The Study On The Impact Of Topographic Forcing On The Disaster Rainstorm In West China

For the regions of complex topography and numerous mountains in West China, it has a great influence on the scope and intensity of heavy rain. Therefore, by depth study for the persistent heavy rains in west China (especially in southern Shaanxi and Sichuan north) in September1to18,2011, the paper use a variety of data for diagnosis and analysis and utilize the GRAPES_meso model to carry on the numerical simulation about the persistent rainstorm in shaanxi and sichuan province during September16to18,2011. By comparing the three sets of sensitivity test results, it can be revealed that the interaction between the airflow and the terrain during the disaster rainstorm, and the results are as follows:1) The analysis of the large-scale synoptic situation shows that:the interaction of different weather system and the development of the circulation result in the heavy rain in West China in September2011. In the region of Ural, from the development of the high pressure ridges, the blocking high appears, and then become a deep high pressure ridge again. In this process, sustained cold air southward transport and clashes with the hot, led to the autumn rain continuously. By the common influence of the double typhoon:Rock and Sonca and the Mongolia high pressure during September16to18, a eastern water vapor channel was formed in The Yellow Sea and The Bohai Sea at850hPa, and it makes a large number of moisture through the North China Plain transported to the West China area. This is also the important vapor source of the storm. Precipitation continued until the blocking high collapse on September19.2) The analysis of mesoscale synoptic systems indicated that:there are4mesoscale rainy-clusters from south to north at south area of Mountain Qin and3rainy-clusters from west to east in the northern Shaanxi during September16-18. A lot of rainy-clusters group in West China region at the same time. It is the important cause of the rainstorm and secondary geological disasters. During the storm, there is widespread and continued potential instability zone (PIZ) in West China. PIZ is consistent with rainy-clusters moving path. It shows that PIZ provides the sufficient and necessary conditions for the heavy rain formation, and topographic force is the most important reason to release potential instability.3) The numerical experiments show that:from Sichuan, to southern and center Shaanxi, the parallel alternate ridges and valleys caused positive and negative vertical wind band, which formed a strong suction effect in the vertical direction, besides a plenty of water vapor convergence occurred in the same area, therefore the rainstorm happened. By the common influence of typhoon and Mongolia high pressure during September16to18, a eastern water vapor channel was formed in The Yellow Sea and The Bohai Sea at850hPa, and it makes a large number of moisture through the North China Plain transported to the West China area. Vapor was divided into two parts by Qinling Mountains:the north one and the south. The north vapor was blocked and uplifted by Qinling Mountains and formed a water vapor convergence center. The south one entered the Hanzhong plain and influenced by the "bell mouth" terrain, which resulted in convergence and upward.4) For cut the Qinling and Daba Mountains in the elevation sensitivity test, two vapor, from north and south, converge into one. It directly into the Sichuan basin under the push of north cold air, and makes precipitation increase in the central and southern Sichuan basin. However, water vapor convergence significantly decreased in the Guanzhong Plain, Qinling Mountains and Daba Mountain area, led to decreased rainfall in these regions. Sensitivity test also shows that altitude and topographic up-down have great influence on rainfall, and the greater surface relief when the altitude is the same, the more it will be rain.5) The sensitivity test of mountain pass shows that:when the mountain pass width in the Qinling Mountains increase to more than55km, it can lead to a large number of moisture which piling up in the Guanzhong Plain poured into the south of Qinling Mountains. It makes850hPa horizontal wind convergence line move toward south. So the precipitation decreased in the north of the Guanzhong Plain and Sichuan Basin, and increased in the south of Hanzhong Basin and the central of Sichuan Basin. In addition, with the appearance of mountain pass, the north wind reinforced. It causes the airflow reduced which around the south of Qinling Mountains, and the "bell mouth" uplifting effect of Han river valley is also relatively weakened. Precipitation is mainly caused by horizontal wind convergence line to uplifting.