Diagnosis And Simulation Of Precipitation Anomalies In Sichuan In July 2018

On July 10-11,2018,there was a strong precipitation process in Sichuan Basin.Affected by this process,the daily precipitation of Pengzhou station in Sichuan broke through the local historical extreme value,and a 50-year catastrophic flood occurred in some tributaries of the upper reaches of the Yangtze River.Based on the hourly precipitation densified observation data of 106 stations in Sichuan Basin and the hourly CMORPH data with spatial resolution of 0.1 °× 0.1 °,the satellite observation data is the hourly equivalent blackbody brightness temperature(TBB)of FY-2E,and the 6h GFS global grid forecast field data with spatial resolution of 0.5 °× 0.5 ° of NCEP,the "7.11 rainstorm" is diagnosed.The main conclusions are as follows:(1)During the "7.11 rainstorm" process,the South Asia High is stronger than that of the whole year,and the basin is located in the strong divergence area on the right side of the high-altitude jet entrance area;the superposition of the North China High Pressure and the Subtropical High prevents the westward trough from moving eastward,resulting in a long duration of the precipitation process;At the same time,due to the effect of the typhoon on the south side of the Subtropical High,the southeast air flow on the south side of the Subtropical High increased,increasing the intensity of water vapor transport to the basin;After the cold air intruded into Sichuan Basin,the Southwest Vortex was induced in the south side of the shear line.The lower shear line and the Southwest Vortex had a good corresponding relationship with the area of heavy precipitation area.The most important feature of the "7.11 rainstorm" is that the shear line and its cold air intrude into Sichuan Basin and induce the Southwest Vortex.(2)Based on the analysis of potential vorticity and atmospheric heat source(sink),it is found that there are two kinds of precipitation with different properties in the process of precipitation.The nature of precipitation is relatively complex at 20:00 on the 10 th.The cold and warm air forms a front over the precipitation area,accompanied by obvious secondary circulation,and the latent heat released by precipitation promotes the downward transmission of potential vorticity at high altitude,which makes the intensity of bottom cyclone increase;The precipitation peak at 02:00 on the 11 th belongs to the summer convective precipitation.(3)The Shuman-Shapiro filtering scale separation method can make the high-altitude jet,the trough at 500 h Pa and the shear line and the relatively weak Southwest Vortex better reflect,which proves that this method can effectively separate the synoptic scale system and the sub synoptic scale system in Sichuan Basin;the shear line and the local vortex around the shear line in the disturbed flow field correspond to the disturbed low-pressure area;the heavy precipitation center occurs in the low-pressure convergence area near the disturbed shear line;In the pseudo-equivalent temperature disturbance field,the temperature-humidity gradient large value area and the warm-humid area on the south side are superimposed with the water vapor disturbance convergence center,and there will often be heavy precipitation corresponding to it.The results show that the large value area of temperature-humidity gradient between the positive and negative centers of the pseudo-equivalent temperature disturbance field is often corresponds to the location of the shear line in the disturbance flow field,and the heavy precipitation center occurs in the warm and wet area to the South and southeast of the disturbance shear line;the negative phase coincidence area of the disturbance height field and the disturbance water vapor flux divergence field is very strong for the heavy precipitation area,and the change of precipitation intensity corresponds to the change of intensity in the low-pressure region and the convergence center of water vapor disturbance.In the period of large precipitation,the disturbed low-pressure area and the disturbed convergence center of water vapor are superposed,and both of them are strong.When the precipitation tends to stop weakening,the centers of the two are separated,and the intensity is obviously weakened.The "7.11 rainstorm" is produced under the joint action of the shear line,the southwest vortex and the sub synoptic scale disturbance caused by them.(4)From the perspective of the distribution of vorticity advection,the positive vorticity of 700 h Pa was transmitted from north to south to the Sichuan Basin during the occurrence and maintenance of heavy rain.From the vertical distribution analysis of the regional average values of vorticity budget,the contribution of vorticity stretch term to the positive vorticity budget of the middle and lower troposphere is relatively large.It is proved that the stretch term of vorticity makes the lower troposphere maintain the positive vorticity for a long time,resulting in the circulation situation of "low level convergence and high level divergence".Therefore,the vorticity stretch term is the important factors for maintaining the stability of rainstorm.