中等風切環境下颱風強度的演變-低層環境風風向的角色

Intensity Change of Tropical Cyclone Embedded in Moderate-Sheared Environment: The Role of the Low-Level Flow Direction

李宗勇(Tsung-Yung Lee)

指導教授 ： 吳俊傑

國立臺灣大學/理學院/大氣科學研究所/碩士(2019年)

https://doi.org/10.6342/NTU201902608

摘要

垂直風切被視為限制颱風強度發展的重要環境因子，並在過去研究多用深層風切（Deep-Layer Shear；DLS）代表環境垂直風切。然而，相較於弱風切以及強風切環境，作業用模式對於中等DLS環境（10-20 knots）下以及最大風速達到70 knots以上之颱風，強度預報誤差較大。因此，了解在中等DLS環境下，颱風在不同背景流場下的發展，對於颱風強度預報十分重要。
過去觀測結果顯示低層環境風向對於颱風強度亦扮演重要角色。本研究透過高解析度WRF理想模式，並在控制組實驗最大地表風速達到70 knots時植入背景流場，以檢驗低層環境風向在中等風切下對颱風強度的影響。敏感性實驗結果顯示，改變低層環境風向會改變海表熱通量與邊界層相當位溫距平的分佈。而所有實驗組在背景流場植入之後的前12小時，皆先增強後減弱，並在第12小時之後，強度演變呈現分歧。當低層風與DLS指向相反，增強速度較快但增強時期較短；當低層風與DLS指向相同，增強速度則較慢而增強時期較長；當低層風指向風切左側時，渦旋強度較弱、增強速度則較慢且增強時期較短。透過內核結構分析發現，增強中的渦旋其眼牆回波、位渦、中高層暖心結構軸對稱度皆上升。較軸對稱的眼牆結構能提供最大風速半徑之內較多非絕熱加熱量，較有效於建立濕熵徑向梯度。以非軸對稱觀點分析，下風切左側的海表熱通量與上風切處邊界層內相當位溫增加將有利於渦旋增強。而位於上風切左側與下風切右側之眼牆對流活動則對於軸對稱眼牆之建立十分重要，特別是抑制通風效應以及提供較軸對稱的非絕熱加熱。因此，軸對稱眼牆之建立對於風切環境下渦旋增強扮演重要角色。另外，本研究亦發現，最大風速半徑之外的雨帶活動不利於渦旋增強。

關鍵字

颱風強度 ； 中等深層風切 ； 低層風風向 ； 軸對稱度 ； 海表熱通量

並列摘要

Environmental vertical wind shear is generally considered to be a factor inhibiting tropical cyclone (TC) intensification. Operational forecast errors for intensity are significantly higher for hurricane-stength TCs (maximum wind speed ≥ 70 knots) embedded in an environment with moderate deep-layer shear (DLS; around 10-20 knots) than for those in weaker or stronger shear. Therefore, it is important to consider the impact of the background flow on TC intensity in a moderately-sheared environment.
From the observational standpoint, the low-level flow direction is shown to be important for TC intensification. In this study, high-resolution idealized simulations using the Advanced Research Weather Research and Forecasting (ARW-WRF) model are conducted to examine the impact of low-level flow direction on intensity change. The sheared background flow is superimposed onto the control simulation when the maximum surface wind speed exceeds 70 knots. Analyses from the sensitivity experiments show that the change of low-level wind direction lead to the change of the distribution of the surface heat flux anomalies and the purtabation of boundary layer equivalent potential temperature. All of the members weaken during the first 12 h, then followed by very different intensity evolutions. The members with counter-shear-oriented (shear-oriented) low-level flow can reintensify faster (slower) but with shorter (longer) intensification period. On the other hand, left-of-shear-oriented low-level flow can lead to a weaker vortex. It is shown that the inner core structure including eyewall reflectivity, potential vorticity and upper-level warm core are more axisymmetric for the intensifying members than the weakening members. The more axisymmetric eyewall provides more diabatic heating inside the radius of maximum wind (RMW), thus favorable for the increase of radial gradient of moist entropy and intensification. Higher surface fluxes in the downshear-left quadrant and larger boundary-layer equivalent potential temperature at upshear region are more favorable to intensification. The enhanced upshear-left and downshear-right convection results in more axisymmetric eyewall structure and suppresses the ventilation effect. Both of these two effects are favorable for TC intensification, while the rainband outside the RMW is unfavorable for TC intensification.

並列關鍵字

tropical cyclone intensity ； moderate vertical wind shear ； low-level flow direction ； axisymmitricity ； surface heat flux

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