In this study,NCEP/FNL reanalysis and WRFV3.9.1 are used to investigate the influence of the intensity of the monsoon depression on the rainstorm process in Guangdong from 27 August to 1 September,2018.On August 27,the rainstorm was in the development stage,the precipitation distribution was relatively scattered,and the maximum value of the daily precipitation was 80 mm/day.Starting from August 28,the heavy rainfall was concentrated and tended to shift southward to the Pearl River Delta region,with a maximum daily precipitation exceeding 100 mm/day.The rainstorm reached a mature phase between 29 and 30 August,with a maximum of daily precipitation more than 200 mm/day,and the location of the precipitation area was concentrated in the southeast coastal area of Guangdong,with a northeast-southwest spatial distribution.On 31 August,the rainfall began to weaken,but the daily precipitation remained strong.Above all,this rainstorm process involved much precipitation,a long duration,and a concentrated area,making it a rare extraordinary heavy rainstorm event.Using the physical decomposition method indicates that the monsoon depression and low-level jet provided sufficient water vapor and energy for the rainstorm process.In the mature phase of the rainstorm process,there was a negative entropy anomaly to the south of the rainstorm area,and a positive one to the north.The zero line of entropy anomaly indicates that the tendency of entropy was zero,that is,the value of entropy reached its peak,which means that the accumulation of energy had a maximum value in this area.On the north side of the rainstorm,the increase of the entropy means that the unstable energy continued to grow,while the precipitation on the south side released the unstable energy,that is,the entropy decreased and tended to be stable.The location of the zero line of the entropy change corresponded closely with the spatial distribution of the precipitation.Energy accumulating at the zero line of the entropy change was favorable to the development of the rainstorm.To further verify the influence of the monsoon depression,sensitivity experiments (EXP_500,EXP_700,and EXP_1000) were constructed.Sensitivity experiments were conducted by filtering out the perturbation component in the monsoon depression to change the intensity of the monsoon depression.The results show that when the monsoon depression was strong,the rainstorm was likely to occur and the precipitation was strong,and when the monsoon depression was weak,the precipitation was reduced or even absent.The diagnostic analysis shows that the energy helicity had implications for the development of the rainfall:when the energy helicity was greater,the intensity of precipitation was stronger,and when the intensity of the monsoon depression weakened,the energy helicity decreased and the precipitation weakened.