雲輻射回饋在熱帶氣旋生成過程中的角色

Abstract

先前研究透過理想模擬指出，雲輻射回饋可以加速熱帶氣旋(tropical cyclones, TCs)初期階段的發展。本研究的目標在使用Clouds and the Earth's Radiant Energy System (CERES)與CloudSat衛星觀測，搭配ERA5再分析資料，了解熱帶擾動中雲輻射加熱的空間結構與雲輻射回饋過程，並透過比較可發展(developing, DEV)與無法發展(non-developing, NDEV)為TC的系統，評估雲輻射回饋在TC生成過程中的角色。 CERES的合成結果及column-integrated moist static energy (CMSE) variance收支指出，渦旋中心的雲長波輻射加熱，為渦旋對流組織過程之最大正回饋來源。CloudSat的合成結果及Sawyer-Eliassen (SE)次環流診斷則指出，雲長波輻射加熱可驅動中低層次環流反應，將低層的高濕淨能(moist static energy, MSE)輸往中心，利於中心附近的對流組織。雲短波加熱則驅動出高層次環流，將中層低MSE引入內核而造成渦旋結構的日夜變化。在DEV系統初期階段，中心附近較多的長波雲輻射加熱，可促進對流組織並加強次環流，有利於渦旋增強及TC的生成。而DEV系統中心較活躍的對流活動，則與較濕的環境、較高的潛在強度與較低的垂直風切有關。然而初期輻射差異與後續系統強度改變之間的因果關係目前仍無法確立，有待未來進一步設計理想實驗驗證。

Idealized model simulations in previous studies have shown that cloud radiative feedback can accelerate the development of tropical cyclones (TCs) in their early stages. In this study, we utilize satellite observations from Clouds and the Earth's Radiant Energy System (CERES) and CloudSat, in conjunction with ERA5 reanalysis, to examine the spatial structure of cloud radiative heating in tropical disturbances, and to assess the role of cloud radiative feedback in TC genesis through the comparison between developing (DEV) and non-developing (NDEV) systems in their early stages. The results from CERES composites and column-integrated moist static energy (CMSE) variance budget show that, cloud longwave feedback is the dominant term in convective organization. On the other hand, the results from CloudSat composites and Sawyer-Eliassen (SE) diagnosis demonstrate that, longwave trapped by clouds drives a mid- to low-level secondary circulation, transporting low-level high moist static energy (MSE) into the inner core and favoring the vortex development; while upper-level heating caused by cloud absorption drives an upper-level circulation, leading to the import of mid-level dry air and the diurnal oscillation of vortex structure. The results further show that, the more vigorous convective activity near center in the early stage of DEV systems allows themselves to receive more radiative energy from clouds in the inner core, which can reinforce the secondary circulation and benefit the development of the vortex. The vigorous convection in DEV systems is shown to be related to moister environment, higher potential intensity (PI), and weaker vertical wind shear (VWS). However, the causality between radiation and intensity change cannot be fully validated in this study, and require further verification through model simulations with suitable experimental designs.