聖帕颱風模擬的位渦反演之診斷分析

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题名:	聖帕颱風模擬的位渦反演之診斷分析
作者:	劉豫臻;Yu-Jhen Liou
贡献者:	大氣物理研究所
关键词:	片斷位渦;位渦反演;piecewise potential vorticity;potential vorticity inversion
日期:	2009-07-10
上传时间:	2009-09-22 09:43:54 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	過去許多研究指出，位渦反演是良好的診斷工具，能研究特定天氣系統與現象，而Wang and Zhang(2003)發展出限制較少的位渦反演方法，因此能利用位渦反演法深入研究颱風內核結構，本研究將使用Wang and Zhang 所提出之位渦反演法，及片斷位渦概念對模擬之颱風進行診斷。想藉以瞭解颱風內部結構及非對稱平衡場和強度的關係。平衡流場的部分顯示可良好的反演原始颱風的流場，但在低層輻合和高層輻散會有比較大的誤差，低層主要是摩擦力從低層解析度較高的模式場內插到解析度較差的虛高座標，高層差異主要為較大的靜力穩定度和絕對渦度為正的假設。儘管有這些限制，結果顯示在較強及快速發展階段都能良好的反演出來。接著使用反演的平衡流場進一步計算準平衡流場，結果顯示，颱風的流場有很大的量是準平衡的，並將流場分成潛熱釋放、摩擦力過程、乾動力過程的貢獻。潛熱會造成下層輻合與上層輻散及眼牆的上升運動，摩擦力作用造成低層的輻合與眼中上升氣流類似 Ekman pumping，而乾動力過程則與風切作用有關，風切作用的次環流可減少大尺度環境風切的破壞性作用，但也可能與颱風流場的不對稱分布有關。為了研究非對稱環流分布，使用片斷位渦概念將內部颱風的位渦擾動分成眼牆正的位渦擾動、眼牆負的位渦擾動及眼的擾動。結果顯示去除眼的位渦擾動後，颱風眼牆環流的分布與大尺度環境駛流風場的方向有關，環境風方向的右(左)側大致上為負(正)的位渦擾動分布，反演出正(負)的非對稱壓力擾動及反氣旋(氣旋)擾動風場，而負的非對稱壓力擾動的分布則和颱風的垂直運動位置一致，三個片斷疊加後，在颱風中心擾動風場方向會與環境風一致。 Many studies demonstrated that potential vorticity (PV) inversion is a good diagnostic tool to study specific weather systems and phenomena. This study uses the PV inversion proposed by Wang and Zhang (2003) with piecewise PV diagnosis to investigate the dynamics of the simulated typhoon for understanding how the intensity of the typhoon is related to the internal structure and the asymmetric balanced flow. Our inversion shows that the balanced flow could well recover the characteristics of the original flow except for the low-level convergence and high-level divergence. They could be attributed respectively to the use of reduced vertical resolution at low levels and the assumptions of the required nonnegative absolute vorticity and larger static stability at high levels, respectively. In spite of these restrictions, the inversion results show that the original flow is well inverted at the strong and fast-developing stage of typhoon. The results show that quasi-balanced dynamics determines the typhoon flow to a large extent compared to the flow predicted by the model primitive equations. With the quasi-balanced flow calculated from PV inversion, the vertical motions can be attributed to different contributions from latent heating, friction, and the dry dynamic processes. The latent heating leads to low-level convergence, high-level divergence and vertical motion of eyewall. The friction causes the radial inflow in PBL to behave like the Ekman pumping. The dry dynamic processes are also influenced by the vertical shear. The shear-induced flow could reduce the destructive effects of the large-scale environmental shear but may be related to the asymmetric distribution of the flow. In order to study the asymmetric flow, the total PV perturbations are divided into positive PV and negative PV on the eyewall, and PV on the eye. It is shown that after removing the PV on the eye, the distribution of the inverted flow for each PV piece is related to the large-scale environmental steering flow. There are positive (negative) PV perturbations left (right) of the steering flow direction that inverts the negative (positive) pressure perturbation and cyclonic (anti-cyclonic) flow. The superimposed inverted flow from the three piecewise PVs inside the eyewall is approximately in the same direction of the steering flow, indicating that the outer PVs may not play an important role.
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