低層平均流場對颱風初期發展之影響

The Influences of Low-Level Mean Flow on the Early Development of Tropical Cyclone

郭昱德(Yu-Te Kuo)

指導教授 ： 李清勝

國立臺灣大學/理學院/大氣科學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202003478

摘要

熱帶氣旋(TC)形成後的初期發展過程，受到許多環境因素之影響。本研究之觀測分析結果顯示，在具垂直風切之環境下，較大的環境低層平均流場(low-level mean flow, LMF）可能會抑制TC的發展趨勢；但若LMF小於6 m s-1，則其對TC發展的影響並不明顯。
利用WRF進行的理想實驗結果亦顯示，在具有垂直風切的環境中、較大的LMF將影響TC的初期發展速率。當環境的垂直風切相同時，若LMF較弱、則模式中的TC仍可發展為較強的系統；但若LMF較大(12 m s-1)、則模式中之TC將逐漸減弱。分析實驗結果顯示，在較大LMF的實驗中，中層渦漩與低層中心明顯分離，TC無法重組其結構並逐漸減弱；然而，造成此現象的機制，則因LMF與垂直風切方向的不同而改變。在LMF與垂直風切反向的實驗中，邊界層空氣與高對流層大氣的風速、風向之差異小於其他組實驗，TC內核區的垂直風切較不易受對流活動之影響而減弱，故此實驗TC受垂直風切的干擾、較其他組實驗更為明顯，而不利TC之增強。另一方面，在LMF和垂直風切同向的實驗中，LMF引起的低層相當位溫之不對稱分布，使對流在TC外圍顯著發展，因而減弱TC中心附近之角動量輻合，而不利於TC之增強。
本研究並利用ERA5再分析資料，分析兩個 MF較大的颱風個案之發展情形；分別為LMF與垂直風切反向的法西颱風(2019)、及LMF與垂直風切同向的桑達颱風(2016)。法西颱風的高層非旋轉風沒有特定的輻散方向，即此颱風之對流較不易改變其周圍之垂直風切；此結果與理想模式者相符。桑達颱風之環境垂直風為7 m s-1，大於颱風內核區者 (4.9 m s-1)，且高層之非旋轉風方向與垂直風切相反；顯示在此環境下，較容易透過對流活動減弱TC內核區的垂直風切，降低垂直風切對TC發展之影響。

關鍵字

熱帶氣旋 ； 垂直風切 ； 平均流場

並列摘要

The early development of tropical cyclones (TCs) is affected by multiple
environmental factors. The observation and analysis results of this study show that in an environment with moderate vertical wind shear, a large low-level mean flow (LMF) may affect the developing trend of a TC. However, the effect of LMF on TC’s development is not obvious if the environmental LMF is less than 6 m s-1.
Idealized WRF experiments also show that a large LMF will affect the early development of a TC, under an environment with moderate vertical wind shear. Under such an environment, the model TC in each experiment can develop into a strong system if the LMF is weak. However, the model TC will gradually weaken if the LMF is large (12 m s-1). With the mid-level vortex being sheared apart from the low-level center, the model TC with large LMF is unable to organize its structure and weaken gradually. Mechanisms of these phenomena are different depending on the directions of LMF and vertical wind shear. In the experiment in which the LMF and the vertical wind shear are in opposite directions, the momentum difference between boundary layer and upper-level is smaller than those of other experiments. Therefore, the convective activity is less able to reduce the vertical wind shear at the inner core region; the TC in this simulation thus is less likely to intensify. On the other hand, in the experiment with the LMF and the vertical wind shear having the same direction, the LMF generates asymmetric low-level equivalent potential temperature distribution, fueling the convection to develop at TC’s outer region, which is unfavorable for the TC’s development.
On the other hand, this study analyzes the development of two typhoons using the ERA5 reanalysis data to verify the above hypothesis. For typhoon Faxai (2019), with the LMF and the environmental vertical wind shear in opposite directions, the upper-level irrotational (divergent) winds have no specific direction. The convection of Faxai thus is less likely to reduce the vertical wind shear at TC’s inner core region, which is consistent with the results of idealized experiments. Typhoon Songda (2016), which has the LMF and the environmental vertical wind shear in the same direction, has a weaker vertical wind shear (4.9 m s-1) over its inner core region than that of its environment (7 m s-1). The direction of upper-level irrotational wind is opposite to that of the vertical wind shear, showing that convection is likely to have reduced the vertical wind shear at TC’s inner core region. Therefore, the influence of vertical wind shear on TC development is reduced under such an environment.

並列關鍵字

tropical cyclone ； vertical wind shear ； mean flow

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