Observation Of Stable Isotopes Of Water Vapor And Precipitation Near The Ground In Nanjing

Atmospheric water vapor and precipitation are the important components of the atmospheric circulation,while its stable hydrogen and oxygen isotopes are also served as important carriers in the research of global or regional atmospheric circulation.Among them,precipitation isotopes has been widely used in the study of various processes of the water circulation and has achieved fruitful results.However,as a climate indicator,precipitation isotopes still has certain limitations.First of all,raindrops may be subject to evaporation process when dropping,which may cause isotope dynamic fractionation,and may lead to shift to the information record of water vapor source area.In addition,water vapor itself is a mixture of different sources before the forming of precipitation.Therefore,long-term and continuous monitoring of stable isotopes in water vapor will help us better understand the changing mechanism of stable isotopes in hydrological processes.This work was essential for the accurate interpretation of the climatic significance of isotope records.In recent years,with the popularization of spectral isotope technology,we have been able to continuously observe water vapor isotopes.In this study,we take Nanjing that located in the Eastern part of China as the study area,where the climate was typically affected by the East Asian monsoon.Here we use the daily mean values of the stable isotopes in precipitation and water vapor,which were collected from the continuous observations from September 2011 to December 2018.And we further(1)systematically analyzed the seasonal variation characteristics of the stable isotope composition in water vapor and precipitation;(2)study the local meteorological factors that affect the stable isotopes of water vapor and large-scale atmospheric circulation;(3)simulate the effects of re-evaporation under clouds and local water vapor recirculation on stable isotopic compositions of precipitation.The main conclusions of this research are as follows:(1)In terms of seasonal changes,the stable isotopes in water vapor are significantly different from those in precipitation.Values of?¹⁸O_vand?D_vare high in spring,and low in autumn and winter,while relatively low values are presented in summertime;d-excess_vand d-excess_pshowed similar seasonal variation patterns,which are characterized by low values during summer monsoon and high values in autumn and winter.On the seasonal scale,the d-excess_vvalue is higher than the d-excess_pvalue in the next year.The annual average?¹⁸O_v,?D_vand d-excess_vare-16.93‰,-115.97‰and 19.51‰respectively.(2)The influencing factors of water vapor isotopes in different seasons are obviously different.In spring,autumn and winter,?¹⁸O_vhas a strong correlation with local meteorological factors such as surface temperature,water vapor concentration and surface pressure.The results indicate that the index should be more significantly affected by local climatic factors;During summertime that affected by monsoon activities,?¹⁸O_vweakly correlated with local meteorological factors and the low values of?¹⁸O_vare mainly controlled by the upstream convective activity.In addition,results from the backward trajectory simulation also showed that in the summer monsoon seasons,the low values of?¹⁸O_vare mainly related to the upstream convective leaching during the large-scale water vapor transport process,and the reason for the lowest value of d-excess_vis related to the ocean water vapor source in the season.The isotope dynamic fractionation is weaker.Throughout the year,the?¹⁸O_vvalues in spring are the highest.The reason may be related to the lower isotope fractionation at relatively high temperatures as the relatively high d-excess_vvalue is caused by the kinetic isotope fractionation under dry conditions.The?¹⁸O_vvalues in autumn and winter are the lowest,which may be due to the strong fractionation of isotopes at low temperatures.In addition,due to the strongest isotope fractionation in the continental water cycle under dry climate conditions,the d-excess_vvalue of the season is at the highest level.(3)The seasonal variation trend of water vapor and precipitation stable isotopes in precipitating days in Nanjing was basically the same.Hydrogen and oxygen isotope values of water vapor and precipitation were the highest in spring and relatively low in other seasons,while their highest d-excess values were both exist in autumn and winter,and the lowest d-excess values occurred in summer.The?¹⁸O and d-excess difference between precipitation and water vapor indicate that the raindrops were affected by the surrounding unsaturated air,thus occurred fractionation to a certain degree,causing the stable isotopes in raindrops continuously enriched during their precipitating process;while the stable isotope of water vapor in surrounding atmosphere were relatively depleted.Based on the Rayleigh model of isotopic equilibrium depletion,this study calculated the sub-cloud raindrop evaporation.Results showed that the sub-cloud evaporation of raindrop was weakest in summer,strongest in spring,followed by autumn and winter.(4)Meteorological factors such as precipitation,temperature and relative humidity all had varying degrees of influence on the sub-cloud raindrop evaporation.When the rainfall is small,the sub-cloud evaporation of raindrop was more significant;when the temperature was too high or too low,the sub-cloud evaporation of raindrop was not obvious;as the relative humidity increases,the sub-cloud evaporation of raindrop gradually weakened.(5)Using the observed isotope data and modified Stewart Model,the effects of sub-cloud raindrop evaporation on the stable isotopes of precipitation and water vapor in Nanjing were estimated.Results showed that the annual averaged raindrop evaporation rate on precipitation days in Nanjing is?11%.The raindrops evaporation rate was largest in spring,and were relatively low in summer,autumn and winter.There existed significant linear correlations between the evaporation fraction of the raindrop and changes in precipitation??¹⁸O,??D,and?d in different seasons.(6)The influence of various meteorological parameters on the model simulation results was analyzed.There was a significant impact of relative humidity on the d-excess in precipitation.For every 5%increase in relative humidity,?d value of precipitation increased by an average of 3.6‰.The influence of temperature on d-excess was relatively small,for every 5°C increase in temperature,the average?d value of precipitation decreased by 1.4‰;The effect of raindrops diameter on the d-excess of precipitation was that for every 0.2mm increase,?d value increased by1.5‰.(7)Utilizing the binary isotope mixing model,the utilization rate of local water vapor recycling was estimated.The arithmetic average of the local water vapor recycling rate in Nanjing was 11.4%.The average utilization rate was the lowest in summer,the highest in autumn and winter,and in the middle in spring,and its variation range in autumn and winter was much greater than in other seasons.