Effects Of Water Vapor Transport Of Meiyu On Stable Isotopes In Atmospheric Vapor In The Typical Area Of The Middle And Lower Reaches Of The Yangtze River

Hydrogen and oxygen stable isotopes are important tracers for different forms of hydrological cycle due to remarkable isotopic fractionation when water phases change.In addition,isotopic records derived from ice cores,stalagmites and tree rings can be used to reconstruct paleoclimate.However,factors affecting stable isotopes in precipitation over the East Asian Monsoon region are extremely complex,which is the reason why climatological implication of isotopic records from stalagmites in the monsoon region remains unclear.As a unique weather and climate phenomenon in East Asia,Meiyu is the product of the evolution of eastern Asia atmospheric circulation from the spring to summer.Few studies have been investigated on the evolutionary characteristics and influencing mechanisms of atmospheric water vapor stable isotopes during the Meiyu period.Based on atmospheric water vapor isotope observation system(Picarro L2120-i)established in the Station for Observing Regional Processes of the Earth System(SORPES)of Nanjing University,the evolutionary characteristics and influencing mechanisms of atmospheric water vapor stable isotopes during 2013 to 2016 at Nanjing during the Meiyu period are investigated,from the perspectives of large-scale circulation(changes in moisture source and the convection process along the air mass trajectories),interactions between mesoscale cold and warm air masses,local convection activities,and so on.The observations show that large fluctuations of atmospheric water vapor stable isotopes(?18Ov)during the Meiyu period in Nanjing may be related to the complicated transport processes of Meiyu water vapor.Preliminary conclusions are drawn as follows.(1)Changes of moisture source and the convection activities along the air mass trajectories are the main reasons for the variation of atmospheric water vapor stable isotope in its downstream region.When moisture comes from distant regions such as the Indian Ocean and the Bay of Bengal,long-distance vapor transport causes gradual depletion of water vapor ?18Ov due to continuous rainout processes.In addition,strong convection in the vapor source region and along the air mass trajectories would aggravate the degree of depletion of water vapor ?18Ov.By contrast,when water vapor comes from the nearby western Pacific,the degree of rainout of air mass during its transport is weaker due to the shorter transport distance,which leads to the enrichment of water vapor stable isotopes.(2)Meiyu is a quasi-stationary frontal convective rainband over the middle and lower reaches of the Yangtze River,formed by the interactions of warm moist air mass from the south and dry cold air mass from the north.When the intensity of the southern warm moist airflow is weaker than the northern dry cold airflow,water vapor with high?18Ov value dominates the study region,leading to ?18Ov of near-surface is not severely depleted or even enriched during the precipitation event.(3)In addition to the effects of large-scale circulation factors and interactions between mesoscale cold and warm air masses on the atmospheric water vapor stable isotopes of Meiyu,local factors also affect the water vapor stable isotopes,such as sub-cloud processes(the diffusive exchanges of stable isotopes between raindrop and vapor,the re-evaporation of the falling rain),the evapotranspiration process,unsaturated subsidence of air mass,and so on.At synoptic timescale,these local factors cannot be ignored.