【作者】 毕宝贵；

【导师】 李泽椿；

【作者基本信息】 南京气象学院 ， 气象学， 2004， 博士

【摘要】 我们在实际天气预报业务中，常常为报准西北地区东部、华北地区的暴雨大伤脑筋，这其中重要原因之一可能就是中尺度地形及与地形有关的下垫面对暴雨的影响问题。因此，本论文就是从2002年6月8-9日（简称“02.6”）西北地区东部强降水（尤其陕南地区大暴雨过程）个例出发（中央气象台虽然文字预报服务材料对陕西南部和中部持续出现的强降雨过程做出了较准确的预报，但强降雨落区具体位置预报与实况相比还有一定的偏差，强度预报偏弱），采用多种常规和非常规资料，通过观测分析、数值模拟、诊断分析等手段对该大暴雨过程，特别是对其中的β中尺度系统和中尺度地形作用做了较全面、深入的研究。作为对比，本文还对2002年6月24-25日发生在北京地区的强地形雨进行了分析，并讨论陕南、北京地区两地暴雨的异同点以及地形作用的共性和个性，为两地暴雨预报提供有益的参考，得出了一些很有意义的结果： 1 大尺度环流背景分析表明：（1）“02.6”强降水与6月上旬越赤道气流和季风爆发密切相关，携带大量水汽的偏南气流与冷空气于6月8日交汇在西北地区东部，导致了这次强降水的发生；（2）200hPa的副热带西风急流、500hPa副高以及850hPa的低空急流的配置非常有利于本文分析之陕西强降水的发展与维持。大尺度形势分析表明，东高西低形势场、低空急流的建立和高低空形势的配置决定了这场降雨出现在西北地区东部。与暴雨区相联系，存在一支横越低空急流的经向垂直环流，暴雨区处于该垂直环流的上升支；（3）偏南和偏东气流水汽通道在西北地区东部交汇，水汽的辐合积聚主要在对流层低层和行星边界层内完成；（4）整层的视热源<Q₁>高值区在暴雨区附近呈东北-西南向分布，与切变线走向非常一致，降水产生的凝结潜热释放是强降水区大气的主要热源。同时，在大尺度上升运动区中低层存在一个条件对称不稳定建立的机制，使得在暴雨区，既存在深厚的对流不稳定机制，又存在水汽输入机制和对流不稳定的触发机制，从而形成强暴雨。 2 β中尺度系统及降水成因分析：除了以上有利的大尺度背景场外，陕南强降水过程对应的中尺度系统是非常明显的，且无论对流层低层还是高层的流场和高度场都有很好的配置。β中尺度的天气系统的发生发展是直接造成强降水的原因。这种系统的发生发展具有很强的局地性和突发性，主要取决于当地的地形条件及大尺度背景场，其中，地形和对流不稳定是重要的原因之一。中尺度结构分析表明，本次暴雨具有明显的中尺度特征，是由三支气流共同作用的结果，分析发现（1）地面东南风和地形在这次大暴雨过程起主要作用；（2）东路冷空气主要是通过中低层侵入陕南地区的，并与地面冷式锢囚锋的形成密不可分；（3）夜博士论文:中尺度地形对陕南暴雨的影响研究间垂直次级环流发展加强可能有两个原因，一个是由于地形山谷风的作用，另一个是降水的潜热释放激发了上升运动:（4）位势不稳定层结的形成与低层暖湿平流的发展有很好的对应关系;（5）本次暴雨的水汽主要靠偏南风急流将孟加拉湾和中国南海的水汽输送至西北地区东部，为暴雨的发生提供了必要热力条件。位涡分析结果表明低层正值位涡扰动的存在是暴雨发生发展的重要条件，而凝结潜热释放和地面潜热输送是位涡扰动的主要原因。东路冷空气对陕南强降水的影响尤为重要。同时，由于冷空气是从中层侵入西北地区东部，这样就形成了较强的对流不稳定层结，有利于对流降水的发生。3地形热力作用的影响主要表现在:（1）地面感热、潜热通量和温度的区域性分布与高原东部特殊的地形分布有关，地表通量和温度的等值线与地形等高线大致平行，平原低洼地区和山脉所在地特征明显，从而可以说明地形热力状况在陕西强降水中发挥重要作用;（2）不考虑地面潜热通量的作用时，模式对降水的模拟与实况偏差较大，700hPa的中尺度低压和850hPa的背风气旋不能形成，从而导致由中尺度背风气旋所引起的陕南大暴雨也不能模拟出来。同时，散度垂直剖面图也表明，除去地表潜热的作用后，没有出现低层的强辐合和高层的强辐散，低层辐合只出现在秦岭北坡和渭河河谷，秦岭南坡和汉江河谷被辐散气流所控制;（3）地表感热使山脊降水减少，使平原、山谷降水增加，这与夜间的山谷风环流密不可分。 4地形动力作用的影响表明:（l）大巴山的各部分对降水的影响都很大，尤其是大巴山西部山脉影响最大。大巴山的整体高度降低并非是其他试验线性叠加，降水减小程度比线性叠加更大，这说明各个地形对降水的作用是非线性的;（2）秦岭使大巴山和汉江河谷、陕北降水增加，使山脉本身降水减少。其中，秦岭山脊的作用是关键性的，秦岭山脉对降水的影响主要是通过地形产生的垂直次级环流实现的;（3）低层东南风沿汉江河谷喇叭口地形动力抬升与冷锋过山产生的垂直扰动的叠加激发了垂直上升运动的强烈发展，从而导致陕南强降水的发生;（4）锋面翻越秦岭是一个不连续过程，与Keuler等（1992）的结论相一致，锋面翻越大巴山过程则明显不同;（5）山脉除了对冷锋移行速度有影响外，对冷锋强度的变化也有重要影响。山脉对上层锋的影响不大，这是因为山更多还原

【Abstract】 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 <Q1> 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更多还原