Study Of Genesis And The Structure Change Of Radar Reflectivity During A Squall Line In Yangtze-Huaihe Region

In the summer, China’s Yangtze-Huaihe region in the East Asian summer monsoon, westerlies systems and the Western Pacific subtropical high, such as the large scale system of the favorable cooperation, making MCSs occur frequently, which easily lead to heavy rain, thunderstorms, high winds, hail and other severe local convective weather.On the 4th to 6th July 2013, Yangtze-Huaihe region appeared the longest since the onset of Meiyu in 2013, with the maximum range, the intensity of the precipitation and thunderstorm continued the longest time of a torrential rain and thunderstorm weather process. In this paper, based on a variety of meteorological data (including the fusion of Chinese automatic station and CMORPH precipitation products hourly precipitation data, NCEP/NCAR global objective analysis data and Doppler radar and live radar reflectivity data), the macroscale circulation background, the radar reflectivity evolution characteristics of formation and development structure of a squall line with heavy rainfall. And then, this squall line process was numerically simulated by using WRF model, with assimilating multiple Doppler Radar data by using 3DVAR module of ARPS model to improve the initial field. The characteristics of mesoscale vertical structure, the evolution of radar echo and the reason of system splitting and change was analyzed by using numerical simulation results.The squall line process occurred in a wide divergence zone at 200hPa, before the trough by west air current and more than 20m/s between strong wind speed district at 500hPa, the south side of the shear line and the north side of the low-level jet at 850hPa. Middle and low level water vapor is sufficient, the existence of the southwest low-level jet transport a large amount of water vapor to the convection region. Convective instability of the lower atmospheric layers during the precipitation period provides an unstable energy for the continuous development of convection.The squall line initially was related to a clusters of convective system on the coastal border region of Shandong and Jiangsu provinces. Based on the analysis of simulation results, the results show that, in the early stages of the process, new convective cells were initialized in parallel on south of the east-west band. The convergence of the cold outflow from the decayed band on the north and low-level southwestern warm moist air was conducive to triggering newborn convective cells and then merged into the main rain band, which led to a transformation of the long squall line from a quasi-east-west banded structure to the short northeast-southwest banded structure. During later stages, the low-level shear reinforcement led to a emergence of the strong vortex sheet. The unstable of vortex sheet, resulted in the increasing of small perturbations in the shear line and an increment of small convective cells, which was unable to organize into a band and ultimately led to a loose of band. Meanwhile, the instability of the vortex sheet and the weakness of radar reflectivity had caused a more irregular precipitation.