Abstract: Due to the demand of nowcasting of convective weather in North China (including Beijing, Tianjin, and Hebei Province), the authors use the averaged fields of NCEP 1°× 1° final analyses during May to August of 2000-2005 to analyze the climatic background of convective weather in warm season in north China. The averaged fields include mean circulation of the lower and upper troposphere, mean humidity, temperature, pseudo?equivalent potential temperature, convective available potential energy (CAPE), convective inhibition (CIN), and so on. The results show that the climatic background in May and June is different to that in July and August. In May and June, cold air mass from north is more active, and North China is mainly influenced by disturbances of westerly wind systems in mid?latitudes, in contrast, in July and August this area is influenced by both disturbances of westerly wind systems in mid?latitudes and disturbances in lower latitudes. There are more hailstorms and thunderstorms in May and June, but more heavy rains in July and August. The mean daily variation of relative humidity of lower troposphere reveals that there is one dry line (dew point front) in the afternoon to the east of Beijing, and the averaged streamlines of 850 hPa indicate that there exists a large?scale convergence line in boundary layer at the north of Beijing. The above results provide nowcasting of convective weather in north China with a climatic background.

Key words: convection, climatic background, diagnosis, dry line, convergence line of boundary layer

摘要： 由于华北地区（北京、天津、河北）对流天气临近预报的需要，利用2000?2005年5?8月每日4次1°×1°NCEP最终分析资料的平均场分析了该地区暖季对流性天气的气候背景。分析内容包括：高、低空平均环流，湿度、温度、假相当位温、对流有效位能（CAPE）、对流抑制能量（CIN）等多年各月各时次的平均场和气候日较差分析。结果表明，华北地区5、6月与7、8月的对流天气是发生在两种不同的气候背景下。5、6月份北方冷空气比较活跃，华北地区受中纬度西风带扰动影响为主；7、8月华北地区受中纬度扰动和低纬度扰动的共同作用，从而造成5、6月份和7、8月份对流的性质有显著差异，前者的对流发生在变性的大陆冷气团中，常出现冰雹和雷雨大风天气，后者的对流发生在锋前热带季风气团中，易出现暴雨天气。因此，5、6月份和7、8月份的对流天气临近预报应该采用不同的预报思路。00 UTC和06 UTC近地面层?975 hPa相对湿度场的差异表明，午后华北地区容易出现干线（或露点锋）的气候特点。00 UTC和06 UTC的850 hPa平均流场对比还表明，在北京地区的北侧存在热力作用下产生的地形性边界层辐合线的气候特点。上述两点为华北地区初始对流的临近预报提供了气候背景。

关键词: 对流, 气候背景, 诊断, 干线, 边界层辐合线