Characteristics And Possible Causes Of Atmospheric Water Recycling In The Middle And Lower Reaches Of The Yangtze River

The middle and lower reaches of the Yangtze River(MLRYR)are located in the East Asian monsoon region,with large annual precipitation and high variability.Especially in recent decades,in the context of global climate change,although the total amount of precipitation in the MLRYR has not changed significantly,extreme precipitation events have increased significantly,and the resulting meteorological disasters have also increased.In the summer of 2020,the MLRYR experienced the strongest precipitation since 1960,which caused severe floods,causing direct economic losses of 178.96 billion yuan and causing 219 deaths or missing persons.This is a typical example.However,what changes have taken place in the precipitation characteristics in the MLRYR?What is the reason?There is no clear understanding yet.From the perspective of the water vapor source of precipitation,this paper divides the precipitation in the MLRYR into a component from the local evaporation contribution(evaporation contribution)and a component from the advection contribution from external regions(advection contribution)with the aid of a precipitation recirculation rate(PRR)evaluation model.and the water vapor source of precipitation can be divided into local evaporation and external water vapor transport.On this basis,we analyze the change characteristics of the PRR and the various components of the atmospheric water cycle in the MLRYR,and use linear regression and trend analysis to study the relationship between each component and the external forcing factor,and then analyze the possible reasons for the change of precipitation in the MLRYR.Finally,the water cycle characteristics and possible causes of the super strong Meiyu process in the MLRYR in the summer of 2020 are analyzed.Details as follows:Firstly,the calculation scheme of the contribution of water vapor to the regional grid evaporation was optim ized based on the Eltahir precipitation recycling rate evaluation model,the PRR evaluation model was reconstructed,and the change characteristics and main effects factors of the PRR in the region were calculated and analyzed.We mainly use the NCEP/NCAR atmospheric circulation reanalysis data from 1961 to 2017 and the precipitation data provided by CPC to calculate the PRR in the MLRYR from 1961 to 2017,and analyze the variation characteristics and reasons.The results show that the PRR in the MLRYR is the largest in autumn(0.3),the second in winter(0.28),and the smallest in summer(0.23);the PRR shows an upward trend in all seasons(except summer,all other seasons pass the 95%significance test),the largest in autumn(0.38),and the smallest in summer(0.09);The reasons for the changes were further analyzed from external forcing factors such as sea temperature and circulation.When the SST of the Northwest Pacific is high(the SST of the Middle East Pacific is low),it promotes the local upward movement,which is conducive to the development of cyclones,which suppresses the water vapor input from the West Pacific,the South China Sea and the Indian Ocean Bay of Bengal.The anticyclone anomaly near Lake Baikal suppresses the water vapor input from the westerly belt,the 500hPa height field in the middle troposphere(except in autumn)shows a "+-" dipole distribution from north to south in East Asia,which inhibits the northward transport of water vapor and makes the PRR in the MLRYR higher;on the contrary,it makes the PRR lower.For local factors,the PRR in the MLRYR is mainly affected by precipitation and cloud cover.Secondly,the components of the atmospheric water cycle in the MLRYR are separated and their variation characteristics are analyzed with the help of the evaluation results of the PRR.The results show that:the water vapor input presents a downward trend in the annual average and each season,while the overall precipitation presents an upward trend,in which the precipitation in spring and autumn shows a small downward trend,while the precipitation in summer and winter shows an upward trend(summer passes the 95%significance test);The contribution of precipitation advection to the precipitation in the MLRYR is about 77%,which is the most important source of precipitation,and its variation characteristics are basically consistent with the precipitation variation characteristics.The evaporation contribution shows an upward trend in the annual average and each season,indicating that the evaporation contribution to the precipitation will become more and more important in the future.Combined with the decrease of water vapor input and the increase of contribution term of horizontal flow,it can be concluded that the precipitation efficiency becomes higher.With climate change,the water vapor entering the MLRYR will be "easier" to condense and fall,especially in the annual average,summer and winter;Combined with the increasing trend of evaporation contribution,it can be seen that the local water cycle intensifies,which is particularly significant in all seasons(except winter).Finally,according to the super strong Meiyu in the MLRYR in 2020,the characteristics of precipitation recycling in the middle and lower reaches of the Yangtze River in summer of 2020 are analyzed.The results show that:the precipitation has the characteristics of long duration,large precipitation magnitude and stable rain belt,the PRR is obviously smaller than that in previous years,the water vapor input is more from June to mid-July,and then decreases relatively,the corresponding effective precipitation rate is abnormally larger from June to mid-July than that in previous years(almost twice of that in previous years),and then returns to the average level of previous years;Furthermore,from the perspective of water vapor conditions,atmospheric instability and atmospheric uplift conditions,it is conducive to precipitation from June to mid-July,which makes a large part of the water vapor from the external advection to the MLRYR form precipitation and fall down compared with previous years.After the middle of July,it returned to the average level of previous years.In general,the PRR in the MLRYR has shown an upward trend in all seasons in recent decades(except for summer,the other seasons passed the 95%significance test),indicating that the local water circulation in the region has intensified.The amount of precipitation is increasingly dependent on local evaporation,but the increase in total precipitation is not significant due to the overall decrease in the transport of external water vapor.The unusually high summer rainfall in 2020 may be due to the increase in early evaporation and more water vapor has been stored in the atmosphere.With the advent of the monsoon,a large amount of foreign water vapor has poured into the MLRYR,which is conducive to triggering precipitation conditions.Under the combined action of internal and external water cycles,extreme precipitation is more likely to form.