中等垂直風切下熱帶氣旋結構演變與初始強度的關係

Abstract

熱帶氣旋（TCs）在垂直風切（VWS）下的演變一直是天氣預報中的重要議題。在本研究中，VWS指的是深層風切，即200 hPa與850 hPa之間環境平均流場的差異，通常被認為對熱帶氣旋的發展具有不利影響。這種環境影響對氣象機構預報構成重大挑戰。先前的研究通常將影響熱帶氣旋的VWS分為三種類型：弱、中等和強烈VWS。其中，遭遇中等VWS的熱帶氣旋是最難預測的（Rios-Berrios and Torn 2017; Lee et al. 2021）。 近期關於熱帶氣旋在VWS下的研究主要集中在雷達觀測分析和理想模型研究。大多數觀測分析針對的是主要颶風，而使用理想模型的研究通常採用冷啟動方法，即將一個理想渦旋置於初始帶有風切的環境中，這些研究較少探討環境VWS變化對熱帶氣旋的影響，除了Finocchio和Rios-Berrios（2021）主要針對強烈VWS下主要颶風的演變進行分析外，對於弱熱帶氣旋在中等VWS變化中的研究仍存在空白。本研究旨在透過時變點降尺度方法（TVPDS; Onderlinde and Nolan 2017）填補這一空白。這項技術能在天氣研究與預報模型（WRF）中模擬隨時間變化的垂直風切背景流場。使用這種方法，我們先在無垂直風切的環境下模擬一理想渦旋的正常發展及增強，並選取三個不同的強度時間點（TD、TS和TY）作為實驗模擬的初始強度。這些實驗在兩種不同大小的VWS（7.5和10 ms⁻¹）下進行，以探究它們在遭遇中等VWS時的發展。 結果顯示，兩個較弱熱帶氣旋（TD和TS）的實驗組在遭遇VWS後，渦旋表現出了明顯的渦旋傾斜和渦旋進動（vortex precession）。然而，初始強度較強的實驗顯示出較小的渦旋傾斜和相對較弱實驗更短的渦旋進動週期。與先前研究結果一致（Rios-Berrios et al. 2018; Fischer et al. 2023），進動持續時間（24至48小時）明顯短於之前冷啟動的理想模擬研究所觀察到的（48至96小時）。基於位渦收支分析，我們發現渦旋進動和渦旋傾斜減少是由兩個渦度最大值相互環繞並合併引起的，這與Rios-Berrios et al.（2018）的分析部分吻合。同時，渦旋傾斜的減少主要由低層渦度中心（LLC）的位移主導，而非中層渦度中心（MLC）。一個是由MLC周圍的對流活動生成，另一個是由新渦度最大值提供的位渦流入維持的原始LLC。相比之下，較強熱帶氣旋（TY）的實驗組在整合VWS後對渦旋的影響明顯較小。雖然它們的風速演變與較弱實驗組別一樣都有發展停滯的情形發生，但發展停滯的狀況是主要由風切所造成的乾空氣入侵所引起。VWS越強，我們發現乾空氣的入侵效應也越強。 整體而言，我們的初步發現突顯了初始帶有垂直風切的平均流場下的渦旋發展與初始無風切渦旋遭遇垂直風切環境時的渦旋發展之間的差異。渦旋進動的持續時長以及低層渦度中心（LLC）在驅動渦旋傾斜減少和整體渦旋運動中的主導角色，是影響熱帶氣旋在VWS下演變的關鍵因素。未來的研究應基於不同環境條件，進一步探討熱帶氣旋在VWS下的不確定性，並深入分析乾空氣入侵與渦旋合併的機制。

The evolution of tropical cyclones (TCs) under vertical wind shear (VWS) has long been an important issue in weather forecasting. VWS, in this study, refers to the deep-layer shear, which is the difference in environmental mean flow between 200 hPa and 850 hPa, mostly regarded to be detrimental to TCs. This environmental influence poses significant challenges to operational forecasts. Previous studies, in general, categorized the VWS exerted on TCs into three types: weak, moderate, and strong VWS. Among these, TCs encountering moderate VWS are the most challenging to predict (Rios-Berrios and Torn 2017; Lee et al. 2021). Recent studies on TCs under VWS have primarily focused on radar observation analysis and idealized dynamical model runs. Most observations have been used to inspect major hurricanes, while idealized modelling studies often employ a cold start approach, which involves placing an ideal vortex in an initially sheared environment. These approaches are short of investigations into TCs changing environmental VWS, except for the work by Finocchio and Rios-Berrios (2021), which mainly examined the evolution of major TCs under strong VWS. This leaves a gap in the study of weak TCs experiencing moderate VWS changes. This study aims to compensate by employing the time-varying point-down-scaling method (Onderlinde and Nolan 2017), a technique that can implement time-varying and vertically sheared background mean flow in an idealized environment in the Weather Research and Forecasting model (WRF). Using this approach, we initiate an unsheared idealized TC and select three different intensity time stamps (TD, TS, and TY) as the initial intensities for the numerical experiments. These experiments are conducted under two different magnitudes of VWS (7.5 and 10 ms-1) to investigate their development when encountering moderate VWS. The results show that the two experiments with weaker TCs (TD and TS) underwent vortex precession with significant vortex tilt after the integration of VWS. However, the experiments with stronger initial intensity exhibit smaller vortex tilt and shorter vortex precession period relative to the weaker ones. Consistent with results from previous studies (Rios-Berrios et al. 2018; Fischer et al. 2023), the duration of the precession (24 – 48 hrs) is noticeably shorter than those observed in former idealized cold-start studies (48 – 96 hrs). Based on the potential vorticity budget, we found that the vortex precession and vortex tilt reduction is caused by the two vorticity maxima orbiting and merging with each other, which partially agrees with Rios-Berrios et al. (2018). Meanwhile, the tilt reduction is primarily contributed by the displacement of the low-level vorticity center (LLC), rather than the mid-level vorticity center. One of the vorticity maximums is generated by the convective activities around MLC, and the other one stems from the original LLC sustained by the PV inflow from the new vorticity maximum. The stronger initial intensity can also make a difference in the tilt reduction process via a stronger magnitude of the PV inflow. In contrast, the experiments with a slightly stronger TC (TY) show a much lesser impact on the vortex after the integration of VWS. Although their intensification is delayed similarly to the weak experiments, this behavior is primarily attributed to dry air intrusion in our analysis. It has been observed that stronger vertical wind shear (VWS) is associated with more intense dry air intrusion. Overall, our initial findings highlight the differences between vortex development under initially sheared mean flow and when an initially unsheared vortex encounters a vertically sheared environment. The duration of vortex precession and the dominant role of the low-level vorticity center (LLC) in driving vortex tilt reduction and overall vortex motion are also crucial factors in investigating the tilt reduction process. As to the ongoing work, studies based on different environments are needed to investigate the uncertainty of TCs under VWS. The mechanism of the dry air intrusion and vortex merging should also be further investigated.