台灣地區冬季鋒面系統之結構特徵研究

Abstract

本研究分析1994年11月至2002年2月期間，影響台灣地區且具有顯著鋒面特徵之13個上滑冷鋒個案，依鋒面南側近地層垂直於鋒面的水平風場分布，將上滑鋒區分為暖型與冷型，探討其鋒面之結構特徵、綜觀環境配置及其對台灣地區天氣之影響。 分析顯示，台灣附近700 hPa等高線多為緯向分布，該層等高線與地面鋒面的交角普遍較小，因而該層垂直於地面鋒面的風向量不易大於地面鋒面移速，氣流易出現相對於鋒面上滑的情形，形成上滑鋒。暖型上滑鋒受到太平洋高壓脊西伸至台灣鄰近海面影響，鋒前為南風分量；冷型上滑鋒受到大陸高壓籠罩台灣地區影響，鋒前與鋒後均為北風分量。暖型與冷型上滑鋒在地面鋒面附近均存在正渦度與輻合最大值，且鋒面區上有最大上升運動，唯暖型上滑鋒上述各項運動參數均較冷型上滑鋒略大。兩類鋒面之雲區均主要分布在鋒上與鋒後，唯暖型上滑鋒鋒前仍有些許雲量，此應與鋒前近地層南風分量帶來較暖濕空氣，且鋒前有較大之低層輻合與較大之上升運動有關。 暖型與冷型上滑鋒在台灣北部與東北部均有較顯著降水，但冷型上滑鋒的降水強度較大；暖型上滑鋒在東南部鋒上的降水強度有另ㄧ大值，此乃因鋒前南風分量帶來暖濕空氣，且東南部位在迎風面，因地形舉升故有較大降水。暖型與冷型上滑鋒通過西部地區期間的溫度下降速率（氣壓上升速率）均較東部地區大（小），冷型上滑鋒通過期間溫度下降速率與氣壓上升速率則較暖型上滑鋒大。暖型上滑鋒通過前後風向普遍由南風距平轉為北風距平，冷型上滑鋒通過前後普遍仍為北風距平，唯冷型上滑鋒通過後北風分量些微增強。暖型上滑鋒通過前混合比較冷型上滑鋒大。 最後，分析發現反聖嬰年與正常年期間，太平洋高壓脊東退，在大陸高壓影響下台灣附近冷鋒鋒前風場多由北風主宰，此環境特徵較利於形成冷型上滑鋒；聖嬰年間，太平洋高壓脊西伸至台灣東方鄰近海域，台灣附近冷鋒鋒前風場則以南風分量為主，此環境特徵較有利於形成暖型上滑鋒。

Characteristics of frontal systems in the Taiwan area during the winter of 1994–2002 (Nov-Feb) were investigated in this study. Thirteen representative anafronts were identified. These cases were classified into warm-type and cold-type anafronts according to their horizontal wind field distribution in the area to the south of the fronts in the lower troposphere. During this period, no katafront was identified. This domination of anafronts was probably due to the fact that the angle between 700 hPa flow and the surface front was very small in the subtropics, so that the surface front usually moved faster than wind speed at the upper levels, resulting in a relative upslope motion of warm air along frontal interface. The southerlies (northerlies) prevailed in the prefrontal areas of warm-type (cold-type) anafronts during the period of dominance of the subtropical ridge (Mongolia high). Maximum cyclonic vorticity, convergence, and upward motion occurred in the areas near surface fronts for both types with slightly larger magnitudes in the warm-type than those in the cold-type. Clouds mainly concentrated over frontal and postfrontal areas in both types of anafronts. For the warm-type, clouds also extended into the prefrontal area because of more humid and warmer southerlies as well as stronger convergence and upward motion in the lower troposphere. Significant precipitation occurred over northern and northeastern Taiwan during the passage of both types of anafronts, whereas it was larger in the cold-type than that in the warm-type. For the warm-type, significant rainfall also occurred over southeastern Taiwan because the orographic lifting of the warmer and moist southerlies. The temperature (pressure) decreased (increased) faster over western Taiwan than that over eastern Taiwan, and the decreasing (increasing) rates of temperature (pressure) were larger for the cold-type than for the warm-type. The southerlies changed to northerlies during the passage of the warm-type anafronts, whereas the northerlies prevailed during the passage of cold-type. The mixing ratio before the frontal passage was larger for the warm-type than that for the cold-type. Analyses also showed that during the normal and La Niña years, large-scale flows favored the occurrence of cold-type anafront because the dominance of Mongolia high pressure. Whereas during the El Niño years, large-scale flows favored the occurrence of warm-type anafront because of the dominance of stronger subtropical high pressure with prevailing southerlies.