A Study On The Atmospheric River And Its Relationship With The Regional Heavy Precipitation In China

The atmospheric river(AR)has been proved to be closely related to precipitation in various countries and areas.Previous studies have found that a considerable number of ARs affect East Asia each year.However,these studies do not systematically reveal the mechanism of ARs affecting regional heavy precipitation in East Asia,and the impact of ARS on precipitation in China is still unclear.To this end,based on multi-source observation data,global reanalysis data and the Weather Research and Forecasting Model simulation,this study investigates the characteristics of ARs affecting China and its influence mechanism on heavy precipitation in different weather and climate regions.According to the meteorological industry standard,China is divided into four climate zones with distinct rainfall characteristics: Zone 1(mainly Northwest China),Zone 2(Southwest China),Zone 3(Northeast China,North China and the Huang-Huai region)and Zone 4(South China and the middle and lower reaches of the Yangtze River).Then,the characteristics of ARs and their impact on the heavy rainfall in Zone 2,Zone 3 and Zone 4 are studied in detail.The results show that there are 954 AR events in China in recent 20 years.There are obvious seasonal characteristics in the average/maximum integrated horizontal water vapor transport(IVT),and the length and width of ARs.June to August are the large value months of length,and July to September are the large value months of width.The average IVT and maximum IVT are the largest in June.The durations of 83% AR events are no more than 48 h,and the high incidence area is Zone 4.An obvious positive spatio-temporal correlation is detected between ARs and heavy precipitation,especially in Zone 3.The heavy rainfall event and the frequency anomaly of AR events show evident positive correlation in Zone 3and Zone 4,while almost no correlation in Zone 2.Heavy rainfall events with ARs may occur all year round in Zone 4,while mainly in summer half year in Zone 2 and Zone 3.The average intensity of heavy rainfall with ARs is higher than that without ARs.There is an obvious positive correlation between the average/maximum IVT and length of ARs and the rainfall intensity in Zone 3.The rainfall intensity in Zone 4 increases with the maximum IVT,length and width of ARs,but the rainfall intensity in Zone 2 has little correlation with those four factors.Through the study on four AR cases in Zone 2(one case),Zone 3(one case)and Zone 4(two case),we find that the AR transport channels from the low latitude oceans and from the middle-high latitude continent present a upward motion and downward motion,respectively.The uplift of warm-wet AR channel and the sinking of dry-cold AR channel jointly cause heavy rainfall.Water vapor converges in the low levels and is transported to the middle and upper levels through strong vertical upward motion.The amount of net water vapor income directly determines the rainfall intensity.Low-level jet cores and water vapor flux convergence centers appear in the lower layer of ARs,with steep energy front areas on both sides.On the north side,the high-level large-value area of potential vorticity extends downward,with obvious frontogenesis in the lower layer,which is conducive to the continuous strengthening of low-level cyclones and the maintaince of strong upward motion.The AR causing heavy rainfall in Zone 3 is quasi north-south oriented,and its main water vapor sources are the South China Sea and the oceans on its south.Its strong IVT center lasts the longest,and the rainfall is the strongest in the four events.In contrast,the ARs in the other three cases are quasi east-west oriented,and the water vapor mainly comes from the Indian Ocean.The height of the maximum water vapor flux center of AR and the height of the low-level jet core are the lowest in Zone 4(850 h Pa),followed by Zone 3,and the highest is in Zone2(700 h Pa).The energy front areas of the three AR events in Zone 2 and Zone 4 show a steep structure,while that in Zone 3 tilts southeastward.Furthermore,two typical AR precipitation events in the Asian-Australian monsoon region are investigated in detail,i.e.the "2016.07" event in China and the "2016.08" event in Australia.Both events are caused by the cut-off low.The AR is the main carrier of water vapor,and the infiltration of high-level dry-cold air from the middle-high latitudes plays an important role in stimulating heavy precipitation.The difference of the two events is that the AR in China under the background of summer monsoon is stronger,wider and more capable of carrying water vapor than the AR in Australia under the background of winter monsoon.Different atmospheric circulation backgrounds lead to different heights of the jet and the center of water vapor transport.The water vapor transport center and the low-level jet core of the AR in China are at about 850 h Pa and are located in front of the energy front.But,the water vapor transport center of the AR in Australia is at about 700 h Pa,and there is no low-level jet near the water vapor transport center.The water vapor flux and θse of each water vapor channel in "2016.08" event in Australia are obviously smaller than those of the "2016.07" event in China.In addition,due to the lack of topographic uplift and dry climate background,the net water vapor income and rainfall efficiency in the heavy rainfall area in the "2016.08" event are much lower than those in the "2016.07" event.The typhoons that landed on China mainland with AR(AT)and without AR(NT)from 1986 to 2015 are counted.The climatological characteristics of large-scale circulation and water vapor field of AT/NT are revealed through composite analysis.The results shows that the average onshore maintenance time of AT is longer than that of NT.After the landfall of AT,the contour lines between the typhoon and the subtropical high are dense,and the typhoon gradually approaches the baroclinic front area when moving northward in front of the middle latitude trough.The low-level jet and the super low-level jet last for a long time.The vertical extension of the strong wind core in the typhoon vortex area is high,and the strong water vapor transport channel is connected with the typhoon.After the landfall of NT,the contour lines between the typhoon and the subtropical high are sparse.There is no long-wave trough,and the typhoon is disconnected from the strong water vapor channel.The water vapor transport and convergence in AT are substantially stronger than in NT.The water vapor convergence presents a symmetrical structure in AT,while an asymmetric structure in NT.After the landfall of AT,the water vapor transport has remained strong in the southern boundary,and the total water vapor income in the typhoon area decreases slowly,while it decreases rapidly after the landfall of NT.Within 48 hours after the landfall of AT,the low-level cyclonic circulation structure of AT is complete,and there is net water vapor input from the four boundaries.However,about 24 hours after the landfall of NT,the cyclonic circulation structure of NT has been damaged,and the water vapor input is weaker than that in AT.Moreover,the mechanism of topographic effect on the "7.20" extreme rainstorm in Henan Province in China is analyzed,and the modulation effect of the AR water vapor transport on the rainstorm is also studied.The results show that the large-value areas of both the IVT and divergence of integrated horizontal water vapor transport(IVTD)have a good spatial correspondence with the heavy rainfall area.In the two stages of the heavy precipitation,the regional averaged hourly rainfall and the regional averaged IVT and IVTD show bimodal distribution,and the heavy precipitation has a significant positive(negative)correlation with IVT(IVTD),with both correlation coefficients reaching 0.84.The center of the AR water vapor flux and the low-level jet core are both below 700 h Pa.In the first stage,the θse isolines below 800 h Pa are dense before the rainfall peak,and the unstable stratification is mainly in the low levels.In the second stage,the θse isolines between 800 h Pa and 500 h Pa are dense before the rainfall peak,and the unstable stratification is mainly in the middle levels.The terrain sensitivity experiment shows that the decrease of terrain leads to the change of the precipitation echo shape in the first stage and the destruction of the convective train effect along the low-level jet.The strong water vapor is more westerly and northerly transported,and the water vapor convergence center and strong upward motion in the AR area also move westward and northward,resulting in the northwestward deviation of heavy rainfall area.The lower the terrain is,the more obvious the deviation is.