Gaoyuan qixiang (Aug 2024)
Extreme Characteristics and Formation Mechanism of Heavy Rainfall in the Middle Reaches of the Yellow River from July to August 2022
Abstract
The extreme characteristics and formation mechanism of the sustained heavy rainfall process in the middle reaches of the Yellow River from July to August 2022 were analyzed in detail using precipitation observation data from national and provincial meteorological stations， ERA5 reanalysis data from the European Medium Range Weather Forecast Center， and terrain elevation data from the National Environmental Information Center of the United States.The results indicate that： （1） a total of 11 heavy precipitation processes occurred in the middle reaches of the Yellow River during the research period， characterized by long duration， large cumulative rainfall， short precipitation time intervals， and high overlap of heavy rainfall areas.The standardized anomaly of accumulated precipitation at 18 stations exceeds 2.5， the maximum hourly rainfall intensity exceeds 50 mm/h during multiple processes， exhibiting significant extreme characteristics， and the extreme of precipitation is stronger in August than that in July.The distribution of heavy rain bands is closely related to the topographic characteristics of the middle reaches of the Yellow River.（2） From July to August， the standardized anomaly near Lake Baikal in mid to high latitudes Asia reached -2.5 to -1.5 （Exceeding -3.5 in August）， and the low trough is abnormally strong compared to the same period in previous year.During processes of heavy precipitation， the western ridge of the subtropical high is in a westward moving state.The ridge line of the subtropical high and the north-south boundary of the northern boundary have a large swing in July.Each north-south oscillation of the subtropical high combines with cold air from Lake Baikal， triggering a series of heavy precipitation processes.The ridge line and northern boundary of the subtropical high slowly retreat to the south in August with a small amplitude， and persistent heavy precipitation occurred at the edge of the subtropical high.（3） The water vapor come from the Bay of Bengal， South China Sea， and East China Sea， with an integrated standardized anomaly of 2.5 for the entire layer of water vapor flux in August.PWAT maintains 40~60 mm， with its standardized anomaly ranging from 2.5 to 3.5 in the southern part of Inner Mongolia and the northern part of the Shaanxi region， locally exceeding 3.5.The water vapor conditions are significantly stronger in August than in July.（4） There is an unstable stratification above the middle reaches of the Yellow River， characterized by upper dry cold and lower warm wet.Frontogenesis is formed north of 36°N in the middle and lower layers， and the upward movement occurs throughout the northern layer.The vertical velocity standardization anomaly reaches -2.5 to -0.5， which is corresponding to strong precipitation area.（5） The analysis of frontogenesis function in August shows that there were frontogenesis before and during the occurrence of precipitation in the northern part of the middle reaches of the Yellow River.The increase and decrease in frontogenesis values are consistent with the trend of precipitation intensity variations.The deformation term contributes significantly to the total frontogenesis， while the tilt term contributes significantly to the total dissipation.The process with high precipitation intensity results in higher frontogenesis extension height， while the process with low intensity results in lower frontogenesis height and relatively smaller frontogenesis value.
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