利用有效擴散率診斷綜觀尺度天氣系統

Abstract

本研究利用有效擴散率診斷槽線形狀窄化與變寬的過程、南極極渦波動破碎，以及大西洋的熱帶氣旋生成過程。有效擴散率的概念與邊界層動力的K理論是一致的，假想次網格次度的大氣運動和分子擴散行為類似，所以可以診斷次網格尺度的混合程度。利用非輻散的風場平流被動追蹤物質，根據被動追蹤物質的結構計算有效擴散率，有效擴散率愈大，表示大氣流場處於愈劇烈混合的狀態，愈小則表示混合愈微弱。由槽線的個案研究，顯示最大的有效擴散率出現在羅士比波破碎的區域，而核心渦旋的有效擴散率最小。由南極極渦的個案研究，顯示有效擴散率可以詮釋飛機觀測的臭氧濃度在很短的距離內有很大幅度的變化。 中層的相對溼度是熱帶氣旋生成的重要因素之一，近來研究指出有發展的熱帶擾動在隨著東風波移動的座標上，有氣旋式封閉流線結構，此結構又稱為育兒袋，育兒袋不僅有利於累積渦度，還可以保護濕對流，使其不易受到乾空氣逸入而消散。本研究用有效擴散率診斷中高層乾空氣逸入作用對大西洋的熱帶氣旋生成影響，診斷個案為2010年大西洋颱風季源自於非洲東風波的熱帶擾動。研究結果顯示在700hPa以上，有發展的熱帶擾動位於混合程度比較微弱的大氣流場，沒有發展的擾動則是位於劇烈混合的大氣流場，混合程度和有發展的相比可以達到5倍以上。有效擴散率配合衛星水氣影像，顯示沒發展的熱帶擾動在中高層有乾空氣逸入，而有發展的擾動幾乎沒有乾空氣逸入。由熱帶氣旋生成的個案研究，顯示有效擴散率分析和類似育兒袋熱帶氣旋生成理論是相輔相成的。本研究顯示有效擴散率配合衛星水氣影像，對於分析大西洋的熱帶氣旋生成是很有用的診斷工具。

The effective diffusivity diagnosis is used to obtain a basic insight into the two-dimensional transport and mixing properties of synoptic scale events such as trough thinning processes, wave breaking of Antarctic polar vortex and both a developing and non-developing disturbance during the 2010 Atlantic hurricane season. Effective diffusivity is used as a mixing diagnostic on a passive tracer subject to diffusion and advection by the non-divergent part of observed winds. Large/small effective diffusivity corresponds to strong/weak mixing. For the trough event, the filaments are regions of large effective diffusivity, which suggests strong mixing. For the Antarctic polar vortex, the effective diffusivity shows that strong mixing occurs at the edge of stratospheric polar vortex. This is in good qualitative agreements between effective diffusivity and measurements of chemical tracers taken by the NASA ER-2 aircraft. The tropical cyclone formation has long been recognized to form in regions of approximately circular flow. Recent studies showed that a region of recirculation, also called pouch, along a propagating Easterly wave occurred in all genesis cases. We use effective diffusivity as a diagnosis that can identify the transport and mixing properties around the tropical disturbances. The effective diffusivity diagnosis is used to evaluate the intrusion of dry air into Gaston, which did not re-develop into a tropical cyclone, and the intrusion of dry air into Karl, which did become a tropical cyclone and later a major hurricane. The northwestern region of Gaston shows a strong mixing region at 700hPa and 500hPa, allowing mixing dry air into Gaston to be quite effective. On the other hand, the Karl is in a weak mixing environment where the Karl is slightly affected by the dry air. The results show that the effective diffusivity diagnosis of mixing can be useful in understanding the tropical cyclogenesis.