| 摘要: | 2019年在科技部的資助下，我們總共發表了5篇期刊文章(http://mpipom.ihs.ncu.edu.tw/RecentPublications.php)。本計畫為下列兩篇已刊登之期刊論文之延續：2019a: An observational analysis of ocean surface waves in tropical cyclones in the western North Pacific Ocean與 2019b: Young ocean waves favor the rapid intensification of tropical cyclones - a global observational analysis。了解熱帶氣旋增強的條件，特別是快速增強，是個非常有挑戰性的議題。這個問題涉及到複雜的氣旋內部動力過程、環境條件和彼此之間的交互作用等等。然而，對於預報改善則是有益的。在期刊2018a中，我們研究了在西北太平洋熱帶氣旋中的海面波浪。在期刊2018b中，我們將分析擴展到全球熱帶氣旋，並且首次證明了海面波浪將透過交換係數與大氣低層水氣得幅合產生交互作來增強氣旋。我們分析了23年全球的觀測資料，包含超過16,000個熱帶氣旋強度變化、上層海洋特徵、海表面波浪與低層大氣的水氣幅合等。與前人研究不同，我們並沒有發現較厚的上層海洋與/或較溫暖的海溫和氣旋的快速增強有明顯相關。相反地，我們發現氣旋的快速增強與低層水氣幅合和海面年輕波浪的海氣交換係數有顯著的高相關性。這與前人使用模式模擬的結果一致，氣旋主要透過水氣與海表面通量的潛熱釋放來增強強度。在70%快速增強的氣旋中，其移動速度與受制的年輕波浪會產生共振，使得年輕波浪在氣旋的第二象限中變得飽和。年輕的波浪在氣旋中心會產生相對較少的混合和冷卻效果，進而增強氣旋。本計畫尋求進一步的資金來繼續我們的耦合海洋-颱風模擬和數據分析。 ;In 2019, MOST-funded research has resulted in a total of 5 papers; please visit http://mpipom.ihs.ncu.edu.tw/RecentPublications.php. This proposal will continue the work of two of these papers: Zhang & Oey 2019a: An observational analysis of ocean surface waves in tropical cyclones in the western North Pacific Ocean. Journal of Geophysical Research Oceans, 124, 184-195, https://doi.org/10.1029/2018JC014517; and Zhang & Oey, 2019b: Young ocean waves favor the rapid intensification of tropical cyclones - a global observational analysis. Monthly Weather Review, 147, 311-328, https://doi.org/10.1175/MWR-D-18-0214.1; they can be downloaded from http://mpipom.ihs.ncu.edu.tw/RecentPublications.php. Identifying the condition(s) of how tropical cyclones intensify, in particular rapid intensification, is challenging, due to the complexity of the problem involving internal dynamics, environments and mutual interactions; yet the benefit to improved forecasts may be rewarding. Ocean surface waves and sprays modulate surface heat and moisture fluxes in tropical cyclones, which in turn can alter storm’s intensification. In paper 2019a, we studied surface waves in western North Pacific tropical cyclones. In paper 2019b, we extend the analysis to global tropical cyclones and demonstrate, for the very first time in the literature, how surface wave-induced surface exchanges are coupled to atmospheric low-level moisture convergence to induce intensification. To make the analysis more tractable, we focus near the sea surface, by examining 23-year global observations comprising over 16,000 cases of tropical cyclone intensity change, together with upper-ocean features, surface waves and low-level atmospheric moisture convergence. Contrary to the popular misconception, we found no statistically significant evidence that thicker upper-ocean layers and/or warmer temperatures are conducive to rapid intensification. Instead, we found in storms undergoing rapid intensification significantly higher coincidence of low-level moisture convergence and a dimensionless air-sea exchange coefficient closely related to the youth of the surface waves under the storm. This finding is consistent with the previous modeling results, verified here using ensemble experiments, that higher coincidence of moisture and surface fluxes tends to correlate with intensification, through greater precipitation and heat release. The young waves grow to saturation in the right-front quadrant due to trapped-wave resonance for a group of Goldilocks cyclones that translate neither too slowly nor too quickly, which 70% of rapidly-intensifying storms belong. Young waves in rapidly-intensifying storms also produce relatively less (compared to the wind input) Stokes-induced mixing and cooling in the cyclone core. A reinforcing coupling between tropical cyclone wind and waves leading to rapid intensification is also proposed in the paper. This proposal seeks further funding to continue our coupled ocean-typhoon modeling and data analysis. |
