冬季冷鋒個案之數值研究

A Numerical Study of a Winter-time Cold Front

簡方菁(Fang-Ching Chien) ； 林勝峰( Sheng-Feng Lin)

《大氣科學》 32卷2期 (2004/06) Pp. 141-160

摘要

本文使用MM5 模式針對1996年1月7日至9日的冬季冷鋒個案進行數值模擬，以研究其三維結構並探討台灣地形及地表/海面通量對其之影響。該冷鋒呈東北東—西南西走向之淺鋒面結構，鋒後冷空氣受到洋面之海面通量影響而產生變性，使低層呈現濕冷的環境。鋒面在抵達台灣後，導致溫度明顯的驟降。衛星雲圖顯示該鋒面通過台灣時產生變形，沿東、西岸南下，鋒面前緣出現索狀雲之細長結構。
中尺度模式 MM5 的模擬顯示，鋒面抵達台灣地區後，在北部迎風面受到地形的阻擋呈現滯留，其兩側沿著台灣東、西部南移。鋒後沿西部地形而向西偏轉之氣流在台灣西北部出現輻合並造成高壓脊，使此區出現東北風向之地形噴流。西南部由於較弱的風切並且缺乏地形強迫的抬升運動，使鋒面通過此區期間並無雲冪及降雨的發生。沿東部海岸南移之鋒面，鋒後北風受到科氏力的偏轉而使冷空氣往山脈東側堆積，在近岸處形成高壓脊，增強此區之非地轉北風而加速鋒面南移。鋒面雲系在通過東部的花蓮以後，於外海形成一空洞無雲的區域，模式結果指出西風跨越中央山脈後在山脈東側形成背風波，在此區形成下沉運動是主因。海峽處之鋒面則受管道效應影響而加速南移，以致鋒面速度以台灣海峽中線最快，東部沿海次之，西部沿海最慢。
無台灣地形的敏感度測試指出，在沒有中央山脈的阻擋下，通過台灣本島之鋒面結構較為完整，清楚顯示前述台灣地形扮演之各種重要角色。而在無地表通量的模擬中，鋒面的溫度梯度因無海面通量的影響而能維持強度，加上強的壓力梯度與水平風切，鋒面呈現較控制模擬強且淺的結構，同時，鋒後較為穩定之大氣更抑制了對流雲之發生。

關鍵字

冷鋒 ； 地形 ； 海面通量

並列摘要

This paper presents a numerical study of a cold front that passed the Taiwan area during 7-9 January 1996. The front, with a shallow structure, was significantly modified during its journey over the ocean. When arrived at Taiwan, it caused a sharp temperature drop at many surface stations, with a few rain showers. Satellite pictures showed a distortion pattern of the front by the steep mountain ranges of Taiwan.
MM5 simulations showed that the front was delayed on the windward side of the mountain, with its east and west counterparts moved southward along the coasts. In the northwest of the island, a northeast barrier jet appeared because of convergence of cold air behind the front. Further to the south, there were no clouds and rainfall because of the leeward rain shadow effect and the flat terrain where topographical lifting was lacking. In the east, cold air gradually obtained a westward component toward the coast as it moved southward along the coast. The stable cold air was blocked to the east of the high mountains, resulting in a pressure ridge along the coast and a southward acceleration of the air and the front. There was a cloud free region, shown both in satellite pictures and model simulations, over the ocean to the east of Hua-Lian after the frontal passage. This was related to the lee-wave subsidence after the upper-level air passed over the high mountains. The speed of the front was the fastest over the Taiwan Strait because of the channel effect, the second fastest in the east coast, and the slowest along the west coast.
In the sensitivity studies, Taiwan terrain was proved to be the most important effect in modifying the front. Nonetheless, the surface heat and moisture fluxes from the ocean were also important in the modification of the front.

並列關鍵字

Cold front, Terrain, Surface fluxes