Analysis Of The Source And Cause Of Water Vapor In The “accumulation Effect” Of Heavy Rain In The Flood Season In South China

Based on the conception of "Cumulative Effect" of torrential rain(CETR)by using the 740 stations' daily precipitation observational data from 1960 to 2012 in China,we choose the south China as a sample region to study the relationship between CETR and floods season precipitation.Through the statistical analysis of observations,three indexes including continuous time(Ld),control area(Ar)and precipitation contribution rate(Qs)are used to difine CETR,which means the cumulation of many torrential rain processes.Then the relationships between CETR and simutaneous total precipitation in south China are analyzed in pre-flood season and later flood season respectively.It is showed that both on interannual and interdecadal scale,above three indexes are correlated to simutaneous total precipitation in south China in pre-flood season and later flood season respectively.On the amount of precipitation which is caused by CETR,the later flood season is affected more significant than the pre-flood season.While on the distribution of precipitation is opposite.In conclusion,both in pre-flood season and latter flood season,CETR in south China greatly influences and even determines the amount and distribution of precipitation.When persistent rainfall occurs frequently over South China,meso-scale and micro-scale synoptic systems persist and expand in space and time and eventually form meso-scale and long-scale weather processes.The accumulation of multiple torrential rain processes is defined as a CETR event.Daily reanalysis datasets collected by the National Centers for Environmental Prediction-Department of Energy(NCEP-DOE)during 1979—2014 are used to study the anomalous features and causes of heavy CETR events over South China.The results show that there is a significant difference in the spatial distribution of the heavy CETR events.Based on the center position of the CETR,the middle region displayed middle-region-heavy CETR events while the western region displayed west-region-heavy CETR events.El Nino events in the previous period(December,January,February,March(DJFM))are major extra-forcing factors of middle-region-heavy CETR events,which is beneficial for the continuous,anomalous Philippine Sea anticyclone and strengthens the West Pacific Subtropical High(WPSH),extending it more westward than normal.The primary water vapor source for precipitation in middle-region-heavy CETR events is the Tropical Western Pacific Ocean.The major extra-forcing factor of a west-region-heavy CETR is the negative anomaly in the southern Tropical Indian Ocean(TIO)during the previous period(DJFM).This factor is beneficial for strengthening the cross-equatorial flow and westerly winds from the Bay of Bengal to the South China Sea(SCS)and early SCS summer monsoon onset.The primary water vapor source of precipitation in the west-region-heavy CETR is the southern TIO.Then,4-times daily reanalysis data collected by NCEP/NC AR from 1979-2014 are used to analyze the main atmospheric moisture sources of pre-flood season(April-June)CETR events over South China.Using a Lagrangian particle dispersion model—Hybrid Single Particle Lagrangian Integrated Trajectory Model(HYSPLIT)—the major atmospheric moisture sources of 66 CETR events for the period 1979-2014 are identified and compared.In this study,five atmospheric moisture sources—the land area(LD),Indian Ocean(IO),Pacific Ocean(PO),South China Sea(SCS),and South China(SC)—are defined and the effects of these moisture sources on pre-flood season CETR events over South China are quantitatively estimated.The results indicate that during the pre-flood season,Indian Ocean water vapor transport accounts for 33%of the atmospheric moisture generated in CETR events,thus dominating precipitation over South China.In pre-flood season CETR events,western Pacific Ocean water vapor transport(17%),SCS water vapor transport(18%)and SC water evaporation(25%)also play an important role.Furthermore,a significant transition in water vapor transport occurred in the 1990s,at which point water vapor transport from Indian Ocean increased significantly,while water vapor transport from Western Pacific Ocean significantly declined.In addition,a significant positive anomaly in SCS water vapor transport spurs CETR events to the east,while a significant positive anomaly in Indian Ocean water vapor transport facilitates the CETR events to the west.Additionally,a center point of CETR events over the middle of South China forms when western Pacific Ocean water vapor transport follows a significant positive anomaly.