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Analysis of the Circulation Situation of the Rainfall Process during Flood Season from August 2 to August 14, 1998

2018-05-23  |   Editor : houguangbing  
Category : Natural Sciences
The rainfall process from 2 to 14 August.

The rainfall in this period was formed by two weather processes. From August 2nd to 6th, it was affected by the subtropical high and Changjiang-Huaihe cycone. From August 7nd to 14th, it was affected by High-altitude cold vortex, Ground North China cyclone and Mongolia cyclone. The two weather processes had led to the northeast of China heavy rain and the mainly rain belt located in the middle and lower reaches of Nenjiang River and the main stream of Songhua River. The main rain dates were from August 2nd to 3, August 5th to 6 and August 9th to 12.

Analysis of Circumfluence Situation

In the 500hPa high altitude of Eurasia at 8 o'clock on August 2nd, a low value channel extended from the northwest to southeast of the Lake Baikal area to the central part of Inner Mongolia and showed a transverse trough over the Northeast. At the same time, there was a cold center coordination in this low value area. The northeastern part of Heilongjiang province was a warm high pressure ridge. On the surface weather map at 8 o'clock on August 2nd, there were a Mongolia cyclone in the north of Hohehot Municipality and a weak and unclosed low value area in the northeastern region. There was a traverse shear that existed in the rear of the subtropical high. At 20 o'clock on August 2, the subtropical high moved northward and the West Extension ridge line moved to the vicinity of 30° N. The center of the subtropical high was relatively strong. A large amount of water vapor from the Pacific Ocean and the cold air from the Lake Baikal combined in the Songhua River, which caused a heavy rain or even rainstorm. In August 3rd, the cold air forces north of Lake Baikal were constantly invading to the south, which caused the low pressure trough forming a closed low pressure circulation in the western part of northeast China and in front of it. There was a cold center coordination. At this time, the Mongolia ground cyclone which located in the northern part of Hohhot also began to expand to the East. There was still a wind shear after the subtropical high and the rainfall was still larger. In August 4th, due to the guiding role of the subtropical high, the Jianghuai cyclone went northward along the edge of the subtropical high and reached the northeast area on August 5th that merged with the cyclone of Mongolia. The force was strengthened , covering the whole northeast region. Aloft the weast of northeast was stil a vortex and the rest areas were in the high-level trough. At this point, the rain began to increase in the northeast, which lasted until August 6th. The high altitude vortex weakened. The closed circulation disappeared and the rainfall decreased slightly. Due to the cold air of Lake Baikal moved to south, in August 9th a newborn high altitude vortex began to accelerate moving to eastward, forming the northeast cold vortex over the northeastern region. Thus the rain began to increase. On the ground, a newborn Mongolia cyclone moved to east in August 9. With the combination of occluded cyclone in Northeast China, the powers significantly enhanced. Because of the sea of Okhotsk blocking anticyclone, preventing the eastward shift of northeast cold vortex and causing the cold vortex moving eastward slowly. It had been hovering in the western Nenjiang basin of the northeast of China. This state continued until August 11th. On the Euro Asia 500hPa high altitude weather map in August 11th necv centre was in the north western region, followed by a cold tongue. The East was the Sea of Okhotsk blocking high pressure and the west was warm blocking high pressure. On the ground weather map at 8 o'clock on August 11th Mongolia cyclone had developed to its heyday and formed Occluded cyclone. There was a new cold front in the back and the rain had been great. 500hPa high altitude circulation in August 12th had some adjustments and the blocking anticyclone weakened. The scope of Northeast Cold Vortex narrowed, but it's center was still located over the Nenjiang River Basin. In August 13th, due to the high-handed sphere of influence of the north of Lake Baikal expanded, causing blocking high pressure ridge disappearing that formerly existed near the Okhotsk Sea. Northeast Cold Vortex weakened and retreated to the Northwest, which merged with the low pressure of Lake Baikal. So the northeast region was at the end of the trough and the rainfall was reduced. Finally, the ground Mongolia cyclone weakened and the rainfall process ended in August 14th.

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