The Characteristics Of Precipitation Conversion And Recycle Over Arid Regions In Northern Hemispheric And Their Changes Under Global Warming

Water Cycle is a crucial subject in climate change as well as hydrological science.It not only links to the weather phenomena,like rainfall and snow,but also to the global energy budget.On the one hand,because lack of advective vapor,the property of total vapor from local water cycle in arid region is a crucial scientific question,on the other hand,evapotranspiration is a main variable in water cycle,the estimation of evapotranspiration is the key to quantitative evaluate the water cycle.However,comparing with the same latitude regions,the higher air temperature and less soil moisture make the estimation of evapotranspiration in arid region has an inconvenient uncertainty.How to estimate the evapotranspiration in an objective and accurate way is another scientific question needs to explore.In this paper,based on the multi-reanalysis data and model outputs,the different evapotranspiration estimations are compared and analyzed,a more reliable modified evapotranspiration estimation is proposed.Also,the spatial and temporal characteristics of water cycle over Northern Hemispheric arid regions during recent 30 years are analyzed,mainly on the features of the precipitation conversion and recycling.Further,the changes of water cycle during different periods(early period: 2017-2036,middle period: 2047-2066,late period: 2077-2096)of 21 st Century is forecasted.Using Penman-Monteith equation and Brubaker precipitation recycling model,the precipitation recycling ratios(PRR)in China-Mongolia(CM),West Asia(WA),North Africa(NAF)and North America(NA)are calculated,the averages are 5%,12%,18% and 8% respectively.It represents that the less precipitation corresponds with the higher PRR.However,the trend of PRR is 0.43% 10a-1 in WA and indicates that precipitation recycling makes up the decreasing trend of precipitation under the net output of advective vapor.During the recent 30 years,the trends of PRR in NAF and NA are-0.45% 10a-1and-0.26% 10a-1,corresponds with the increasing trends of Penman evapotranspiration(0.07 mm day-1 10a-1)and precipitable water(0.12 kg m-2 10a-1),and decreasing precipitation(-0.02 mm day-1 10a-1),represents that the vapor is enhanced though the evaporation from land but do not convert into actual rainfall.The PRR decreases with increasing precipitation and enhancing advective vapor transport in NAF implies that the property of local evaporated vapor converts into actual precipitation becomes less.Generally,the main vapor source is advective transport and precipitation recycling affects as compensation.Using Penman evapotranspiration estimations(PET)and the correcting evapotranspiration(CET)by considering soil moisture,the precipitation recycling in Asian-African arid regions is calculated with Dynamic Recycling Model.The results indicate that limited by the soil moisture,CET is less than PET in arid regions.And the trend of ET-P indicated the aridity trend is catabatic in CM and NAF.It can be explained as that the increasing advective vapor leads the PRR reduced.And it can be treat as this mechanism: the decreasing PRR links with the enhanced advective vapor transport,and this will lead more precipitation and ease up the drought.On the contrary,the increasing PRR indicates less of advective vapor transport and even the local vapor makes up the reduce precipitation,the arid trend will still enhance.The typical precipitation processes in CM is simulated using WRF and FLEXPART model and analyzed.On the one hand,the results indicate that during the recycling process,evapotranspiration transports vapor from land to atmosphere from afternoon,PRR enhances and reach to the maximum at evening.Correspond with the maximum PRR,precipitation,evapotranspiration and precipitable water conversion rate(PWCR)will reach to the top at the evening.Then,as the temperature cool down,evapotranspiration decreases,PRR reduces from night to the morning.On the other hand,changes of precipitation,evapotranspiration and PRR have 10 days' quasi-period.And the negative mechanism is still fit for daily scale precipitation in CM: the decreasing precipitation enhances the evapotranspiration,then increased evapotranspiration leads more vapor into atmosphere as precipitation recycling,and makes up the reduced precipitation.Further,the vapor contributed by zonal transport is the mainly vapor source to arid region rainfall,reaches to more than 40%.And vapor contributed by meridional is about 7% ~ 20%.Moreover,when the meridional vapor transport enhances,the amount of rainfall is larger,vice versa.By analysis the precipitation recycling during 21 st Century,the results indicate the precipitable water(PW)will increase obviously.However,PRR and PWCR will decrease consistently,especially under RCP8.5,the negative anomalies reach to-2.7% and-0.9% respectively.Over the typical arid region,PW will increase significantly,the maximum increasing trend reaches to 1.85 mm day-1 10a-1,meanwhile,the decreasing PRR and PWCR represents the advective vapor transport will enhance and speed up,indicate the vapor cycle is accelerated,nevertheless,the indistinctively changes of ET-P under different emission scenarios in CM indicates that the accelerated vapor cycle will not lead humidification in arid region.