关键词: 对流, 气候背景, 诊断, 干线, 边界层辐合线

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