2017年6月1-2日梅雨鋒面雨帶降水與運動場特徵分析

Abstract

2017年6月1日夜間至2日清晨，一條梅雨鋒面自東海緩慢南下，進入臺灣陸地後於北部地區滯留近10小時，帶來超過600毫米的驚人降水。本研究透過地面測站、探空、雷達及EC再分析資料，解析鋒面雨帶的結構，並試圖找出造成雨帶滯留並給大臺北地區帶來超強降雨的可能原因。 依據鋒面的移動趨勢，本論文將梅雨鋒面雨帶分成三個時期：第一次南下期、準滯留期以及第二次南下期。(1)第一次南下期，雨帶面積最廣且回波強度大。於雨帶西側之對流結構較為垂直，伴隨鋒前對流併入主雨帶，有較強回波發展。雨帶進入陸地後移速減緩，同時回波強度逐漸減弱。(2)準滯留期，雨帶面積較小，對流結構呈傾斜，有利雨帶維持。鋒前對流於西北沿岸併入主雨帶，雨帶短暫再增強並於臺北盆地北側降下強降雨。在此期末 (1日2300 UTC至2日0040 UTC間)，雨帶自大屯山區北退至北海岸，此時雨帶面積增廣且回波增強。(3)第二次南下期，於2日0200至0300 UTC間，在桃園沿岸出現明顯對流胞合併情形，使雨帶在登陸後重新增強，並於臺北盆地內降下時雨量近100毫米的強降雨。 風場分析顯示，在第一次南下期與準滯留期間，鋒前低層噴流風速維持在25至28 m/s，強度無顯著變化。鋒後西北風強度分別在第一次南下期末期(約1日1800 UTC)與準滯留期末期(1日2300 UTC)減弱，前者可能導致雨帶停滯在大屯山區，後者則使雨帶重新北退至北海岸。而2日0000 UTC後伴隨強西北風自海峽進入北臺灣，導致雨帶於0100 UTC起再度南下，並在0200 UTC時重新進入大臺北。此時海峽上低層噴流與雪山山脈西側屏障噴流均增強至30 m/s，且伴隨強大輻合，有利雨帶再增強。這可能是第二次南下期，雨帶於臺北盆地內降下比第一次南下期與準滯留期更強降雨的主要原因。

From the night of June 1 to the early morning of June 2, 2017, a Meiyu front came south from the East China Sea and entered Taiwan area. The front stayed in the northern Taiwan for nearly 10 hours, bringing in more than 600 mm of staggering rainfall. In this study, the structure of the frontal rainband is analyzed through ground station, sounding, radar, and EC reanalysis data, and attempts to find out the possible causes of the rainband being trapped and bringing torrential heavy rainfall to Taipei. Based on the movement characteristics, this study divides the frontal rainband into three periods: the first period of going south, the quasi-stationary period, and the second period of going south. (1) In the first period of the southward movement, the rainband has the widest area and the intensity of the echo. The convection structure on the west side of the rainband was relatively vertical, and with the pre-frontal convection merged into the main rainband and produced strong echo. However, after entering land, the movement slowed down, and the echo intensity weakened. (2) Quasi-stationary period, the rainband possessed a smaller area and the convective structure was tilted to the north which is conducive to the maintenance of the rainband. The prefrontal convection was merged into the main rainband along the northwest coast, which briefly re-strengthened and received heavy rainfall on the northwest coast of Taiwan. At the end of this period, the rainband retreated and re-intensified and brought heavy rainfall to the area. (3) In the second period of the southward movement, the merge of rain cells along the coast was pronounced. The rainband re-intensified after landfall and with torrential rainfall. Dual-Doppler synthesized horizontal winds showed during the first southward period and the quasi-stationary period, the prefrontal low-level jet was maintained at 25-28 m/s, and the intensity did not change significantly. Weakened post-front northwesterly at the end of the first southward period and the quasi-stationary period may cause the rainband to stagnate in the Datun Mountains and later to retreat to the north of the coast. In third period, the low-level jets on the strait were enhanced to 30 m/s, and with strong convergence, it was conducive to the reinforcement of the rainband and brought heavy rainfall to the Taipei Basin.