2008年“7.22”襄樊特大暴雨的天气学机理分析及地形的影响

Analysis of the mechanism for “2008.7.22” excessive rain event in Xiangfan with a focus on the terrain effect.

中文关键词:  特大暴雨，西南低涡，低空急流，高空急流，机理分析，地形影响

英文关键词:Excessive rain, Southwest China vortex, Low-level jet, Upper-level jet, Analysis of mechanism, Terrain effect

中文摘要:

      利用常规观测、地面逐时降水、NCEP再分析资料和卫星雷达资料，对2008年7月22日发生在鄂西北襄樊的特大暴雨过程从对流层高层一直到地面的天气形势特征进行了较为系统的分析，结果表明，对流层高层稳定的辐散系统、近地层稳定的辐合系统以及中低层发展深厚的西南低涡、不断加强的低空急流是造成这次特大暴雨的主要影响系统。重点分析了200 hPa强辐散中心形成的原因及其在降水中心上空稳定、停滞、加强的机制，认为高空急流右后侧的风速辐散区与西风槽和南亚高压反气旋环流之间的风向开口区两种辐散作用的叠加是造成强降水中心上空强辐散中心的主要原因，而高层西风槽在东移过程中突然停滞并加深，从而导致强辐散中心一度稳定、停滞则是强降水持续发展的重要机制。在近地层，由一个已经发展的对流云团外围出现的强偏北下沉冷出流沿浅薄地形河谷区侵入襄樊附近，当偏南暖湿气流不断加强北进时，一方面受到冷出流的横向阻挡，另一方面又受到大巴山地形的纵向阻挡，两种阻挡作用交汇于襄樊上空，使低层出现强辐合中心并稳定维持，在高层强辐散的共同作用下出现深厚的上升运动。地面低压倒槽和准静止锋的稳定维持以及边界层内大气斜压性的增强也有利于中尺度对流系统在该区域的维持和发展。中低层发展深厚的西南低涡和不断加强的低空急流为强降水中心输送了充足水汽，从西南低涡的东北侧不断分裂出β中尺度对流云团和强回波单体，并沿低层切变线移动到襄樊上空后叠加在高空强辐散、低层强辐合的有利动力作用下而得到进一步发展，并最终形成特大暴雨。

英文摘要:

      An excessive rain occurred in Xiangfan City in northwest Hubei Province on July 22, 2008. Based on the data of conventional observation, surface hourly precipitation, the NCEP reanalysis data, as well as the satellite and radar data, weather diagnosis and analysis on this event are carried out. The results show that the stable divergence system in the upper troposphere, the stable convergence system in the surface layer, the deep southwest China vortex in the middle-low levels and the strengthening low-level jet are the main influence systems in this event. With an analysis focused on the emergence of strong divergence center in 200 hPa and the mechanism of its stabilization, stagnation and strengthening, we find that the superposing of strong divergence center on the right side of the upper-level jet and the wind direction peristome between the westerly trough and the south Asia high anticyclone circumfluence has a main contribution to the strong divergence center appearing above the strong precipitation center. The growing of the high-level westerly trough after a sudden stop in its eastward moving results in the stabilization and stagnation of strong divergence center, which is the essential mechanism for strong precipitation maintaining. In the surface layer, a strong north sinking cold outflux emerging from the outside of a developed convective cloud cluster intrudes to near Xiangfan along the shallow valley. When the south warm-wet flow strengthens and moves northward, it is obstructed breadthwise by the cold outflux and lengthways by the Great Ba Mountain, which brings a strong convergence center in low-levels and helps for its stabilization. With addition to the effect of strong divergence in high levels, thick ascending movement comes forth. The stabilization of the surface inverted trough of low pressure as well as quasi-stationary front and the strengthening of baroclinicity in the boundary layer are favorable to the maintaining and developing of meso-scale convective systems in this region. The deep southwest China vortex in the middle-lower layers and the developing low-level jet feed strong precipitation center with sufficient water vapor. The meso-β-scale convective cloud clusters and strong convective cells breaking up continually from the northeast side of the vortex move along the low-level shear line to Xiangfan, where they develop further under the favorable dynamical action of high-level strong divergence and low-level strong convergence and finally induce an excessive rain.