長生命期對流系統與西北太平洋季風槽間的多重尺度交互作用：從衛星觀測資料的觀點

Abstract

本研究透過追蹤衛星降水技術統計對流系統的生命期及其結構特徵，探討長生命期對流系統與西北太平洋季風槽的關係。我們提出長生命期系統的數量與季風槽的強度存在正回饋機制的假設：季風槽提供低層渦度促使更多的渦度熱塔形成，透過渦度熱塔的合併得以延長對流系統的生命期；而長生命期系統可以透過在中層大氣的非絕熱加熱使低層位渦增加，並進一步透過跨尺度過程來維持季風槽的強度。為了驗證上述假設，我們首先結合多衛星反演的網格化降水資料（IMERG）、向日葵八號（Himawari-8）觀測的亮度溫度、以及歐洲中期天氣預報中心的第五版再分析資料（ERA5）定義出在2001到2019年間16個季風槽事件中的對流系統及潛在渦度熱塔結構，並且統計其生命期、降水、系統尺寸大小在季風槽發生時的變化。結果顯示，相較於季風槽建立前的時期，在季風槽建立後，生命期在兩天以上的長生命期對流系統數量增加了兩倍，且水平尺度在500公里以上的系統其發生機率有明顯提升；透過數量及尺寸的增加讓長生命期系統的降水貢獻從16.9%提升至34.7%，而降水熱區主要集中在菲律賓東北側向西南延伸的區域，與季風槽發生區域高度重疊。具有潛在渦度熱塔結構的長生命期系統在季風槽建立後的數量增加最為明顯，而短生命期或是對流結構不同的長生命期系統的增加則較少。上述統計結果確認了季風槽建立後長生命期系統數量的增加與渦度熱塔的增加有關，未來希望可以進一步檢驗渦度熱塔的合併與對流系統生命期的關係，並探討不同生命期對流系統的非絕熱作用對於底層位渦的效果，以及維持季風槽強度的跨尺度過程，以完成正回饋機制的驗證。

This study investigates the relationship between the long-lived convective systems and the northwest Pacific monsoon trough by tracking the satellite precipitation product to analyze the lifetime and the convective structure of the convective systems. Here we hypothesize positive feedback between the formation of the long-lived systems and the maintenance of the monsoon trough intensity. The positive vorticity provided by the monsoon trough promotes the formation of vortical hot towers (VHTs), and the merging of the VHTs enhances the lifetime of the convective systems. The long-lived systems can increase the low-level potential vorticity through diabatic heating at the mid-level troposphere. The increase of the low-level potential vorticity might maintain the intensity of the monsoon trough possibly through the upscale process. To verify the above hypothesis, the precipitation from Integrated Multi-satellitE Retrievals for GPM, the brightness temperature from Himawari-8, and the vorticity fields from the ECMWF reanalysis version 5 are used to identify the convective systems and the potential vortical hot tower (PVHT) structures in 16 monsoon trough events from 2001 to 2019. By contrasting the 20-day periods before and after the establishment of the monsoon trough, the number of the long-lived (≥ 2 days) systems increases by 2 folds; among them, the systems with horizontal sizes larger than 500 km increase most significantly. The precipitation contribution of the long-lived system grows from 16.9% to 34.7% owing to both the increasing system number and the horizontal size, and their precipitation hotspots aggregate significantly from the northeast of the Philippines extending to the southeast, which is highly consistent with the location of the monsoon trough. The long-lived systems with PVHTs structures have the largest increase in number after the monsoon trough establishes, compared to the systems with shorter lifetimes or with different structures. The current confirms that, when the monsoon trough is present, the increasing number of long-lived systems is related to the increasing number of PVHTs. In the future, the relationship between the merging of the VHTs and the systems’ lifetimes will be investigated, and the effects of increasing low-level potential vorticity from diabatic heating released by different systems and the upscale process of maintenance of the monsoon trough will be further examined.