Spatio-temporal Characteristics And Influencing Factors Of The Proportion Of Recycled Moisture During Summer Half Year In The Arid Region Of Northwest China Based On LMDZ_iso

Local moisture recycling plays an important role in the land water cycle.Although the absolute amount of recycled moisture is limited in the arid region of Northwest China,the contribution to regional precipitation?that is,moisture recycling rate?should not be ignored.Based on the data of stable isotopes of precipitation and related meteorological elements?temperature,pressure,specific humidity,etc.?simulated by the LMDZ_isoso model,a global atmospheric circulation model nested with isotopic modules and based on the isotopic nonequilibrium fractionation hypothesis,the simplified Craig-Gordon model and Rayleigh fractionation principle,the isotopic values of precipitation moisture,recycled moisture and advection moisture were simulated.At the same time,with the help of isotopic mixing model,the spatio-temporal characteristics,action mechanism and influencing factors of the contribution of recycled moisture to precipitation in summer from 1979 to 2007 in the arid region of Northwest China are analyzed.Thus,a breakthrough had been made in quantitatively evaluating the moisture recycling rate in the arid region of northwest China by using the stable isotope technique.At the same time,the relative contribution of recirculating moisture and advection moisture to precipitation was compared and analyzed,and the importance of advection recycling in regional atmospheric advection equilibrium was further clarified,that was,the formation of precipitation in local areas and the provision of a small amount of precious water resources for arid areas.It was helpful to understand the process of water cycle and land-air circulation in the inland of Northwest China in summer.The interaction provided a valuable reference and provides ideas for the application of isotopic simulation in hydrology and water resources.The main conclusions of this paper were as follows:?1?The seasonal and interannual variations of?¹⁸O and?D in precipitation moisture,advection moisture,surface evaporation moisture and plant transpiration moisture in summer half year during the study period showed that the heavy isotopes in moisture in July and August were relatively rich,and that in April and October were relatively poor.The heavy isotopes transition from relative dilution to relative enrichment from April to July and from relative enrichment to relative dilution in August to October.The trend of year-by-year enrichment in the interannual.As far as the spatial distribution of moisture isotopes is concerned,on the whole,in April,July,October and summer,the?¹⁸O and?D in the moisture are relatively poor in mountainous areas,and are relatively enriched in the oasis and desert areas around the mountains.The distribution of?¹⁸O and?D is similar in April and October,and the enrichment degree of?¹⁸O and?D in July is higher than that in April and October.On the whole,the enrichment degree of?¹⁸O and?D in plant transpiration moisture is the highest,the spatial distribution of?¹⁸O and?D in precipitation moisture and advection moisture is similar,and the enrichment degree of?¹⁸O and?D in surface evaporation moisture is the lowest.?2?The seasonal and interannual variations of the contribution rate of advection moisture,surface evaporation moisture and plant transpiration moisture simulated by LMDZ_iso?free?and LMDZ_iso?nudged?showed that the contribution rate of advection moisture to precipitation was the highest,ranging from 79.9%to 99.4%and 77.4%to98.7%,respectively,and increased gradually in spring and summer?frem April to July?,but decreased gradually in summer and autumn?from August to October?.The interannual contribution to precipitation was ranging from 90.1%to 94.5%and 88.2%to 93.6%,respectively,and increased year by year,and its interannual contribution was ranging from 302 mm to 485 mm and from 26.5 mm to 508 mm,respectively.The contribution rate of surface evaporation moisture to precipitation was the lowest,which was ranging from 0.01%to 14.1%and 0.01%to 8.2%respectively,and the seasonal fluctuation was obvious.and decreased gradually from April to June,and increased gradually from July to August,and then decreased gradually from September to October.The interannual contribution was ranging from 0.1%to 0.7%and 0.2%to 0.8%,respectively,and the interannual contribution was ranging from 2mm to 8 mm and 3 mm to 10 mm,respectively.The contribution rate of plant transpiration moisture to precipitation was between advection moisture and surface evaporation moisture.It was between 0.05%to 15%and 0.16%to 21.2%in season,decreased gradually from April to July,and increased gradually from August to October,and interannual contribution was between 5.2%to 9.7%and 5.9%to 11.3%,and 24 mm to 47 mm and 26 mm to 51 mm,respectively.On the whole,the contribution rate of plant transpiration moisture was higher than that of surface evaporative moisture,but the sum of them showed spring and summer season?from April to July?decreased gradually,summer and autumn season?from August to October?increased gradually.And the interannual contribution rate showed a downward trend year by year.The contribution rate of each moisture to precipitation in summer half year was uneven in space:the contribution rate of advection moisture was generally higher,and the higher value?from 85%to 100%?appears in the mountain area,and its contribution was higher than 200 mm.The lower value?from55%to 80%?occured in the Oasis and the desert areas,and its contribution was less than 50 mm;When the contribution rate was between 80%and 85%,the contribution was from 50 mm to 200 mm.Compared with the contribution rate of plant transpiration moisture,the contribution rate of surface evaporative water vapor was lower in most areas?from 0 to 5%?,and its contribution was less than 5 mm;but the contribution rate was higher near mountain area?from 5%to 20%?,and its contribution was from 5 mm to 100 mm.On the whole,the contribution of advection moisture and surface evaporative moisture to precipitation was positively correlated with its contribution rate,that is to say,it was higher in mountainous areas and lower in oasis and desert areas.The contribution of plant transpiration moisture to precipitation was negatively correlated with its contribution rate.When the contribution rate was from 15%to 35%,the contribution was less than 5 mm,and when the contribution rate was from 0 to 15%,the contribution was between 5 mm and 100 mm.?3?Isotopic fractionation will occur in the process of moisture transport in the west wind,and with the change of moisture transport intensity,its fractionation degree will be different,which leads to the change of the enrichment degree of isotopes in foreign moisture.Therefore,the temporal and spatial contribution rate and contribution amount of advection moisture to precipitation change.In July,?¹⁸_advO and?_advD in advection moisture were relatively enriched,and the contribution rate and contribution amount were higher.From April to July,?¹⁸_advO and?_advD transition from relative dilution to relative enrichment,and transition from relative enrichment to relative dilution from July to October,indicated that the precipitation vapor formed by advection moisture changed from less to more and from more to less,respectively.In addition,most of areas of the arid region of northwest China are in the transition zone between westerly circulation and subtropical high pressure zone,which is affected by the northward movement of subtropical high in summer,and the westerly current is unstable in the area south of 45°N.The moisture content in most areas of southern Xinjiang was less,the contribution rate and contribution rate were low,and the precipitation in southern Xinjiang was not as much as that in northern Xinjiang.In April,the water and heat conditions in the study area were poor,which restricted the isotope fractionation of moisture from surface evaporation and plant transpiration,thus?_ev¹⁸O and?_evD and?_tr¹⁸O and?_trD were relatively poor.At the same time,surface evaporation and plant transpiration contributed less moisture.In July and August,the water and heat conditions in the study area were better,and the water vapor isotopic fractionation of surface evaporation and plant transpiration was more thorough,as a result,?_ev¹⁸O and?_evD and?_tr¹⁸O and?_trD were relatively enriched,while surface evaporation and plant transpiration contributed more moisture.?4?Precipitation moisture is composed of advection moisture,surface evaporative moisture and plant transpiration moisture,and the contribution of advection moisture to precipitation is dominant by westerly water vapor.The increased of advection moisture transport made precipitation increased,but also made the local recycled moisture ratio reduced in the total precipitation in the arid region of Northwest China.The recycled moisture played a higher compensating role when the precipitation decreased,that was,the contribution rate is higher.Due to local water and heat conditions,moisture recycling contributed greatly in July and August,but overall,advection moisture transport was still the main source of precipitation,so the contribution rate of recycled moisture in this period was relatively low.