A Lagrangian Analysis Of The Water Vapor Transport And Its Variation In East China During Propagation Of Rainy Belt

In this paper,we used the HYSPLIT platform to calculate Lagrangian trajectories of air parcels that reach East China during the northward propagation of the East Asian Summer Monsoon.Our study covers a long period from 1961 to 2010 to ensure the significance of results.To fully explore the Lagrangian trajectories,we performed a clustering on the trajectories.The Evaporation-minus-Precipitation diagnosis and the areal source–receptor attribution of water vapor contribution are also elaborated.We can thus obtain a robust picture for the water vapor transport,including paths,moisture sources/sinks and moisture contribution from different regions to precipitations in EASM.In addition,the Season-reliant Empirical Orthogonal Functions(SEOF)and Wave filtering are used to analysis the trend character,interannual and interdecadal variation of the moisture contribution of moisture source regions,and then clarified the influence of the change of water vapor source on precipitation,determine the main factors.Main findings are summarized as follows:(1)In the pre-monsoon stage with rainfall in South China,the most important moisture transport channels are from West Pacific Ocean(WPO)and Indian Ocean(IO),the proportion of the trajectories being 34.0% and 23.1% respectively,consistent with our general expectation.And the most importrant channel in low level is WPO and in the middle and high levels are IO channel.But main moisture sources are from East China(EC)and WPO,with moisture contribution rates at 34.4% and 26.0% respectively,which mainly come from low level with the contribution rate of these two sources region in low level are higher than 20%.After the onset of the South China Sea(SCS)summer monsoon,the monsoon rain-belt is still in South China,the IO channel strengthens,with the trajectories proportion increasing to 53.0%and the strongest water vapor channel from the low level to the upper level are all Indian Ocean channel.Despite the Indian monsoon strengthening,EC and SCS are,however,the most important moisture sources in this period,their contribution rates to precipitations being both larger than 25%.For Meiyu in the Yangtze-Huaihe River basin,the IO channel is still the most important water vapor channel,which includes 43% of all the trajectories.But the contribution rate from East China to precipitations is the highest(40.2%)and it in low level is27.1%.East China(mainly in low level)becomes the most important moisture source region.When the monsoon is in its terminal stage in North China,the most important moisture channels are the mid-latitude westerlies channel(middle and high levels)and WPO channel(low level),their trajectories proportions being 30.2% and 27.1%,and the local water vapor recycling in East China has the greatest contribution to rainfall in North China during this stage,its contribution rate exceeding 55%,and it mainly comes from low level with the contribution rate reach 43.1%.Local water vapor recycling inside East China plays an important role for the regional precipitations especially in the terminal stage in North China.(2)The moisture source during the northward propagation of the East Asian Summer Monsoon in East China has the obvious trend characteristics.The water vapor contribution in the south source region is decreased and in the north source region is increased.The trend of the moisture contribution leads to the precipitation for East China rainy season increases in the South and decreases in the North.And it is well connected with the weaken of East Asian monsoon.(3)The interdecadel variation of the moisture source during East China rainy season is closely related to the Pacific Decadal Oscillation(PDO).During the pre-monsoon stage in South China rainy season,when the PDO is in positive(negative)phase,the moisture contributions of all the moisture source regions are stronger(weaker)than the mean value.The moisture contribution in EC,WPO,SCS and IO are increased(decreased)by 35%(18%),23%(12%),29%(14%)and 30%(15%)respectively,and the total moisture contribution is increased(decreased)by 30%(15%),which lead to the increase(decrease)of the total precipitation and rainy days in this rainy season.After the onset of the SCS summer monsoon,during the South China rainy seaon and North China rainy season,the total moisture contribution are all weaken(strongen)in the positive(negative)phase of PDO.In South China rainy season,the moisture contribution in EC,WPO,SCS and IO is decreased(increased)by 24%(12%),11%(5%),15%(7%)and 16%(8%),and the total moisture contribution is decreased by 18%(9%);as in North China rainy season,the moisture contribution for the four main moisture source is decreased(increased)by 13%(7%),14%(7%),20%(10%)and 44%(22%).The interdecadel variation of the total moisture contribution leads to the interdecadel variation of the precipitation in the rainy season in East China.(4)The first SEOF eigenvector for the interannual variation of the moisture contribution is closely related to ENSO.As the ENSO is in the positive phase,the total moisture contribution in the pre-monsoon stage in South China rainy season is increased,with the moisture contribution of Pacific Ocean increased by 39.8% and moisture contribution in South Chian Sea increased by 48.4%.The reason is that positive phase of ENSO enhance the walker circulation,and then lead to the anticyclone anomaly around Philippines,so the moisture contribution in south part of west Pacific Ocean is increased.After the onset of the SCS summer monsoon,the positive phase of ENSO lead to the enhance of the Indian monsoon and then rainy belt in East China moves north rapidly,so the rainy days in South China is decreased and the total precipitation decreased by 19.2%.The second SEOF eigenvector mainly shows the interannual variation of moisture source contribution in North China rainy season.As the time series of SEOF2 in its positive phase,the total moisture contribution of North China rainy season is increased by 26.6%,in which the moisture contribution in West Pacific Ocean increased by 31.5%.It is mainly due to the increase of moisture contribution in the northeast of North China.The artic high influences the moisture contribution of North China by the wave train,and then has an important impact on the interannual change of precipitation in North China rainy season.