Characteristics And Causes Of Summer Precipitation And Extreme Precipitation Over Northwest China

Due to the shortage of water resources in northwest China(NWC),NWC is very sensitive to global climate change,and its water resources are mainly contributed by summer precipitation,while observational data show a general increase in extreme precipitation events in NWC.Exploring the variability of precipitation and extreme precipitation events in NWC and their possible causes can not only enhance the understanding of climate change patterns in NWC,but also provide an important scientific basis for water resource redistribution.In this paper,we use the grid data of regional meteorological variables from the CN05.1,daily precipitation from the National Meteorological Station in NWC,the Asian Precipitation High Resolution Observational Data Integration Assessment(APHRODITE),the Climate Prediction Center(CPC),and the Tropical Rainfall Measuring Mission(TRMM)satellite data,as well as the European Centre for Medium-Range Weather Forecasts(ECMWF)circulation data and monthly mean SST data from the Hadley Center,the variability characteristics of summer precipitation and extreme precipitation in the northwest region were studied,and a causal analysis was conducted.The results show that:(1)the spatial distribution of summer precipitation in NWC from 1979 to 2020 shows that the main precipitation is concentrated in the Qilian Mountains and Tienshan Mountains,and the precipitation in the mountains and its contribution to NWC’s summer precipitation are both increasing.According to the changes of water vapor balance,the changes in evaporation and precipitation in Qilian Mountains and Tienshan Mountains have a consistent coupling relationship,indicating that there is a feedback relationship between the increase of evaporation and precipitation and the recent warming.We also found that the increase of precipitation is greater than the increase of evaporation,indicating that there is a net humidification trend in Qilian Mountains and Tienshan Mountains.(2)Using the water vapor balance equation to separate the contributions of each branch,we found that the increase of precipitation in Qilian Mountains is more contributed by the increase of the zonal and vertical water vapor transportation,while the increase of precipitation in Tienshan Mountains is more contributed by the increase of meridional water vapor advection and local evaporation.(3)Based on the analysis of four high-resolution precipitation data,the accumulation of daily precipitation greater than or equal to the 95th percentile in summer(R95TOT)and the maximum daily precipitation in summer(RX1day)generally decrease in a southeastnorthwest direction,with relatively high daily precipitation in the Tienshan and Qilian Mountains;conversely,TRMM underestimates extreme precipitation in mountainous areas,while APHRODITE and CPC capture the extreme precipitation characteristics in the northwest.The inter-annual increase of extreme precipitation in the northwest is closely related to the weakening of the South Asian high pressure,the strengthening of the western Pacific subtropical high pressure,the strengthening of the westerly rapids,and the formation and maintenance of the mid-and high-latitude Rossby wave,which can be traced to the western Pacific SST anomaly in May,June,and July.Generally speaking,the formation of the mid-and high-latitude Rossby wave is due to the fact that the elevated SST in the western Pacific promotes convective and non-adiabatic heating,which stimulates anticyclonic anomalies and interferes with the enhanced westerly rapids to produce a positive pressure Rossby wave through the Gill response.In addition,it directs more water vapor to converge in the NWC through anticyclonic anomalies in Western Europe and East Asia,and cyclonic anomalies in Central Asia,and enhances upward motion.It is suggested that the results of our research further supported the opinion above.