长江中下游地区暖季中尺度涡旋的特征分析

发布时间：2018-04-24 06:12

本文选题：中尺度涡旋 + 涡旋自动识别方法　；参考：《南京大学》2017年硕士论文

【摘要】：中尺度涡旋是造成长江中下游地区暖季暴雨灾害的主要天气系统之一,它是出现在对流层中低层的,水平尺度在几十到几百公里之间的气旋性环流,并常常与对流活动相伴随。为了加深对中尺度涡旋的认识,本文采用水平分辨率为0.5°×0.5°、时间分辨率为 6h 的 CFSR(Climate Forecast System Reanalysis)再分析资料,利用神经网络算法对1996-2015年共20年长江中下游地区暖季的中尺度涡旋进行了普查,共得到309个中尺度涡旋样本,并从涡旋发生频数的年际、月际变化、生成源地、水平尺度、垂直厚度、生成时刻、持续时间、移动路径、涡旋干湿性质等方面着手进行了统计特征研究。对2015年长江中下游地区暖季的MCS(Mesoscale Convective System)进行了普查,找出了形成于 MCS 中的 5 例湿涡旋,对这5例涡旋进行了合成分析,研究其结构特征;对其中1例持续时间最长的涡旋进行了个例分析,详细地展示了涡旋的演变过程,探讨其发生发展的机制。分析结果表明:长江中下游地区的中尺度涡旋在5、6、7月出现频率基本相同,而8月明显偏少。大多数的涡旋是局地新生的涡旋,凌晨02:00(北京时间,下同)为涡旋的高发期,午后14:00时为涡旋生成的第二个高峰期。大部分涡旋的维持时间较短(30h以内),半径在100km左右,垂直厚度大多小于250hPa,闭合环流最明显的高度在850hPa。因此,长江中下游地区暖季的中尺度涡旋是位于较低层次的浅薄系统。涡旋的移动路径主要有偏北、偏南、向东和准静止四种,其中偏北路径最多。按照涡旋的干湿性质可以分为:湿涡旋、干涡旋和MCV(Mesoscale Convective Vortex,MCV可看作湿涡旋的一种特例)。识别到的涡旋中湿涡旋的数目远多于干涡旋,说明大部分中尺度涡旋的形成都离不开水汽,相态变化和潜热释放在涡旋的形成中占主导作用,干涡旋的水平尺度和持续时间都较小,其形成主要是动力作用的结果。对2015年5个形成于有组织的对流系统中的湿涡旋进行合成分析发现,这些涡旋是上暖下冷的结构分布,它们与成熟期的温带气旋的冷心结构不同,与热带气旋的强暖心结构也不同,而是一类弱暖心的结构,这可能是属于东亚季风区与副热带地区中尺度涡旋热力结构的特点;涡旋的强辐合区、高层的强辐散区和强上升运动区不在涡旋的中心,而是位于涡旋的东侧,这预示着未来新的对流有可能在涡旋的东侧发展,这也许与高低空急流的耦合有密切的关系。涡度收支分析表明风场的低层辐合、高层辐散配置对涡旋的涡度增长起着非常大的作用。在对流层低层,辐合辐散项和倾斜项是正涡度的来源,垂直输送项和平流项抑制局地涡度的增大。低层初始的辐合和地面扰动,会触发对流产生降水,降水释放的潜热有利于涡旋的生成。低空急流除了输送水汽外,还会向中尺度涡旋区输送正涡度,而中层的正涡度强迫有利于地面的减压。因而,涡旋的产生是多个因子共同作用的结果。涡旋产生后反过来影响降水,两者存在正反馈作用。低层辐合、高层辐散的配置以及潜热的释放使得涡旋得以发展和维持。
[Abstract]:The mesoscale vortex is one of the main weather systems that cause the warm season rainstorm in the middle and lower reaches of the Yangtze River. It is a cyclonic circulation that appears in the middle and low layers of the troposphere and between the horizontal scale from dozens to hundreds of kilometers, and is often accompanied by the convective activity. In order to deepen the understanding of the mesoscale vortex, this paper uses a horizontal resolution of 0.5 degrees. The data of CFSR (Climate Forecast System Reanalysis) with time resolution of 6h are reanalyzed. The mesoscale vortices of the middle and lower reaches of the middle and lower reaches of the Yangtze River are surveyed by neural network algorithm for a total of 20 years in 1996-2015 years, and a total of 309 mesoscale vortex samples are obtained, from the interannual and monthly variation of the vorticity occurrence frequency to the formation of the source area. The statistical characteristics of the MCS (Mesoscale Convective System) in the warm season of the middle and lower reaches of the Yangtze River in 2015 were investigated, and 5 wet vortices formed in MCS were found, and the synthetic analysis of the 5 vortices was analyzed. The structural characteristics of 1 cases with the longest duration were analyzed, the evolution process of the vortex and the mechanism of its development were demonstrated in detail. The results showed that the frequency of mesoscale vortices in the middle and lower reaches of the Yangtze River was basically the same in 5,6,7 months, but in August, most of the vortices were local freshmen. The vortex, 02:00 in the morning (Beijing time, the same below) is the high onset period of the vortex. At 14:00 in the afternoon, it is the second peak of the vortex. Most of the vortices have shorter maintenance time (less than 30h), the radius is around 100km, the vertical thickness is less than 250hPa, the most obvious height of the closed circulation is 850hPa., and the middle ruler of the warm season in the middle and lower reaches of the Yangtze River The vortex is a shallow system at a lower level. The moving path of the vortex is mainly North, South, eastward and quasi-static four, of which the northward path is the most. According to the dry and wet properties of the vortex, the vortex, dry vortex and MCV (Mesoscale Convective Vortex, MCV can be considered as a special case of wet Eddy). The number of spin is far more than that of the dry vortex. It shows that most of the mesoscale vortices are inseparable from the water vapor, the phase change and the latent heat release dominate the formation of the vortex, and the horizontal scale and duration of the dry vortex are smaller, and the formation of the vortex is mainly the result of the dynamic action. In 2015, 5 wet systems formed in the organized convection system are wet. It is found that these vortices are the structure distribution of the upper and lower cooling structures. They are different from the cold heart structure of the temperate cyclone in the mature period, and different from the strong warm heart structure of the tropical cyclone, but a kind of weak warm heart structure, which may belong to the characteristics of the mesoscale eddy thermal structure in the East Asian monsoon region and the sub heat zone; The strong convergence zone, the strong divergence area and the strong ascending motion area are not at the center of the vortex, but on the east side of the vortex, which indicates that the future new convection may develop in the east of the vortex, which may be closely related to the coupling of the high and low altitude jet. The growth of swirl vorticity plays a very important role. In the lower troposphere, the convergence and divergence term and the tilting term are the source of the positive vorticity. The vertical transport term and the flow term inhibit the increase of the local vorticity. The initial convergence and the ground disturbance in the lower layer will trigger the convection to produce precipitation, and the latent heat released by the precipitation is beneficial to the formation of the vortex. In addition, the positive vorticity is transported to the mesoscale vortex region, and the positive vorticity of the middle layer is forced to facilitate the decompression of the ground. Therefore, the vortex is produced by the joint action of several factors. The vortex is produced in turn to affect the precipitation, and there is a positive feedback effect. The low layer convergence, the configuration of the high rise divergence and the release of latent heat make the vortex. To develop and maintain.

【学位授予单位】：南京大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P458.1

【相似文献】