July 17th to 21st and July 22nd to 30th were the two continuous rainfall processes that caused the second flood in Nenjiang River. Because the two rainfall processes were closely connected, the change of physical field had a certain continuity. So the two rainfall processes were put together to analyze.
July 17th to 21st affected by the northeast cold vortex and ground Mongolia cyclone. The rain, heavy rain and local rainstorm occurred in the Nenjiang River Basin. The main rainy days were in July 18th ~ July 20th and the mainly rain belts located in the right tributary of Nenjiang.
Affected by the high altitude horizontal trough and ground Mongolia cyclone from July 22nd to 30th, The heavy rain, rainstorm and local heavy rain occured in the west of Northeast China. The main rainy days were in July 24th ~ 25th and July 27th ~ 29th. The mainly rain belts located in the middle and lower reaches of Nenjiang.
On the 500hPa high altitude map at 8 o'clock on July 18th. There was a low vortex from the western part of the northeast to the Lake Baikal area. The northern part of the low vortex was a high pressure system and the cold air in the eastern part of the high pressure system moved to southward along the front of the high pressure. It was transported to the low vortex, making the vortices constantly develop and deepen. On the ground weather map, there was a mature Mongolia cyclone over the Nenjiang River Basin that combined with a high altitude vortex. In July 19th, the forces of the low vortex deepened into the northeast. The force of the high pressure ridge in the East had also increased, preventing the eastward movement of that low vorte. In July 20th, that low vortex forces further strengthened and had become the northeast cold vortex over the northeast area . We could see from the ground weather map that after the cyclone from upstream of the Yellow River was added to Mongolia cyclone, which strengthened the cyclone force. The cyclone center was located in the north-west part of the northeast and it's place located in the west, which caused that the rainfall mainly occurred in the right tributary of Nenjiang. This stable high altitude circulation would continue until July 22nd and would weaken in July 22nd. The ground cyclones also disappeared in July 22nd. At this point, the rainfall had been reduced and temporarily ended.
In July 24th, there was a high altitude transverse trough over the western part of northeast China, in the rear of which the meridional circulation was significant. It continuously transported the cold air to the high-level trough. Its power was strong and heavy rain began. The high-level trough had penetrated the northeast and the ground Mongolia cyclone's influence also extended to the western and southern parts of Northeast China on July 25. The high altitude circulation was adjusted slightly in July 27th. A cut off the low vortex formed in the vicinity of Lake Baikal and the high latitude meridional circulation forces weakened. Besides, it became more flat. The power of the Lake Baikal vortex had been extended to the Northeast. Over the northeast area, it was shown as a high altitude transverse trough that located in the Nengjiang basin. At this time, the new round of rain began to increase. On the ground weather map at 8 o'clock on July 27th , the whole northeast area was covered by low pressure area. In July 28th, the Northeast high-level trough began moving out of the west. At this time, only the western areas of the northeast region under the control of high-level trough and rainfall in other areas had decreased. In the meanwhile, the Mongolia cyclones also began to move north-west. This state lasted until July 29th. In July 30th, the vortex of the Lake Baikal moved further north-west and the Mongolia cyclone moved out of the northeast area. Finally, the rainfall process ended.
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