-following coordinate was calculated to examine the interaction between waves and TC. Therefore, the generation terms in vorticity budget can be separated into the balanced dynamics (BD) and nonlinear dynamics (ND). The balanced dynamics included the low-frequency waves interaction and the low- and high-frequency waves interaction. Nonlinear dynamics indicated the nonlinear processes of the convective scale and sub-scale variations. The dominated terms in vorticity budget were the balanced dynamics for four TCs in June 2004, which means that the contributions of low-frequency waves and the low- and high-frequency waves interaction created large-scale favorable environmental conditions for TC genesis. The large contributions of low-frequency waves in TC C, D, and E corresponded to the the active waves in mid- and late June. The vorticity budget can identify the relation between low- and high-frequency waves and TC, which can help us to understand the interaction between waves and TC genesis.

並列關鍵字

tropical cyclongenesis ； tropical waves ； vorticity budget

參考文獻

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