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

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.