Analysis Of Multi-scale System And Numerical Modeling Of A June 2005 Torrential Rain Over South China

The rainstorm is an important synoptic system during flood season at South of China, the research of the interactions and their structures of multiple scale systems can post the dynamical and physical mechanism which triggers and maintains the rainstorm, and improve the understanding and forecasting of the rainstorm at South of China. Using NCEP reanalysis data, routine observation, cloud images and hourly precipitation observation, this paper investigates the multi-scale interactions and their structures of the rainstorm on June 2005 at South of China by analyzing, scale separation and numerical simulation.The rainstorm on June 2005 over southern China is an intensive rainfall which maintained a long time, influenced extensive area and caused the flood of Zhujiang River. The investigation of this rainstorm has significant meaning on the research, forecast and disaster mitigation of rainstorm over South of China.First, this paper analyzes the rainfall characteristic, the change of circulation and the structure of front using observation data. Then, this paper separates and analyzes different scale component of the wind of NCEP reanalysis data by filter. Finally, the paper briefly discusses the change of the element in the front, and focuses on the relationship among the daily change of the low-level wind, the change of cloud and the development of the rainfall. The contrastive experiments of the strength of the monsoon and the transportation of vapor show that the monsoon and vapor play a key role during the process to maintain and strengthen the rainfall. This paper investigates multi-scale major influent systems and their interaction which caused the rainstorm, and draws the following major conclusion:1) The analysis of the observation shows the especial distribution characteristic of the rainfall, the abnormity of the front structure, and proposes a conceptual model of the configuration of the systems in the process. The especial characteristic of the rainfall is that the rainfall has two major zones, possesses apparent diurnal variation and the zones would move northward. The abnormity of the front structure shows that the structure is similar to the structure of the Meiyu front, but possesses many differences: the front near ground is not apparent, and the cold air invades from middle level primarily. The configuration of the systems is that the process was impact by many different scale systems, in the favorable monsoon and planetary-scale circumstance, the stationary front maintained at South of China and the meso-scale system continually evolved and resulted in the long time heavy rainfall.2) The analysis of the different scale systems derived from the scale separation of the wind by filter finds that the scale of the impact system on the different rainfall zone is distinct. In synoptic scale, the low-level wind is shear wind and its spatial and temporal moving is consistent with that of rainfall. In meso scale, the low vortex is the wave whose scale is about 1000km, and moves along the line of wind shear and connects with the strong convective precipitation in the front. In small scale, the maintain and development of the wave whose scale is about 400km are corresponding to the convective precipitation in the warm region ahead the front and have significant impact on the rainfall in warm region.3) The numerical simulation indicates the difference between diurnal variation of the flow at South of China and the diurnal change of the circulation of the sea-land wind is the primary reason of the difference between the diurnal variation of the rainfall during this process and that of the nightly rainfall. The diurnal variation of the low-level wind at South of China reflects the change of the flow due to the diurnal uneven distribution of the temperature. The variation caused the local convergence in the near ground wind, which trigger the generation of strong rainfall in the instable warn region ahead the front.4) The contrastive experiments of the strength of the monsoon and the transportation of vapor show that the strength of the monsoon impacts on the location of the rainfall, and the strength of the water vapor posts positive feedback on the vertical movement and rainfall. The strength of the monsoon has impact on the location of the convergence which is the sufficient dynamical condition for rainfall, and transports lasted warm wet water vapor to supply the water vapor needed by rainfall.In summary, the interaction of different scale systems is the key reason for the whole rainfall zone generated and maintained at South of China. The South Asia high in high-level and prevention of the eastern moving of the trough in zonal wind by the arrival of the monsoon and the abnormal western and southern location of the subtropical anticyclone form the favorable background for the development of different scale systems. Under this circumstance, the cold air move to south from middle-level and joint the strong warm wet water vapor from south, then form the shear in low-level and generate the ascend movement near the front. Due to the shear in low-level, the low vortex will arise from the shear wind and move eastern alone the line of the wind shear. This is the direct factor induce the convective strong precipitation in the front. The moving northern of the rainfall zone is the contribution of the moving northern of the shear line and the moving eastern of the low vortex along the shear line. Due to the diurnal variation of the temperature, the diurnal variation of the low-level wind merges into the mean wind to form the meso-scale systems which triggers the strong convection and heavy rainfall in the warm region ahead the front where the potential instability accumulated.The research on the rainstorm at South of China in the past was limited by investigating single scale system or investigating isolated the different scale systems, and lacked of systematic research on the interaction and interrelationship among different scale systems which caused the rainstorm at South of China. By numerical simulation and scale separation, this paper systematically investigates the interaction and interrelationship among different scale systems which caused the rainstorm. This work makes a considerable supplement to the deficiency of the research in the past, and gives a guideline to the further research. The most important point is the new methods and the new conclusion such as the especial characteristic of the rainfall, the diversity of the scale of the influent systems, and the diurnal variations of the low-level wind to the diurnal variation of the precipitation have significant scientific meaning to the exploration for the dynamical and physical mechanism of the rainstorm at South of China.