- 期刊
武夷山地形和海洋邊界層在梅雨鋒面南下過程中扮演的角色
The Impact of the Mountain Wui-I and Marine Plantary Boundary Layer on a Mei-Yu Front
摘要
本研究採用Penn State Univ./NCAR所發展之MM5模式，針對1998年梅雨季豪雨實驗IOP 4（6月4日至6月5日）個案進行數值模擬研究，探討武夷山地形和海洋邊界層在梅雨鋒面南下過程中可能扮演的角色。模擬結果顯示，梅雨鋒面在武夷山東側形成局部大風速區及冷舌結構。此一冷舌結構有利於鋒生過程而增加跨越鋒面方向的相當位溫梯度，並進一步使得鋒面在台灣海峽北部呈現『界面』的特性，此時鋒面帶的位置也就是輻合帶的位置。模式中鋒面的發展高度大約自900至650 hPa，鋒面在900 hPa以上隨高度的增加而向冷空氣的一方傾斜，這和一般鋒面的結構是一致的。900 hPa以下鋒面則受到邊界層內潛熱通量傳送的影響而被破壞，並且在鋒後冷空氣區域形成另一個大的相當位溫梯度，此一相當位溫梯度和邊界層頂所在的位置一致，並且隨高度向暖空氣一方傾斜。此一現象意味著鋒面界面的舉升機制在900 hPa以上才可以發生作用，而此時若直接由地面的空氣塊舉升來估計對流可用位能，勢將因此而產生顯著的偏差。另外，近地層東北風受到地形的阻擋效應而在武夷山東側形成一高壓脊，其往下游方向的氣壓梯度力則形成沿著地形一側發展的大風速區。因此當鋒面靠近台灣海峽時，低層動量場有可能因為武夷山地形的阻擋作用而大幅領先850 hPa質量場所在的位置。在這情形下，中尺度對流系統的激發機制將取決於近地層大風速區前緣輻合線的位置，而不是850 hPa鋒面所在的位置。
並列摘要
The MM5 nonhydrostatic model developed by Penn State Univ./NCAR was used to investigate the impact of the Mt. Wu-I on a Mei-Yu front during IOP4 in the Mei-Yu heavy rainfall EXperiment in 1998(MYEX98). The simulation results showed that the local northeast wind maximum and cold tongue were formed along the coastal line east of the Mt. Wu-I. The cold tongue was favorable to the frontogensis processes and then enhanced the temperature contrast across front. Thus the frontal exhibited as a material interface which was favorable to trigger the mesoscale convective system. The slope of the front was opposite in vertical. The front tilted to the cold air side above 900 hPa but to the warm air side below 900 hPa. It was mainly due to the latent heat flux transport in the marine planetary boundary layer, which played the roles on warming and moistening the frontal structure below 900 hPa. The blocking effect of Mt. Wu-I resulted in the formation of the local wind maximum as the front moving approach Taiwan Strait. The local wind maximum was ahead of the position of the front on 850 hPa by 100km, and the convergence of its leading edge might serve as an indicator for triggering the convective system.