Analysis of summer precipitation trends in the lower reaches of the Yangtze River from 1961 to 2020
The lower reaches of the Yangtze River(28°-33°N,116°-123°E)are economically developed and prone to frequent summer(June-August)flooding disasters.Studying the variability of summer precipitation in this region is of great significance.In recent decades,summer precipitation in this area has increased significantly.This study quantitatively analyzes the factors influencing this trend using precipitation term decomposition,based on observed summer precipitation from 1961 to 2020,TC best track datasets,and hourly mean reanalysis data from the European Centre for Medium-Range Weather Forecasts Reanalysis Version 5(ERA5).The results show that:(1)From 1961 to 2020,the average annual precipitation increase reaches 3.54 mm/year(passing the 95％significance test),with a growth rate of 38.1％.The variance contribution of the linear trend accounts for 20.7％of the total variance of summer precipitation.The increasing trend of daily precipitation reached 0.04 mm/day(passing the 95％significance test),which is the primary for the significant increase in summer precipitation in this region.(2)Precipitation term decomposition reveals that the variability of summer precipitation is influenced by vertical velocity,horizontal motion,water vapor,and evaporation.Among these,significant upward trends in water vapor and vertical velocity contribute to the increase in daily precipitation by 0.039 mm/day(passing the 99％significance test)and 0.019 mm/day(passing the 95％significance test),respectively.This is primarily re-lated to the increased vertical gradient of water vapor and the enhancement of vertical rise velocity.(3)The tem-perature of the lower atmosphere has risen due to the ground warming,while the upper atmosphere has cooled due to the phase change of the Asia-Pacific Oscillation(from a positive phase to a negative phase).The ability of the atmosphere to retain moisture is directly proportional to temperature.The intensified temperature difference between the upper and lower levels of the atmosphere increases the vertical gradient of water vapor,providing a-bundant moisture for increased precipitation.Additionally,abnormal positive vorticity in the upper atmosphere(200 hPa)and abnormal negative vorticity in the lower atmosphere(850 hPa)indicate that large-scale circula-tion does not favor the strengthening of upward motion.However,the anomalous convergence in the lower atmos-phere,combined with increased instability energy due to mesoscale system variations,offers favorable dynamic and thermodynamic conditions for enhanced vertical rise velocity and increased convective precipitation in the summer.Previous studies have lacked quantitative analysis of the dynamic and thermodynamic factors affecting summer precipitation trends in the lower reaches of the Yangtze River.This paper quantifies the contributions of water vapor and vertical velocity variability to the summer precipitation trend,providing a theoretical basis for un-derstanding precipitation changes in the context of global warming.
lower reaches of the Yangtze Riversummer precipitationprecipitation-term decompositionspecific humidity vertical gradientvertical velocity
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