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.