两次陕北暴雨过程热力动力机制诊断

利用常规气象观测资料、NCEP FNL分析资料（水平分辨率为1°×1°,时间分辨率为6 h）,对2013年7月21-22日和2014年7月8-9日两次陕北暴雨过程成因进行热力动力诊断,结果表明:两次陕北暴雨与高低空急流关系密切,暴雨带位于低空急流左侧的水汽辐合区,“0721”过程低空急流更强,在高低空急流耦合的强上升运动区（延安）出现大暴雨。降水前期,两次过程大气均存在对流不稳定,切变线触发对流,产生强降水,而其释放的凝结潜热加热形成中低层大气的热力不连续面,湿斜压性及锋生增强,造成整层饱和大气的抬升,维持强降水。“0721”过程前期对流降水的潜热释放更大,由此反馈的低空急流及锋生更强,出现大暴雨天气。广义对流涡度矢量垂直分量很好地描述了两次暴雨过程高低空急流耦合作用以及凝结潜热释放增强的锋生作用,其变化趋势能够反映降水的发展和减弱过程。暴雨出现在湿热力平流参数垂直积分大值中心及南侧的高梯度区,大值中心出现后约6 h会产生强降水,这对于强降水落区的预报有一定指示意义。

Diagnosis of Thermal and Dynamic Mechanisms of Two Rainstorm Processes in Northern Shaanxi

Based on the conventional meteorological observations and 6 h 1°×1° NCEP FNL analysis data, two heavy rain processes occurred on 21-22 July 2013 and 8-9 July 2014 in northern Shaanxi are diagnosed with synoptic method and dynamic diagnosis method. It shows that both processes can be attributed to the intersection of the warm moist air flow along the edge of subtropical high at 500 hPa and the cold air brought from plateau troughs. Low-level jet plays an important role, as it provides adequate water vapor and water vapor convergence lifts on the left side of the shear line. The vertical secondary circulation produced by the coupling of upper-and low-level jet is an important triggering factor. Heavy rainfall in "0721" process occurs mainly at Yan''an where strong coupling of upper-level jet and low-level jet is located. There is strong convective instability in the atmosphere in the initial stage of precipitation during both rainstorm processes. Convergence lifting from the low-level shear line triggers convection energy, resulting in strong precipitation. The latent heat of condensation released by the precipitation extends downward to the middle atmosphere, and leads to thermal discontinuity of the middle and lower atmosphere. Atmospheric wet baroclinicity and frontogenesis significantly enhances, causing the uplift of whole layer saturated atmosphere and strong precipitation, finally producing heavy rainfall process. Because the convective precipitation is stronger and latent heat released is greater in "0721" rainstorm process, the feedback of the low-level jet and the middle atmosphere frontogenesis is stronger, and therefore the precipitation is heavier. The vertical component of generalized convective vorticity vector describes the enhancing of vertical wind shear very well, and describes the frontogenesis which is increased by condensation latent heat that released by water vapor phase transition in the middle and lower layers very well. Therefore, the changing trend of generalized convective vorticity vector can reflect the development and decrease of precipitation. The large value center and high gradient area on the south side of the vertically integrated moist thermodynamic advection parameters is consistent with rainstorm fall area, and it appears about 6 hours before the precipitation, indicating it can be used to effectively forecast regional precipitation.