浙江中北部枯梅年和丰梅年大尺度环流和结构特征差异的对比研究

Comparative Study on Large-scale Circulation and Structural Characteristics During Rich and Rare Meiyu Years in North-Central of Zhejiang Province

利用实况雨量和客观分析资料,对2007、2008年浙中北梅汛期降水的大尺度环流形势和结构特征上存在的差异进行了比较。结果表明,2007和2008年分别是浙中北枯梅年和丰梅年降水的典型年份,前者梅雨带在淮河流域,后者梅雨带在浙中北地区。虽然对流层高中低3层,两者的影响系统相同,但枯梅年副热带西风急流和南亚高压东部脊线位置偏北,丰梅年则偏南;枯梅年低层来自孟加拉湾的偏西风在南海转向北上后输送至淮河流域,未与和来自西太平洋的东南气流结合在一起,丰梅年在南海转向后的西南气流向东北方向输送至闽浙海陆交界处,与副高西南侧的东南气流合并起来;枯梅年副高偏西,588线西脊点在118°E附近,阻止了北方弱冷空气的南下,丰梅年副高偏东,西脊点位于124°E左右,有利于在闽浙海陆交界处形成一个水汽通道。在梅雨锋结构上,锋区温度相对较低,也没有明显的温度梯度,但相当位温和水汽的梯度十分显著,枯梅年相当位温密集带呈纬向分布,且异常偏北,丰梅年则偏南并呈东北西南走向。在涡散度结构上,丰梅年高层辐散和低层辐合都强于枯梅年,正涡度区的伸展高度也更高。

With available data of rainfalls, the large-scale circulation and structural characteristics during rich and rare Meiyu years in north central of Zhejiang Province in 2007 and 2008 are comparatively analyzed. The resuhs are as fellows. （1） The years of 2007 and 2008 are, respectively, tbe typical rare Meiyu year and rich Meiyu year, with the former＇s rainbands being situated in Huaihe river basin and the lattcr＇s rainbands in north central of Zhejiang Province. （2） Although it is the same weather system in troposphere, the location of subtropical westerly jet and ridge line of southern Asia high is northerly in rare Meiyu year while southerly in rich Meiyu year. In rare Meiyu year, the west flow of low-level which comes from the Bay of Bengal steers in South China Sea and transports water vapor to Huaihe river basin, does not combine with the southeast flow which comes from southwest of subtropical high; while in rich Meiyu year, the southwest flow from South China Sea transports to Zhejiang and Fujian coastal regions and combines with southeast flow from western Pacific. The subtropical high is westerly and the west ridge point of 588 line is close to 118°E and it blocks the weak cold air in rare Meiyu year while in rich Meiyu year, the subtropical high is easterly and the west ridge point of 588 line lies at 124°E, favorable to form a water vapor channel in Zhejiang and Fujian coastal regions. （3） Comparative analysis of the structure of Meiyu front between rich and rare Meiyu years indicates that the temperature is relatively low and there is no obvious temperature gradient, while the gradient of equivalent potential temperature and moisture is quite significant. In rare Meiyu year, the dense band of equivalent potential temperature has a zonal distribution and is abnormal northerly, while in rich Meiyu year, it is southerly with a distribution from southwest toward northeast. With respect to the vertical profile, the convergence on low-level and divergence on upper-level in rich Meiyu year are even more obvious than those in rare Meiyu year, with an extended region of positive vorticity.