The Influence Of Water Vapor Transport On The Precipitation Variability And The Atmospheric Pollution In China

The influence of water vapor transport on the precipitation variability and the atmospheric pollution in China is studied in this paper via various statistical analysis methods,mainly based on the observation data,the NCEP/NCAR reanalysis data,and the ERA-interim reanalysis data.Firstly,the climatic characteristics,the interannual and interdecadal variation characteristics of the water vapor transport in China are analyzed.And then the interdecadal variation characteristics of the water vapor transport in Northwest China and its influence on the rainfall are studied.In addition,the impacts of El Nino events on the summer water vapor transport and rainfall in China are also discussed.Finally,the influence of water vapor transport and humidity conditions on the formation of haze over central-eastern China is discussed.The main conclusions are summarized as follows:(1)The water vapor transport has obviously interdecadal variabilities in four seasons in China and has important influence in rainfall.In spring,the water vapor transport and rainfall over southern China is characterized by a decadal weaking trend since the 1980s.In summer and in autumn,the water vapor transport presents a decadal weaking trend since the mid-1970s.Meanwhile,the rainfall over eastern China changes from the so-called flooding in the north and drought in the south mode to the opposite one.In winter,the water vapor transport orinating from Bay of Bengal and the South China Sea shows a decadal weaking trend since the mid-1980s,and the rainfall over eastern China changes from less in the north and more in the south mode to the opposite one.The influence is significant of El Nino on the interannual variation of water vapor in four seasons in China.The water vapor transport in China is weak in the developing summer,but plentiful moisture orinating from Bay of Bengal is transported into South China.In the decaying spring and summer,plentiful moisture orinating from northwest Pacific Ocean is transported into South China or Yangtze valley.(2)Annual and seasonal precipitation presents a significant rising trend in Northwest China from 1961 to 2015.The annual and seasonal actual evapotranspiration(ETa)and moisture transport also show the similar rising trend.Moreover,the annual and seasonal precipitation recycling ratio in most areas of Northwest China presents a significant increasing trend,indicating that land surface hydrologica cycle in the Northwest China is strengthened,especially in the southern xinjiang and the Tibetan plateau.In the western region of Northwest China,both of the increasing of ETa and moisture transport are benefit for the increasing of precipitation in spring,summer and autumn,but the contribution of the increasing of moisture transport is grater than the increasing of ETa.The precipitation in the eastern region of Northwest China presents decreasing trend since that the decreasing trend of moisture transport is grater than the increasing of ETa.In winter,the precipitation in most areas of Northwest China presents increasing trend due to the increasing of moisture transport and ETa,but the contribution of the increasing of moisture transport is grater than the increasing of ETa.(3)The eastern Pacific(EP)El Nino and central Pacific(CP)El Nino events have different impacts on China summer water vapor transport and rainfall.In the decaying summer,extra tropic pacific evaporation increased significantly.the western North Pacific subtropical high(WNPSH)tending to extend westward of and the establishment of East Asia/Pacific(EAP)teleconnection benefit for the southwesterly winds over the western side of the WNPSH carrying abundant water vapor from the Pacific to the central-eastern China.Meanwhile,the double-blocking set up in Eurasia high-mid latitudes and the northerly air between its low trough and its upstream blocking high tends to carry more water vapor from the Arctic Ocean to Northwest China and the north part of North China.The positive net moisture income anomalies are found in most areas in China,providing favorable moisture conditions for the rainfall.Two rain belts are formed in the south and north of China.In the CP El Nino decaying summer,the WNPSH tends to extend westward and northward,contributing to transport of the water vapor originating from the strong Pacific evaporation into Jianghuai region and causing the increase in the net moisture income in Jianghuai region.Hence,El Nino events not only influence the rainfall anomalies in Yangtze River valley and Huai River valley,but also influence the rainfall anomalies in North China,Northeast China and Northwest China.(4)The haze days show obviously rising trend from 1961 to 2012 over central-eastern China.Because of the weakening of the East Asian winter monsoon system since the mid-1980s,the moisture sourced from the Pacific increases to be transported into the central-eastern China,benefiting for the formation of haze.In addition,the relative humidity decreases in response to increases in temperature,which may be an important reason for the increasing wintertime haze days in Jiang-huai Region and South China.Moreover,the moisture condition is favorable for the formation of haze in the El Nino winter,but the moisture condition is opposite in the La Nina winter.Since 1980s,the occurrence frequency of persistent haze events has an obviously rising trend in the Beijing--Tianjin--Hebei(BTH)region.Southwesterly,southeasterly and westerly transport paths contribute to transport the pollutants and moisture into BTH region.Additionally,descending motion in the mid-lower troposphere contributes to a reduction in the height of the planetary boundary layer(PBL),which generated an inversion in the lower troposphere.This inversion trapped the abundant pollution and moisture in the lower PBL,leading to high concentrations of pollutants.