| Based on kinds of conventional and non-conventional observation data, a sudden extremely heavy rainfall during June 8-9, 2002 in the southern part of Shaanxi province was investigated by means of observation analysis, numerical simulations, and diagnosis, especially the meso- systems' structure and the effects of meso-scale topography. By contrast, another case heavily influenced by terrain as well during June 24-25, 2002 in Beijing district was analyzed and similarities and dissimilarities between the two cases , generality and individuality of topographical effects on torrential rain were discussed. The following conclusions can be drawn:1. The analysis of the large-scale synoptic situation shows that strong precipitation is closely related to the explosion of monsoon and the intensification of cross-equatorial flow which bring a lot of vapor and meet with cold masses at the eastern of northwest district on 8th,June causing extremely heavy rainfall; that subtropical high-level jet at 200hPa, subtropical high at 500hPa and low-level jet at 850hPa are the weather backgrounds favorable to strong precipitation ; that the pattern of eastern highs and western lows and the establishment of a low-level jet and the coupling between upper-and low-level patterns that determine that the rain occurred in the east of the Northwest China (on average, this is the rainy season for the south of China, but not for the Northwest China); that water vapor comes from southerly and easterly flow which converge at the eastern of northwest district with convergence mainly in lower levels and PBL; that the high value of the whole-level apparent heat source is near the area of large rainfall in the direction of northeasterly-southwesterly agreeable to shear line very well and the condensation latent heat releasing is main heat source with vertical advection item playing key role in Q1 and Q2; that there is a vertical secondary circulation crossing low-lever jet whose ascending branch is at the area of large rainfall; that the construction of convection instability and conditional symmetry instability results that there is not only deep thermal instability, but also moisture influx and triggering mechanism of thermal instability causing strong torrential rain.2. The analysis of meso- scale synoptic systems indicated that the occurrence and development of the local and paroxysmal meso- scale synoptic systems led to this rainfall besides the helpful macro-scale conditions. The evolution of the meso- systems depended mainly on the topography and the macro-scale conditions, among which the topography and convective instability were thought to be the most important. The above analysis showed that this heavy rainfall eventtook on obvious meso-scale characters and was the combination of the three flows, so we can deduce: (1) surface southeasterly wind and topography may play the main role in this event; (2) the easterly cold air rushing into the southern Shaanxi province in the lower troposphere was strongly relative to the form of surface cold-front type of occlusion; (3) there were two reasons for the secondary circulation's strengthen in the evening, one was the effect of a mountain-valley wind and the other may be latent heat leading to ascending motion; (4) the shape of potential instability stratification corresponded well with the development of the warm-moisture advection; and (5) the vapor providing essential thermodynamics was transported by a southwesterly low-level jet from the Bay of Bengal and the South China Sea. Furthermore, with the effect of terrain, the southern Shaanxi province became the center of this extremely heavy rain process. At the same time, the disturbance of the low-level jet might be the triggering condition for heavy rain. But the surface occluded front crossing Qinling Mountains and strengthening, maybe the direct reason. Dynamic and thermal effect may also play an important role. Results show that positive perturbation of potential vorticity in the lower layers which is pro...
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