The Korea Meteorological Administration (KMA) monitors natural disasters by dividing them into five high-impact weather categories: heavy rainfall, typhoons, heavy snowfall, strong winds and storm surges. In the last decade, 2011 was the year of the greatest damage caused by several heavy rains in Seoul, South Korea. USD 290 million (GDP USD 36,601 in 2015) damages were estimated in Seoul and in adjacent metropolitan areas in 2011 (MPSS, 2016).
In order to analyze the cause of heavy rain event, this study is to understand the occurrence of heavy rainfall using historically climatic records during the summertime throughout the Korean peninsula. First, the datasets during 1778-2016 were lined up in a descending order, and then years were identified with more than two-σ in statistical level (Lee, et. al., 2017). As seen in Table 1, we selected ten significant annual precipitations, which had some dominant features in generating heavy rainfall events.
2. Data and Method
The Annals of the Joseon-Dynasty (Joseon Wangjo Sillok into Korean title) is a book written by the kings' archivist in chronological order for 472-year long (1392-1863) and is inscribed in UNESCO's Memory of the World in 1997. This book contains some of the national risk management responses for disasters associated with high-impact weathers. Seungjungwon Ilgi was registered in UNESCO in 2001 as official document written by the kings' secretariat in 1623-1907. In this record, daily weather description and rain-depth are continuously kept with time records. Archived Monthly Weather Review of American Meteorological Society before 1940 provided useful information about understanding on meteorological facts in 1789 and 1940. After 1907, data are included in the KMA archived data and the ECMWF reanalysis (September 1958 - August 2002) data. The KMA data consists of climatic numerical data, synoptic charts, and meteorological satellite images.
We selected ten annual precipitations beyond two-σ statistical level in 239-year dataset (1778-2016). Each year contains several heavy rain events. According to Maddox et. al. (1979) suggestion, daily heavy rainfalls are selected to more than 100 mm/day in each heavy rain event. And then these selected events have been summarized into significant meteorological features and policy plan against socio-economic impacts. The events before the year 1907 were based on the social response records for natural disaster without meteorological records. Then, counting to the number of days with precipitation was used for the analysis by means of alternative treatment.
There are five types of cases in which rainfall of more than 100 mm/day from June to September is reduced. Among these types, Asian monsoon is classified by Chen et. al., (2007), and is otherwise classified into the forms shown in synoptic weather charts. As seen in the Fig 1, the heavy rain type is as follows: typeⓐ is stationary front associated with the Asian summer monsoon so-called Chang-Ma (e.g., Mei-yu in China, Baiu in Japan), typeⓑ is the stationary front associated with post Asian summer monsoon, typeⓒ is mid-latitude synoptic-scale cyclone, typeⓓ is landing and direct impact on tropical cyclone (i.e., typhoon), typeⓔ is rain-shower caused by the local atmospheric instability on the boundary of mT in summertime.
3. Monthly review of heavy rain events in ten significant annual precipitations
During every June of the selected period, daily rain events of more than 100 mm were recorded only three times. The record of 152 mm on 30th of June 1821 is an unidentified type, while the record of 100.7 mm on 24th of June 1990, and 177 mm on 29th of June are in typeⓐ by the KMA DB. More than 100 mm/day amount of the 1821 cases can be estimated as typeⓐ climatically and statistically. Particularly, a record of 67 mm on 18 June 1879 is written in the SI, simultaneously tracking of low pressure-system was confirmed on the MWR in 1879.
During every July of the chosen period for more than 100 mm/day, 11 events occurred in 1821, 1832 and 1879. After 1907, 15 events occurred as follows; eight in typeⓐ, three in typeⓒ, and four in unknown. Unfortunately, it is difficult to determine the appropriate type before 1907, and four days in early July 1940 owing to the lack of weather chart at that time. However, there are several evidences on long lasting rain and heavy rain events in the SI.
For instance, number of days with rain in July 1821 is the following: 14 days in 1-14; 2 days in 16-17; 4 days in 19? 22; and 7 days in 25?31. It lasted for 13 days during July 1832, and it continued in 18-31 (excluding July 23rd). In July of 1879, a total of 18 days were recorded: 1-2, 8-13, 15-16, and 18-25. In conclusion, these three years display continuous behavior similar to typeⓐ.
Nine events were recorded in August of the specified period. Before 1907, three events occurred and six events after 1907. Two events - 211.4 mm on 4 August 1998 and 177.0 mm on 24 August 2003 - indicated Typeⓒ with strong southwesterly lower level jet, instability at middle layer and divergence in upper layer. Four events which occurred in 6, 8, 14, 20 August 1998 indicated typeⓑ. For example, 332.8 mm on 8 August 1998 was maximal recorded in Seoul. With the changing position of the sun, stationary front over Mongolia slowly moves southward to East-China Sea. The change of astronomical season, followed by weakened solar heat energy, depressed mT.
Six events were recorded in September of that period. 221.1 mm on 3 September 1940 indicated typeⓓ from Bernard and Doucette (1940). Three events on 9-11 September 1990 as typeⓑ were 118.7 mm, 120.0 mm, and 247.5 mm respectively. Particularly, 259.5 mm on 21 September 2010 as the typeⓒ displayed the development of a mid-latitude meso-α low-pressure system in the Shandong Peninsula of China. Consequently, it is intensified while passing through the Yellow-Sea.