Analysis On A Heavy Rainfall Event In The Shaanxi In August 2007

This article analyses the spatial and temporal distribution of the shaanxi heavy rains according to use the data of 720 statistics,the results indicate that the spatical distribution of heavy rain days is consistent with the quantity of annual precipitation at souther Shaanxi is significantly more than that at northern part. the heavy rain days concentrate from June to September, mainly in July,changing trends of heavy rain days maintain steady in recent 40 years, the 47.4%-78.9% of stations occur heavy rains every year, The total precipitation of heavy rains existence 15～20 years of periodic changes. Shaanxi heavy rain mainly happen in June is caused by Bengal though and subtropical high system. The vapor from the Arabian sea and the Bengal reach shaanxi along the southwest airflow of Bengal trough. The Tibetan plateau is high specific humidity in Summer, Shaanxi specific humidity is experiended from small to large and then lager to small change processes influence by Tibetan plateau.After a full understanding of the spatial and temporal distribution of the Shaanxi heavy rains, we select a typical mesoscale rain process and do a carefully research.This article employs the data of NCEP1°×1°and TBB as well as regular precipitation data to diagnose one torrential rain process at superjacent airspace of Guanzhong, Shaanxi Province during August 8th and 9th,2007. The results indicate that this heavy rain process can be defined as a meso a scale torrential rain. The meso a scale torrential rain happened under anagreeable macro scale circulation, the warm moist flowing and higher energy quickly reach the land along the long-distance typhoon "pab", which offers a steady flows of aqueous vapor. East airflow played an important role in this rainstorm. When the torrential rain occurred, the strong convergence in the lower troposphere and strong divergence in the higher level, and this kind of "suction effect" forms a strong ascent actions above the zone of rainstorm.This article also indicates that this rainstorm is generated by mesoscale vortex, based on the analysis of mesoscale band-pass filtering, the torrential rain happened at the convergence zone in front of the mesoscale vortex, low-level low pressure perturbation and low temperature perturbation triggered the occurrence of heavy rain; Radar reflectivity factor and MCS system changes are better tracer the torrential rain drop zone and strength, heavy downpours happened under the -70℃cloud sector and 50dBz strong echo region. The spiral degree has a great indicative function in this heavy rains, the movement direction of the spiral degree is consistent with the movement direction of rain-band. Before the rainstorm, the airflow above Guanzhong area is at the state of oscillation instability, the cold air invaded from the higher level to the lower moist air by going down through the streamline field; under 850hPa, MPV1<0, MPV2>0, the cold air force the warm air to went up, this is an agreeable condition for this torrential rain to happen.This article employs the model of WRFV3 to Simulate one torrential rain process of Guanzhong,Shaanxi Province during August 8th and 9th,2007. in the case of the simulated precipitation are consistent with the real precipitation, we do the topographic sensitivity test and analyses, the results of test indicate that topography does not impact of Guanzhong precipition terrain from 08:00 to 14:00 on 8, terrain began to work from 14:00 to 20:00 on 8, it make the precipition increase 20-30mm around xi'an, more than 30mm around baoji, compared with last time, the Qinling Mountains play an obvious role from 20:00 on 8 to 02:00 on 9, the center of precipition increase more than 60mm, Guanzhong rainfall occurred at this period,percipition center is coincide with rainstorm growth center, terrain function reduce gradually from 02 to 08 on 9, the precipition increase 30-40mm, precipitation ranges have somewhat expanded, meanwhile, in the west of Guanzhong, the precipition decrease 30-40mm, precipitation increases in Guanzhong disapper after 08:00 on 9.From the two aspects which contain dynamic and thermal analyzed the role of Qilling Mountains, the results show that Qinling Mountains caused the increase of rainfall in the process of heavy rain, the higher the mountains the stronger precipitation intensity.The change of rainfall drop zone is also a certain relationship with the mountains height. The easterly flow of the south of shanbei anticyclone and the southerly flow over Qinling Mountains convergented at Guanzhong area because the block of Qinling Mountains, much water vapor accumulated in the Guanzhong area, the strong convergence in the lower level and strong divergence in the higher level, unstable atmospheric gathered in north of Qinling Mountains accompanied with a strong upward motion,the heavy rain increased.