[Reading Science] The 'Atmospheric River' Pouring Extreme Rain Threatens the Korean Peninsula

Review of Last July's Monsoon
Reflecting Climate Change, Record-Breaking 'Localized Heavy Rainfall' Continues
Frequent 'Atmospheric River' Phenomenon with Narrow, Long Vapor Inflow Channels
Complex Disasters Mixing Heavy Rain, Heatwaves, and Lightning
"Will Become More Frequent, Fundamental Measures Needed for Dams and More"

[Image source=Yonhap News]

"The Korean Peninsula is the most dangerous." This is the diagnosis from scientists regarding the recent series of extreme weather events such as heavy rains caused by climate change. In particular, the recent monsoon showed peculiar characteristics unlike any previous monsoon. Torrential rain fell throughout the month, causing dozens of casualties including the Osong underground underpass disaster. Foreign media have even launched investigations, questioning "Why do such backward disasters happen in a well-developed country like Korea?" What exactly happened during the last monsoon, and how should we respond? Scientists now advise that it is time to establish fundamental measures to protect the lives and property of the people in response to climate change.

‘Bucket-like pouring rain’ becoming more frequent

"You might say, 'Surely this won't happen again,' but it will become more frequent." This was said by Professor Son Seok-woo of Seoul National University (Department of Earth and Environmental Sciences) at a flood damage countermeasure forum hosted by the Korean Federation of Science and Technology Societies on the 28th of last month. First, let's look at the records left by the monsoon that suddenly began on June 25 and ended a month later on July 26. This monsoon started simultaneously and suddenly nationwide. However, the pattern was completely different from before. Unlike before, when the North Pacific High expanded northward causing the front to move up and down over Jeju, southern, and central regions, this time it did not. It rained continuously for six days from the start, with record-breaking concentrated heavy rain. During this period, Jeju Island saw 49 mm per hour, Gwangju 54.1 mm per hour, and other southern regions experienced tremendous downpours. Except for about one day on July 6, it rained continuously nationwide for nearly a month. Notably, on July 11, a disaster alert text warning of ‘extreme heavy rain’ was sent for the first time this year in the Seoul area. Around 4 p.m. that day, record-breaking heavy rain fell with 76.5 mm per hour at Dongjak Observatory, 72.5 mm at Guro, and 70.5 mm at Geumcheon.

Professor Son explained that this was due to a new type of extreme climate phenomenon called the ‘atmospheric river.’ At that time, a very small-scale counterclockwise atmospheric circulation, i.e., a mesoscale low-pressure system, formed over the Gyeonggi area of the Korean Peninsula. This so-called ‘atmospheric river’ phenomenon caused locally intense heavy rain. He said, "The rain that fell then was not from water vapor coming from the West Sea as usual, but from a very long band of moisture flowing in from inland China, causing heavy rain in a narrow area due to the atmospheric river phenomenon." He added, "On August 8-9 last year, the same cause led to 111 mm per hour rainfall at the same location, Dongjak Observatory, showing similar characteristics, spatial background, and damage scale."

However, this ‘rare’ record-breaking heavy rain repeated again shortly after. From July 13 to 15, the Chungcheong and Jeonbuk regions experienced concentrated heavy rain of up to 530 mm over four days, breaking all past observation records in those areas. During this period, a ‘third-world type’ disaster occurred when buses and other vehicles were submerged in water at the Osong Gungpyeong 2 underground underpass in Chungbuk, resulting in 14 deaths and 10 injuries.

Another characteristic was that it was a complex disaster with repeated heavy rain, lightning, and heatwaves. During the last monsoon, 139 heavy rain warnings and 453 heavy rain advisories were issued. Considering that a heavy rain warning is issued when 90 mm or more rainfall is expected in 3 hours or 180 mm or more in 12 hours, these numbers are beyond imagination. Moreover, unlike the past when rain fell from low clouds of a stationary front, rain fell from tall clouds causing frequent lightning. Also, heatwaves occurred repeatedly when it did not rain. During this period, lightning strikes were 3,354 times on June 27 alone and 3,659 times on June 28, and heatwave advisories were issued 61 times.

The bigger problem is that such extreme rainfall is expected to become more frequent in the future. Professor Son said, "Analyzing the frequency of daily rainfall over 100 mm from 1961 to 2020 shows a steady increase across East Asia, especially a sharp rise in the Korean Peninsula, western Japan, and inland China." He added, "This is contributed to by increased water vapor due to global warming and strengthened atmospheric uplift in East Asia. Although the much-discussed super El Ni?o phenomenon is not unrelated, its correlation is low." He emphasized, "It is a truly important time to prepare practical countermeasures against localized heavy rain."

Fundamental measures needed to prevent damage from abnormal weather

How should we respond to such flood damage? Experts broadly classify flood disaster prevention measures into structural and non-structural measures. Structural measures refer to facilities that prevent flood damage, such as deep rainwater drainage systems (deep tunnels), expansion of rainwater pumping stations and retention basins, installation of backflow prevention devices and flood control pipes. Currently, Seoul City is considering constructing additional deep tunnels in frequently flooded areas such as Gangnam, in addition to the existing Sinwol deep tunnel, to temporarily store rainwater.

Non-structural measures include activating flood risk maps and disaster maps to provide residents with advance information. In Japan, flood risk map information provision during real estate transactions was piloted over the past 20 years and made mandatory from last year. Seoul City's recently introduced real-time urban flood forecasting and warning system is also part of non-structural measures. Professor Jeong Chang-sam of Induk University commented, "It was unfortunate that it did not work well during this heavy rain," adding, "Although this was due to the uncertainty of rainfall forecasts, it is necessary to watch whether improvements are made to make it work better." One-on-one management of disaster-vulnerable people and delivery of evacuation orders through local government officials are also ways to reduce precious human casualties despite property damage.

Especially as extreme climate phenomena intensify, there are calls to completely overhaul the existing river management system and water management system, which have become complacent. Currently, Korea manages design standards for flood frequency uniformly: 50 years for small rivers, 100 years for local rivers, and 200 years for national rivers. However, the hydrological engineering community has long suggested that these standards should be flexibly reorganized considering regional characteristics and intensifying extreme climate phenomena. For example, wetlands could be relaxed to 10 years, agricultural areas to 50 years, and urban areas strengthened significantly to 500 years. Design standards based on flood frequency influence the size of sewer pipes and are a key element in flood management.

Some voices also call for raising the height of existing dams or considering additional construction. Professor Jeong said, "We inevitably have to build more dams to endure droughts and floods caused by climate change," adding, "It is time to raise flood frequency standards, add emergency spillways to existing dams, and consider dam expansion and additional construction." He continued, "It is unacceptable that 50 to 60 people die annually from flood damage in Korea," emphasizing, "Strengthening local government capabilities and prioritizing the protection of citizens' lives and property must be regarded as the most important task. The era of treating flood damage with such complacency is over."