慢速移動颱風之海表面溫度冷卻-潭美颱風(2018)快速減弱

Abstract

熱帶氣旋的快速增強以及快速減弱對於現行的颱風作業預報都是極大的挑戰。有許多原因可能導致熱帶氣旋的快速減弱，然而因海氣交互作用導致的快速減弱以及伴隨之熱帶氣旋內部結構變化卻缺乏深入探討。 本研究的主旨是想藉由2018年潭美颱風個案來分析其快速減弱的過程。從衛星觀測上，潭美颱風在移動速度緩慢期間造成顯著的海洋冷卻。數值實驗上，本研究透過WRF與3-D Price-Weller-Pinkel進行了三組控制組實驗，分別為考慮到三維海洋動力過程(C3D)、僅考慮到一維海洋動力過程(C1D)以及完全不考慮到任何海洋過程(UOC)。海洋方面，潭美颱風移動速度大幅下降至4 m s-1，導致颱風內核附近的海溫大幅下降。透過海洋溫度收支分析發現，移動速度緩慢期間，垂直平流作用為海洋溫度冷卻的主要原因，而混合與逸入作用讓海表面溫度的下降更有效率。如此低的海溫也大幅下降颱風內核的焓通量並且造成焓通量分布的不對稱。 大氣方面，快速減弱期間C3D實驗也出現了顯著的結構變化，颱風內核區域(1.5倍最大風半徑以內)的深對流大幅減少，眼牆大幅損失了非絕熱加熱，最終導致颱風快速減弱。海溫大幅下降引發颱風內核的焓通量下降也使颱風內核邊界層變的穩定，進而形成穩定邊界層。穩定邊界層抑制了颱風內核對流的發展，並且也能從T-PARCII (Tropical cyclones-Pacific Asian Research Campaign for Improvement of Intensity estimations/forecasts)投落送觀測資料發現穩定邊界層的存在。 海洋動力項敏感性實驗結果顯示，缺乏考慮到壓力梯度力的實驗，海洋冷卻受更強的湧升作用變得更加顯著，進而導致比C3D實驗更顯著的減弱。暖渦敏感性實驗結果顯示，有著更深厚混合層的暖渦，可以使因流切逸入與湧升作用導致的海洋冷卻時間延後，進而延後了颱風近中心最大風速降低的時間。 整體而言，潭美颱風快速減弱的過程與原因可能如下：1)潭美颱風移動速度緩慢引發海洋強烈湧升作用，進而導致颱風內核海溫下降；2)颱風內核海溫下降進而導致焓通量減少，限制了颱風內核來自海洋的能量供給；3)焓通量減少導致近地面的相當位溫與虛位溫下降，使邊界層內的大氣變穩定；4)穩定邊界層的形成抑制颱風內核對流的發展，並且大幅降低眼牆之對流加熱，最終導致快速減弱。

Rapid intensification (RI) and rapid weakening (RW) of tropical cyclones (TCs) are challenges for operational forecasting. A number of factors are involved in TC’s RW, however, air-sea interaction in RW and its influence on TC’s inner-core structure remains less addressed. The objectives of this work are to investigate the rapid weakening (RW) processes of Typhoon Trami (2018) in which the satellite data documented a substantial SST cooling along its passage. This cold wake and Trami’s RW occurred as the storm was moving at very slow translation speed. Three numerical experiments are performed in this study: 1) An uncoupled simulation (UOC); 2) a 3-D ocean-coupled simulation, produced by the WRF coupled with the 3-D Price-Weller-Pinkel ocean model (C3D); and 3) a 1-D ocean-coupled experiment, created by the WRF coupled with a 1-D ocean model (C1D). From the ocean responses, Trami’s translation speed is below 4 m s-1, resulting in extreme SST cooling in the inner core. By analyzing the ocean temperature budget, we identify that the vertical advection (i.e., upwelling) is the main reason of ocean temperature cooling, while the mixing and entrainment processes help to efficiently cool down SST. With the extremely low SST, the enthalpy flux dramatically decreases and becomes more asymmetric. From the atmospheric responses, a marked structure change of Trami was found in C3D experiment during the RW stage, in which the convective clouds (CC) and convective burst (CB) within 1.5*RMW (radius of maximum wind) in C3D experiment dramatically decrease, resulting in the loss of diabatic heating and leading to the TC weakening. The enthalpy flux dramatically decreases in the inner core due to the SST cooling during the RW period, while the stable boundary layer (SBL) is formed in the TC’s inner-core region. The expanding SBL coverage stabilizes the atmosphere and suppresses the convection in the inner-core, leading to the storm weakening. SBL is also identified in our analyses of the inner-core dropsonde data from the field program of T-PARCII (Tropical cyclones-Pacific Asian Research Campaign for Improvement of Intensity estimations/forecasts). Sensitivity experiments for different processes of ocean dynamics and ocean warm eddy are conducted. For ocean dynamics sensitivity experiments, we find the experiment without pressure gradient force results in the most extreme cooling due to stronger upwelling effect and leads to stronger weakening processes than C3D experiement. For ocean warm eddy sensitivity experiments, we find thicker mixed layer depth can postpone SST ccoling from entrainment and upwelling effect and lead to the delay of maximum surface wind decrease. In summary, this study presents the following processes of rapid weakening for Typhoon Trami (2018): 1) low translation speed enhances inner core SST cooling of Trami by strong upwelling effect ,2) strong SST cooling in Trami’s inner core leads to a great loss of surface enthalpy fluxes and limit energy supply from ocean ,3) low surface enthalpy fluxes reduce low-level equivalent temperature or virtual potential temperature and stabilize the boundary layer and 4) SBL limits the development of convection and shuts down the release of diabatic heating in inner core.