冬季华南准静止锋的结构和类型特征研究

Structure and Type Characteristics of the Winter Quasi-stationary Front in South China

利用12年（2000～2011年）逐日的FNL （Final Operational Global Analysis）分析资料和逐日的中国站点降水资料,定义了一个冬季华南准静止锋强度指数,并根据该指数挑选出强准静止锋事件,研究了强事件的结构特征、环流分型及其与降水的关系.结果表明：冬季强准静止锋事件多发于1、2月,其发生频次在近12年里呈现明显上升趋势.强准静止锋锋区表现为等假相当位温线、等温线的密集带,但是锋区湿度变化不明显,并有明显逆温,锋区由南北风辐合构成,上升气流主要位于锋区上部,纬向有两个次级环流与锋区相对应,伴随正相对涡度和水汽通量辐合.根据850 hPa风场在锋区的辐合情况,强准静止锋可分为北风辐合型、南北风辐合型、南风辐合型三种类型.在这三种类型中,北风辐合型对应的北方冷空气最强,华南降水最少;南风辐合型对应的南支槽最活跃,华南降水最多;南北风辐合型介于两者之间.冬季华南准静止锋与冬季华南降水有一定相关,在强准静止锋的背景下,降水偏多时,锋区低层的水汽通量辐合和上升运动偏强,华南处于偏强南支槽前.

Using the daily National Centers for Environmental Prediction （NCEP） global final analysis （FNL） data and daily station rainfall data in China for the winters of 2000-2011, we first define an index to measure the strength of the South China quasi-stationary front （SCSF）, and strong SCSF cases are then selected. The structure and characteristic circulation of strong SCSFs in relation to winter precipitation over South China are analyzed by composite analysis. The results show that strong SCSFs are nearly west--east oriented, are more common in January and February, and their frequency of occurrence shows a clear upward trend over the past 12 winters. The frontal zone of a strong SCSF is indicated by a dense band of potential pseudo-equivalent temperatures in the lower troposphere, slanting northward with altitude, with obvious differences across the band in temperature rather than in moisture. In addition, the frontal zone is often associated with a great temperature inversion, positive relative vorticity, and moisture flux convergence of north and south winds. Updrafts mainly appear above the slanted frontal zone, and disturbance zonal circulations with sub-frontal scale updrafts and downdrafts exist at about 850 hPa below the upper-level westerlies. Strong SCSFs can be classified into three types according to their wind convergence patterns in the frontal zone at 850 hPa. Among these three styles, the northerly convergence type is characterized by stronger cold air from the north and less precipitation in South China; the southerly convergence type features strong south winds, a deeper India-Burma trough, and more precipitation in South China; and the northerly and southerly convergence type falls in between these two. Winter rainfall in South China is closely related to a winter SCSF. In the presence of strong SCSFs, stronger moisture flux convergence and an enhanced ascending motion appear in the lower frontal zone over South China, accompanied by a deeper India-Burma trough in the mid troposphere to the west of the SCSF, leading to increased rainfall in South China.