Abstract:Meiyu and tropical cyclones(TCs) in the northwestern Pacific are key factors of the weather and climate in East Asia.Both play important roles in the rainfall in eastern China,especially during the Meiyu period.A large number of statistical and synoptic analyses regarding the interaction between Meiyu and TCs has been carried out.Previous studies have revealed that TCs may lead to the interruption of Meiyu,but less attention has been given to the mechanism of this interruption effect.In the present paper,as a case study,the impact of TC Chebi(0102) on the ending of the Meiyu in 2001 is investigated in terms of the sensitivity simulations with the limited-area numerical model.By using the simulation output with and without the influence of Chebi,the effect of TCs on the western Pacific subtropical high,moisture transport and pseudo-equivalent potential temperature are analyzed,which discloses the mechanism of how Chebi exhibits its impact on the rainfall and ending of Meiyu.
The Weather Research Forecasting model(WRFver3.4.1) is used in this study,and the initial and boundary conditions are extracted from the National Center for Environment Prediction and National Center for Atmospheric Research(NCEP/NCAR) reanalysis data at 1°×1° resolution.The model domain is centered at(26°N,121°E),with a horizontal resolution of 20 km.The horizontal grid dimensions are 110×130.There are 27 levels in the vertical plane,and the top of the model is found at 50 hPa.The model is initialized at 1200 UTC 21 June 2001,and the integration ends at 1800 UTC 25 June 2001.The time step is 60 s,and the results are output at 6 h intervals.The experiment with a bogus vortex via automatic vortex-following algorithm is marked as the control run(CTL).The simulation results show that the model can effectively reproduce the track and intensity variation of Chebi,as well as the distribution and intensity of the rainfall during the late Meiyu period.In addition,the sensitivity run(SEN) was also conducted by removing the TC circulation from the initial fields,so as to investigate the response of the atmospheric circulation and rainfall when the TC effect was not considered in the simulation during the Meiyu period.
Comparative studies regarding the influences with Chebi suggest that the mechanism of Chebi on Meiyu lies in the fact that,as Chebi moves northeastward,the low-level jet axis moves northward and becomes weak above the lower and middle reaches of the Yangtze River.Meanwhile,the coupling between the lower and upper level jets also grows weak,thereby leading to a decrease of Meiyu front and moisture transport from the southwest region.In addition,the western Pacific subtropical high jumps northward and the water vapor is transported from the southwest region to the northern side of the Huai River,consequently reducing the rainfall and causing the ending of Meiyu on June 25.Moreover,the weakening pseudo-equivalent potential temperature horizontal gradient and horizontal component of moist potential vorticity associated with the vertical velocity also contribute to the interruption of Meiyu.On the other hand,without the influence of Chebi,the western Pacific subtropical high jumps late,the low-and upper-level jets remain for a long period of time,and the low-level wind shear and convergence are preserved.The strong ascending flow from the lower to upper levels provides a favorable dynamical condition for Meiyu,and the coupling between the lower and upper level jets is helpful for the enhancement of southwesterly winds at the lower levels.At the same time,the gradient of pseudo-equivalent potential temperature and baroclinic instability are conducive to the increasing of vertical vorticity and occurrence of rainstorms,which results in the maintenance of rainfall in the late Meiyu period,and causes a delay in the ending of Meiyu.

Abstract:Using the NCEP/NCAR reanalysis data,FY2E satellite data and the automatic weather station data,combined with the WRF(Weather Research and Forecasting) model,a Meiyu rainstorm occurred in Jiangsu and Anhui provinces in 2013 was analyzed.Observational data analysis results show that under favorable circulation background and thermodynamic conditions,the heavy rain occurs in the south warm side of Meiyu front,with a common influence of Northeast cold vortex,Meiyu front and Changjiang-Huaihe shear line.The water vapor is mainly from the northwest of South China Sea.Convective instability and symmetric instability locate in the rainstorm zone,which is favorable for the common development of both vertical convection and ramp-up airflow.The rain belt transforms from the ribbon to string shape distributions and moves southward along with Meiyu front.Early precipitation is triggered by the surface mesoscale convergence line,directly affected by two successive meso-α-scale linear convective systems.Later rainfall is triggered by the ground warm shear line.There are a string of meso-β-scale convective systems developing eastward along the shear line.High-resolution model outputs analysis results reveal that the large scale non-geostrophic forcing also plays an important role in triggering strong convection during the rainstorm.The ground convergence line produces a banded lower layer convergence zone,resulting in strip continuous distribution of ascending motion,which gives rise to linear convection system and banded precipitation.Additionally,the ground convergence line can form two mesoscale vertical secondary circulations in the north and south of rainstorm area,which is the precipitation enhancement mechanism.The local disturbance of warm shear line produces local strong convergence at low altitude,thus making strong ascending and weak descending interval distribute along the shear line.And then those convective systems develop in the strong ascending regions,bringing local heavy precipitation.

Abstract:Using SST data from Hadley Centre,the NCEP/NCAR reanalysis data along with 740-station precipitation data in China,combined with the lag correlation and other methods,this paper analyzes the characteristics of summer precipitation in the Yangtze River Basin in non-ENSO years and investigates the characteristics of Rossby wave packets and their possible impacts on summer severe precipitation events in the Yangtze River Basin in non-ENSO year 1993.Results show that there exist strong interannual and interdecadal variations for summer precipitation in the middle and lower reaches of the Yangtze River in non-ENSO years.Among non-ENSO years,1993 is a special year when the strongest flooding occurred in the Yangtze River Basin,which is chosen as the case for study.It is found that the wave packets in 1993 are originated from the south of Caspian Sea,propagating eastward to the middle and lower reaches of the Yangtze River.These wave packets disperse energy eastward,inducing accumulation of energy over China,facilitating the occurrence of summer severe precipitation in the Yangtze River Basin.A comparison is made for non-ENSO year 1993 and ENSO year 1983.It is found that the route along which the wave packets propagate in non-ENSO year 1993 locates more southern than that in ENSO year 1983.These results are helpful for us to understand the forming mechanism of summer severe rainfall events in the Yangtze River Basin in non-ENSO years.

Abstract:Based on the daily precipitation data in Ningxia,the NCEP/NCAR reanalysis data and NOAA sea surface temperature(SST) data from 1961 to 2015,this paper analyzes variation of the days of extreme precipitation(DEP) in Ningxia in summer and its causes.Results show that:(1)DEP in Ningxia in summer in the past 55 years has been slightly reduced,but the abrupt change of the variability occurs in 1994 from big to small.Variations of DEP in each month are different.In June,the abrupt change of DEP occurs in 1982,and DEP increases significantly after 1982.DEP in July is slightly decreasing.In August,the abrupt change of DEP occurs in 1995,and DEP decreases significantly after 1995.(2)The dipole pattern in North-Northeast China and the western Pacific at 500 hPa in June,the teleconnection patterns in West Siberia-Mongolia-subtropical China and EAP at 500 hPa in August,as well as wind field at 700 hPa over Ningxia with anomalous easterlies in June and northerlies in August,are the direct reasons leading to the extreme precipitation events.(3)The positive SST anomalies over the Philippines in June are favourable to the "+-" dipole pattern in North-Northeast China and the western Pacific at 500 hPa.The negative SST anomalies over the tropical eastern Pacific in August are good for 500 hPa West-low-East-high height distribution pattern in China,which excites EAP teleconnection wave train.The combination of cold air and water vapor conditions results in the frequent occurrence of extreme precipitation events in Ningxia.