Variations And Causes Of Regional Daily Precipitation Extremes During The Flood Season In South China

Based on the daily precipitation data from both 753 stations in China and the Meteorological Information Comprehensive Analysis and Process System(MICAPS),the Southwest Pacific Ocean Tropical Cyclone(TC)optimal path data from the Tokyo Typhoon Center in Japan,and the National Centers for Environmental Prediction/National Center for Atmospheric Research(NCEP/NCAR)reanalysis,regional mean daily precipitation extreme(RDPE)events and non-extreme events in South China and the associated circulation anomalies are investigated.Depending on whether the occurrence of a RDPE event is affected by a TC,the RDPEs are classified into two categories including TCfree-RDPE and TCaff-RDPE events.The results are as follows.During the first flood season(April-May-June)in South China,the frequency of RDPE events has increased significantly since the early 1990s.The TCaff-RDPE events are only about 6%of the total RDPE events and mainly occur in June,while the TCfree-RDPE events frequently occur from early May to late June,and the non-extreme events(the events whose percentiles are from 85 to 90 and from 75 to80)frequently occur in May and June.The circulation anomalies in RDPE and non-extreme events show similar distributions.However,different features are clearly seen between these two types of events by big differences in the intensity.Different sources of water vapor are found to be related to different types of precipitation events;the vapor from the South China Sea and the Western Pacific in both RDPE events and the events whose percentiles are from 85 to 90 whereas it is from the South China Sea in the events whose percentiles are from 75 to 80.Futhermore,There's a more significant kinematic frontogenesis in TCfree-RDPE events than the non-extreme events,which is conducive to the release of baroclinic energy.When TCaff-RDPE events occur,the wave energy propagates from the northeastern side of the Tibetan Plateau and the Hexi Corridor region to South China and converges in this area,which favors the development and maintenance of perturbation over South China.Whereas,when non-exreme events occur,the wave energy propagates not very obviously.The apparent diabatic heating and vapor sources in these two types of events are examined,which show stronger net heating over South China and stronger cooling in the north and south sides.Interestingly,very significant sea surface temperature anomalies(SSTA)are observed in the Pacific in TCfree-RDPE events other than the non-extreme events,which look similar to the SSTA pattern of La Nina episodes.Both the anomalous circulation and SSTApatterns in the RDPE events are more significant statistically in large areas than those in the non-extreme events.During the second flood season(July-August-September-October)in South China,the TCaff-RDPE events are about 42%of the total RDPEs and mainly occur in late August,while the TCfree-RDPE events frequently occur in July.When TCfree-RDPE events occur,South China is controlled by abnormal cyclonic circulations,and the warm and moist airflow from the Western Pacific region and the South China Sea merge with cold air from northern in this region,inducing a long narrow zone of water vapor convergence and causing significant and strong ascending motion,in responsible for the occurrence and maintenance of TCfree-RDPE events.Simultaneously,the wave energy propagates from the northeastern side of the Tibetan Plateau and the Hexi Corridor region to South China and converges in this area,which favors the development and maintenance of perturbation over South China.Whereas,when TCaff-RDPE events occur,the southern part of China is dominated by a TC-related stronger anomalous cyclonic circulation.The warm wet air is carried into this area from the Bay of Bengal,the Western Pacific Region and the South China Sea,leading to a large amount of latent heat release over South China and inducing stronger ascending motion there.Furthermore,very significant sea surface temperature anomalies(SSTA)are observed in the central-eastern Pacific,which look similar to the SSTA pattern of El Nino episodes while TCfree-RDPE events occur but similar to the pattern of La Nina episodes when TCaff-RDPE events occur.All the above results are helpful for our better understanding and predicting the occurrence of regional extreme precipitation events in South China.