Extreme Features of Severe Floods in the Middle and Lower Reaches of Yangtze River Basin during Meiyu Period in June-July 2020
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Abstract:
Severe flood occurred in the middle and lower reaches of the Yangtze River Basin (MLYRB) in June-July 2020 (JJ2020). In this paper, the extreme features of the flood are analyzed based on daily precipitation observation at national stations from 1951 and hourly observation at national & regional stations in JJ2020. Both the flood range (anomaly percentage more than 50% or 100%) and the total amount are significantly greater than those in 1998 and 2016, and ranked the second in recent 70 years followed 1954. There are three rainfall centers with maximal accumulated precipitation greater than 1500 mm, including Dabie Mountain area, Southern Anhui Province and southwestern Hubei Province. The flood is closely related to the extreme long-term persistence of Meiyu in MLYRB. The Meiyu period in 2020 lasts for 53 days, 23 days more than the climatology. In JJ2020, positive precipitation anomaly existed in each dekad, which was never occurred before since 1961 and also indicates the extreme features of the flood. Total station numbers in JJ2020 are all the maxima since 1961 at four different rainfall grades (i.e., 10-24.9 mm/day, 25-49.9 mm/day, 50-99.9 mm/day and ≥ 100 mm/day), especially in the latter three grades. Extremity of the flood can also be found in the short-duration heavy rain processes. In JJ2020, the accumulated station number is 1612 and 100 with hourly precipitation over 20 mm and 50 mm, respectively. Both of the numbers are the highest since 1981. At four stations with total precipitation exceeding 1500 mm, frequencies of hourly precipitation greater than 20 mm are 2~4 times to their climatologies. Accumulation of the first 43, 21, 19 and 27 strongest hourly precipitation exceeds their seasonal average, respectively. Above results indicate the significant extremity of the flood in MLYRB existed in the total rainfall, the duration and the short-term intensity.