干侵入的研究及其应用进展

利用常规地面气象观测资料以及NCEP/NCAR再分析资料，对2012年7月21日发生在北京地区的一次特大暴雨过程的干侵入效应进行了定量诊断分析。结果表明：干侵入效应在该过程发生前表现明显，干侵入指数对暴雨落区和降水增幅具有一定的指示意义；干侵入指数的偶极分布特征反映了温带气旋发生发展过程中冷暖气流之间的相互作用，强降水区位于干侵入指数等值线密集带靠近其负值区一侧，当干侵入指数正负中心的零线越靠近，降水越趋于增强。在整个暴雨过程中，中高层干侵入指数正值区不断向中层扩展，当垂直方向上干侵入指数由一致的负值变为正负值相间、特别是中层干侵入指数由负值变为正值时，也即形成中层干冷、低层暖湿的不稳定层结时，易激发对流性降水，使得降水增强、暴雨增幅。对该过程使用等熵坐标系表征的干侵入指数同样能全面刻画干侵入过程，中低层等熵面上干侵入指数较高层等熵面上干侵入指数更具指示意义。另外，整层平均干侵入指数对此次特大暴雨过程有提前6~12 h的响应时间。

Mesoscale aspects of a dry intrusion within a vigorous cyclone

Using datasets of conventional observations and NCEP/NCAR reanalysis, we have quantitatively analyzed the dry intrusion effect in a torrential rain event occurred on 21 July 2012 in Beijing. The results show that the dry intrusion phenomenon was the mostly pronounced before the rainstorm occurred. The dry intrusion index can reveal the heavy rainfall area and rainfall reinforcement. The dipole distribution pattern of dry intrusion index reflects the interaction of the cold and warm air masses during the genesis and development of extratropical cyclones. Heavy rainfall always occurs in the dense and negative dry intrusion index contour zone. The closer the zero contour line is in the plus or minus centers of dry intrusion index, the stronger the rainfall shows. In the whole rainfall process, the positive value center of dry intrusion index in the mid- and upper-level extended steadily to the medium level. In the vertical direction, when the dry intrusion index changed from consistently negative value to alternate positive and negative values, especially the dry intrusion index changed from negative to positive in the middle troposphere, that is, unstable stratification with the dry and cold air in the middle, and warm and humid air in the low levels, convective precipitation is easily triggered, which makes the rainfall reinforced. The dry intrusion index represented by isentropic coordinates can similarly used to depict the dry intrusion process, and has clearer indication significance in the lower and middle isentropic surfaces than in upper isentropic surfaces. In addition, the total layer' s (200-950 hPa) mean dry intrusion index has a 6-12 h response time ahead to this torrential rain event.