Aiming to investigate the weather characteristics and mesoscale system in the heavy rainstorm process on Jul 26, 2017 of Yellow River, the conventional observations and mesoscale numerical model (WRF) and its assimilation system 3DVAR was utilized to simulate the major precipitation period. The difference between the results of model with assimilation observation date of meteorological observation stations and without assimilation were also discussed. the results indicates that upper at 500hPa and shear line at 850hPa form the forward-titling trough which provides favorable instability conditions for generation of rainstorm. stability of subtropical high is beneficial to the water flux transporting to main precipitation zone. the wind shear at 850hPa and surface cyclone are the mesoscale systems. By comparing the simulations with and without assimilation, the result showed that the simulation with assimilation improved obviously the location of heavy rain and spatial distribution of different levels of precipitation. Of which the region with precipitation below 10mm simulated by experiment with assimilation was greatly improved. At the factors of atmospheric circulation field, the intensities of water vapor transportation and surface cyclone have strengthened after assimilation, thus can support more favorable conditions for development of convection's system. its implied that simulations after assimilation improve the spatial structure of initial field, and have an essential effect on simulating mesoscale convective system's structure.
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