Analysis On The Characteristics Of Summer Precipitation Change In The Yellow River Basin And Its Causes

The Yellow River Basin(YRB)is sensitive to climate change,and is ecologically fragile.As the impact of climate change growing,the regional water cycle process there has been significantly changing.Precipitation is the most direct and critical part in the process of water cycle,so it has been widely concerned.Clarifying the spatial and temporal characteristics of precipitation,improving the study in precipitation changes of different intensity and exploring the mechanisms of precipitation changes are beneficial to the ecological protection and high-quality development of the YRB.Therefore,this paper analyzed the characteristics of summer precipitation changes in the YRB from 1982 to 2019,and investigated the causes of precipitation changes from the perspective of the water vapor transport.Further,we subdivided precipitation into extreme precipitation,moderate precipitation and light precipitation.Then,we discussed the precipitation changes of different intensity and the factors influencing them.The main conclusions are as follows:(1)Summer precipitation in the source area of YRB(SYRB)increased by 11.4%during 2003-2019 compared with 1982-2002.Such interdecadal increase of summer precipitation was closely related to water vapor transport.During 1982-2019,the water vapor for summer precipitation in the SYRB mainly came from local evapotranspiration(10.6%)and water vapor transport(84.8%).The external moisture source including Qinghai-Tibet Plateau region(35.8%),central Asia-European continent region(22.5%),South Asia-North Indian Ocean region(14.6%),South China Sea-West Pacific region(6.6%),and North Africa-West Asia region(5.3%).In the past 38 years,local evapotranspiration in the SYRB has not shown significant interdecadal variability.However,the water vapor supply of the central Asia-European continent region,North Africa-West Asia region,and South China Sea-West Pacific region increased during2003-2019.It provided sufficient water vapor for precipitation in the SYRB and resulted in an interdecadal increase in precipitation.Similar to the SYRB,the water vapor for summer precipitation in the upper reach and middle reach of the YRB(UYRB and MYRB)is dominated by water vapor transport(83.4%),and it has a significant interdecadal increase during 2003-2019.However,the variation of water vapor content in the UYRB and MYRB is from water vapor transport and local evapotranspiration.Local evapotranspiration decreased significantly in the UYRB and MYRB during2003-2019,resulting in no significant interdecadal variation in water vapor content,so the precipitation shows mainly interannual variabilities.(2)During 1982-2019,the summer extreme precipitation in the YRB was 101.3mm,accounting for 38.2% of the total precipitation.The extreme summer precipitation variability has increased in the YRB since 2010,and the extreme summer precipitation increased significantly in the MYRB for more than 60 mm in some areas.This variation of extreme precipitation in the YRB is significantly influenced by the East Asian subtropical westerly jet(EASWJ).There was increased summer extreme precipitation in the SYRB,northern UYRB,northern MYRB and lower reach of the YRB(LYRB)with the northward shift of the EASWJ.Over the south-eastern and central MYRB,the increased extreme precipitation was induced by the stronger EASWJ.Due to the different regions significantly affected by the position and intensity of EASWJ and their inconsistent effects over most of the region,simultaneous changes in both occur somewhat antagonistically.Therefore,single changes of EASWJ position or strength were more likely to contribute to extreme precipitation anomalies than co-influence.The anomalous atmospheric circulation pattern was regulated by EASWJ position or strength,resulting in changes of water vapor transport and local convection,which affect the summer extreme precipitation in the YRB.And the equatorial Pacific Ocean and Indian Ocean can make significant changes in the EASWJ through sea-air interaction.(3)From 1982 to 2019,the light precipitation days and moderate precipitation days in the YRB accounted for 92% of the total precipitation days in summer,and the cumulative light precipitation and cumulative moderate precipitation accounting for more than 50% of the total precipitation.The proportion of cumulative light precipitation and cumulative moderate precipitation to the total precipitation showed a decreasing trend,which is more pronounced in the SYRB areas.However,the light precipitation and moderate precipitation in the YRB shows mainly interannual variation.The study shows that both light precipitation and moderate precipitation in the YRB have a significant positive correlation with the lower tropospheric relative humidity.The relative humidity in the SYRB is more significant for light precipitation,and the response to relative humidity is more significant for moderate precipitation in nonSYRB areas.During 1982-2019,the offsetting effects of increased temperature and increased water vapor transport on relative humidity result in no significant trend in relative humidity over the YRB.Therefore,the relative humidity in the YRB shows interannual variation.Meanwhile,there is a significant negative correlation between atmospheric stability and light precipitation,moderate precipitation in the YRB.When the atmospheric temperature differences decrease and atmospheric stability weakens,it leads to the enhanced convection which favors light precipitation and moderate precipitation events,and vice versa.