The vorticity field distribution map of 700hPa at 20 o'clock on July 19th. The center of the positive vorticity was in the vicinity of Tailai that was in south of Heilongjiang Province. The middle and lower reaches of Nenjiang were the most dense regions of the equal vorticity line, which showed that the vortex was the strongest here. This strong positive vorticity area was shaped by the eastward shift of the positive vorticity zone in the southwest of the Nenjiang river and the east of Lake Baika at 20 o'clock on July 16th. The positive vorticity center reached the east of Inner Mongolia at 20 o'clock in July 17th and the power of the positive vorticity zone at 20 o'clock in July 19th was the strongest. Compared with the vorticity position of the high and low levels in July 17th, it could be seen that the center of the positive vorticity center of the high and low layers was basically overlapped. It indicated that the future weather system would be stable. However, the enhancement of the positive vorticity center also indicated that the future weather system would be strengthened. Thus, the rainfall time was changed from July 17th to 21 to July 18th to 20.
At 20 o'clock in July 20th, the positive vorticity center weakened rapidly, but it did not move eastward. It could be seen that the future weather system would be weakened. At the same time, it could be preliminarily judged that the rainfall in the east of the East was weak in the future. It was consistent with the actual small rainfall of the left tributary of Nenjiang. From 20 o'clock July 20th to 20 o'clock July 23rd, there was an obvious negative vorticity region invading south-west from the northeast of Heilongjiang Province.Now the cyclonic bending of the weather system was decreasing and the rainfall was weakened. At 20 o'clock on July 24th, there was a strong positive vorticity area extending northward from the southern part of the Nenjiang river. At 20 o'clock on July 25th, most of the Nenjiang River Basin was controlled by positive vorticity. The direction of the whole positive vorticity region was from the northwest to the southeast, which made the July 24th ~ 25 day a heavy rainfall period. In July 28th, the west of the Nenjiang basin had a positive vorticity advection area moving eastward and most areas of the western region of Nenjiang basin was controlled by the positive vorticity area. The largest center of vorticity was in the eastern part of Inner Mongolia. A new round of heavy rainfall had made July 28th the largest rainfall in the process. After July 29th, the center of the positive vorticity moved to the vicinity of Greater Khingan Range and most of the Nenjiang River Basin began to be in relatively weak positive vorticity area. The rain began to decrease. In July 30th, the center of the positive vorticity was weakened and the back had a negative vorticity region moving eastward. The rainfall was basically over.
Other physical quantities were between July 17th and 30th and the variation of vertical velocity and divergence was also well matched with rainfall.
Through the analysis of the divergence field of 400pha, it could be showed that in this half a month time the areas that over Nenjiang basin was basically high divergence or weak convergence field. The time of the convergence field coincided with the time of the light rain in the region. However, the lower layer was basically a strong convergence field in the stage of heavy rainfall. The intensity of convergence was also weakened in the stage of rainfall weakening.
In the upward movement, we focused on the analysis of July 28th that was vertical velocity distribution in the Nenjiang basin with strong rainfall. The 850pha vertical velocity field at 20 o'clock in July 28th could be seen that the whole Nenjiang basin was a ascending motion area and the center of the rising movement was located in the upper reaches of the Nenjiang River Basin, which was basically in line with the strong rainfall area.
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