| 摘要: | 本篇研究利用WRF模式模擬一颮線系統，首先探討其在通過一理想鐘形山脈(山頂高度兩公里)的過程中，颮線系統內水收支及降水效率如何受理想化地形的影響而變化。先在Eulerian framework 下進行討論，將颮線系統發展的過程分為成熟期、迎風坡、過山期、背風面及消散期等不同時期。結果發現，當颮線系統由成熟期演變至迎風坡及過山階段時，由於系統受到地形抬升的影響，水氣輻合及凝結率皆增強，使得降水效率從原來的50.42%增加至約58.71%；而背風面的下沉氣流使得水氣輻散增加並伴隨有強烈蒸發作用，導致降水效率下降，最後颮線系統逐漸減弱至消散。接著從Quasi-Lagrangian framework的角度進行討論，以每十分鐘一筆的模式資料追隨颮線系統運動以得到降水效率、凝結率、凝固率及蒸發率在瞬時的變化。結果指出降水效率及凝結率在迎風坡皆隨著時間增加，至背風面後快速下降，但由於山岳重力波向上傳輸的作用將水相粒子往更高層傳輸而有凝固現象，使得凝固率在背風面反而有增加的趨勢。若將討論範圍縮小至颮線系統的對流降水區域甚至是單一對流胞時，則此現象更為顯著。
然後進行將地形高度降低至原來高度一半(山頂高度一公里)及沒有地形的兩組敏感度實驗測試。在一公里地形高度的實驗中，由於地形降低使得颮線系統在迎風面受地形抬升影響而增強的作用較不明顯，降水效率增加的幅度也減少。但也因為地形的阻礙較小讓低層冷池得以過山，即使到背風面颮線系統的前緣仍可持續激發出新生對流胞，系統維持較長一段時間才消散。而在沒有地形的敏感度實驗中，則是因為沒有地形的抬升作用，因此於原來有地形實驗中的迎風坡位置，降水效率不但沒有隨時間而增加，反而隨著颮線系統的減弱而有逐漸下降之趨勢。
;In this study, idealized numerical simulations of a squall line traversing a sinusoidal mountain ridge are conducted using the Weather Research and Forcasting model, version 3.2, with 2-km horizontal grid size. The vapor and condensate budgets are examined, and the temporal variation of four microphysics ratios, including precipitation efficiency(PE), condensation ratio(CR), deposition ratio(DR), and evaporation ratio(ER) are calculated during and after the period when squall line interacts with the terrain.
In an Eulerian framework, the whole life cycle of the squall line can be divided into five stages, which include mature, over-windward-slope, over-mountain, over-lee-side and, dissipating stage. When the squall line moves from mature stage to over-windward-slope stage, the corresponding PE increases from 50.42% to 58.71%, due to the increasing horizontal flux convergence of vapor and strong condensation of liquid water. Then, a Quasi-Lagrangian framework is adopted to investigate the “in situ” orographic forcing of the mountain on the microphysics process by following the eastward propagation of the squall line. The result shows that the high PE observed on the windward slope is caused by the increase of cloud condensation and the orographic lifting. On the other hand, the low PE observed on the lee side is a result of strong increase of raindrop evaporation and the decrease of cloud condensation. The vertically propagating gravity waves above the terrain is helpful to transport hydrometeors upward and then let the hydrometeors transform into ice critical or snow, so the DR also shows an increasing trend on the lee side.
Two sensitivity experiments with different terrain height are performed to examine the effect of terrain on microphysics ratios. The half-terrain sensitivity experiment shows that because of the reduced orographic lifting effect, the condensation on the windward slope also decreases, which further results in lower PE. But the lower mountain height makes the blocking- effect occurred at mountain ridge become less significant, so the squall line can traverse the mountain ridge more smoothly and maintain a stronger convective system on the lee side compared to the full-terrain control run. Finally, the result from no-terrain sensitivity experiment shows that without the orographic lifting effect, all of the characteristics associated with the interaction between squall line and terrain disappear. |
