From the 850hPa divergence distribution map at 20 o'clock on July 6th, we could see that the western Nenjiang was a relatively strong convergence field. The convergence field moved to eastward into the Nenjiang basin. When it entered the Nenjiang River Basin, it was controlled by the convergence field. The middle and lower reaches of Nenjiang was obvious divergence field on high. According to synoptic theory, high divergence and low convergence were beneficial to ascending motion. Meanwhile, transporting low-level water vapor and energy into the upper atmosphere was conducive to the formation of a larger rainfall process, which resulted in the heavy rainfall period in the middle and lower reaches of Nenjiang River from July 6th to 7th. After July 7th, this configuration was transferred to the stem channel of Songhua River, to which the rainfall center had also moved. In July 9th, the low dispersion field weakened. Without obvious divergence and convergence, this period of heavy rainfall in this process ended.
Other physical quantities
In this rainfall, water vapor flux, vorticity and vertical velocity were also well matched. The physical fields that easily led to rainfall were basically occured in July 6th ~ 8th. That was the time when the rainfall was the strongest. At 20 o'clock on July 5th,the 850hPa water vapor flux showed that there was strong water vapor flux center extending to the north near the Liaodong Peninsula in the south. At 20 o'clock on July 6th, the strong center of the water vapor flux had extended to northward to the west of the Nenjiang basin and the Nenjiang river basin was in the position of the strongest change in the water vapor flux. At this time, the middle and low level of the Nenjiang river basin also had a strong upward movement, which was beneficial to the transportation of water vapor and energy. After July 7th, The convergence zone of the ground moved to eastward and the ascending area of vertical velocity also moved to eastward. The heavy rainfall area had also transferred to the main stream of Songhua River. In July 9th, the vertical velocity and vorticity decreased at the same time. The rainfall process was basically finished.
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