During the flood season in 1998, there was an abnormal weather in Songhua River Basin with unusual polar vortex, westerlines and some other characteristics of subtropical and tropical systems.
This flood was famous for its long lasting sessions, the cumulative rainfall and dynamiclly stable rainfall. In the 62 days from 14th June to 14th August, the Nenjiang basin had raining almost every day,the average rainfall in the basin more than 2 mm reached to 51 days. In the meantime, there were 5 heavy rainfall processes. The cumulative rainfall of June, July and August was 562 mm, 1.7 times more than ordinary years. In particular, the monthly rainfall of 210 mm in August was 2.06 times than ordinary years. What's more, the rain was always hovering in the Nenjiang Basin which contributed to the major flood disaster in Nenjiang and the main stream of Songhua River.
Rainfall Process
In June to August 1998, due to the upper cold vortex,northwest of Northeast China had been raining continuously, especially in Nen River Basin. During which, there were five heavy rains:
The first rain was on June 14th ~ 24th, affected by the upper cold vortex and the ground Mongolia cyclone and it was the first flood in Nenjiang River. The areas whose accumulated rainfalls were above 100 mms were 58000 square kilometers, taking up 20.6 % of the Nenjiang River Basin. The high value area located in Gan River, Nuomin River, the right tributaries of Nen River, during which,there were two rainstorm centers, one was the Gan River Farm of 249 mms,the other one was Deliqier of 249 mms on the Nuomin River. The rain led to the first flood in Gan River, Nuomin River and Nenjiang River.
The second rain was on July 5th ~ 10th, affected by the upper cold vortex and the combination of the ground cyclone in North China and Mongolia cyclone. There were moderate to heavy rain and local heavy rain. This rainfall was heavier than the first rainfall, mainly distributing in the middle and lower reaches in Nenjiang and Songhua River, where the accumulated rainfall reached above 50 mm covering 129 thousand square kilometers, reached above 100 mm covering about 10 thousand square kilometers, each accounting for 49.3% and 3.6% of the Nenjiang basin. The rainstorm center was in the upper reaches of the Yalu River with the largest cumulative rainfall of 150 mm in the Ha La Su station in the Yalu River. The rain caused the first flood of Yalu River, Tao'erhe and Chaoer River, the tributaries of the Nenjiang River.
The third rain was on July 17th ~ 21st. Affected by the northeast cold vortex and Mongolia cyclone, the heavy rains were mainly on the tributaries of the right side of Nenjiang, while the left tributaries had a small moisture.The strong rainfall region was in Tao'er River and Huolin River, whose accumulated rainfall served as 100~200 mm, and the area which had a rainfall above 100 mm were 220 thousand square kilometers, accounting for 7.8% of the whole basin in Nenjiang. The maximum rainfall center was Tuliemaodu station of Huolin River and its moisture was 220 mm, followed by SuoLun Station in Taoer River of 149 mm, Heiniuquan station had a rainfall of 148 mm, which laid the foundation for the second flood in Nenjiang River.
The fourth rain was on July 22nd ~30th. Affected by the high altitude transversal trough and ground cyclone in Mongolia, there were heavy rainfalls in Nenjiang basin and it was the second heavyest rain after the fifth fainfall. In Nenjiang River Basin, there were about 10 stations whose daily rainfall reached to 100 mm, the largest rainfall of which was Wugongli station of Lu River of 192.6 mm in 25th, followed by Pinghu reservoir in Nuomin River of 191.9 mm in 27th. Those areas whose cumulative rainfall was over 100 mm covered about 106 thousand square kilometers, accounting for 37.6% of the whole Nenjiang River Basin. The heavy rainfall area was located in the Alun River, Nuomin River, YaluHe, and Wuyuer River in the middle reaches of Nenjiang River as well as the right tributaries of its downstream. As for the maximum cumulative rainfall, it was in Wugongli station of the upper reaches of YaluHe of 285 mm. The heavy rainfall had a great impact on the Nenjiang River Basin, which together with the third rains led to second floods in the Nenjiang River Basin.
The fifth rain was on August 2nd ~14th, which was formed by the continuous two synoptic processes: the rear of the subtropical high and the Jianghuai cyclone on August 2nd to 6th merged with the upper cold vortex and ground cyclone in North China on August 7th ~ 14th. During these two synoptic processes, there were heavy rains and local heavy rains in Northeast region, mainly located in the downstream of Nenjiang River and Songhua River. There were three strong rainfall areas with a rainfall more than 300 mm in YaluHe, Alun River and Nuomin River, in the center of which the rainfalls were more than 400 mm. The maximum rainfall of 517 mm was in Fuxin Reservoir of Alun River, followed by Gannan County Station of the same river of 499.5 mm. The areas whose cumulative rainfall was over 100 millimeters coverd about 160 thousand square kilometers, and areas with a rainfall over 200 millimeters coverd 65 thousand square kilometers, and 19 thousand square kilometers area with a rainfall over 300 mm, accounting for 56.5%, 22.8% and 6.6% of the gross area of Nen River Basin. The intensity of this rainfall was the heaviest in the whole flood season. The maximum daily rainfall reached to 100 mm in more than 9 stations, the maximum daily rainfall of 164.1 mm was in Gannan County Station of Alun station in August 9th.This rain caused the third floods in Nen River Basin. In addition, it triggered a major flood in the main stream of the Songhua River togethered with previous rainfalls.
Among these five rains, it was the first one, the fourth one and the last one that contributed to the peak rainfalls.
The information is provided by Disaster Risk Reduction Knowledge Service.
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