Mesoscale Weather Analysis And Numerical Experiment On The Effect Of Terrain On Sudden Heavy Rain In Southeastern Guizhou

This article first analyzes the spatial distribution of precipitation and the over all situation of interannual variations in Guizhou,selects the most representative a rea in Southeast Guizhou,and then obtains the variation characteristics of the pre cipitation trends in Southeast Guizhou by using the Ensemble Empirical Mode De-composition(EEMD),so as to clarify the main precipitation period in Southeast Guizhou.On this basis,a typical case of precipitation in Southeast Guizhou is sel ected for mesoscale synoptic analysis,including the change of circulation situation,the spatial evolution of Black-Body Temperature(TBB)and the tracing of water v apor source in rainstorm area.To explore the role of topography in sudden rainst orm,numerical simulation is carried out on Leigong mountain in Leishan county,southwest of Guizhou,which is the center of the rainstorm.The main conclusio ns are as follows:(1)In the past 36 years,there are three rainy centers in Guizhou province,respectively located in the Xingyi-Anshun area,the Southeast of Guizhou area and the Tongren-Songtao area;and the less rainy area is located in Weining county and Bijie county.The distribution of precipitation shows a decreasing trend from the east and southwest to the north and the surrounding area.The interannual variation is more dramatic,showing an increase trend,which presented a spatial reduction in the central and western regions and increase in eastern regions.The results from EEMD analysis show that the precipitation has the change cycle of 3.4 days,and 121.3 days,as well as182 days.The amplitudes of them all change significantly from April to July,the trend term shows that the precipitation increases from January to early July,and then decreases gradually.(2)Before the sudden rainstorm in southeast Guizhou on May 26-27,2015,there is a process of eastward movement of the wave trough.The southwest low-level jet in front of the trough affected most areas of Guizhou.At the same time,the subtropical high remained stable in South China,creating favorable conditions for the emergence of the jet,making a large amount of water vapor carried by the low-level jet can supply Southeast Guizhou,it provides sufficient water vapor for the occurrence and development of rainstorm.(3)The evolution of sudden rainstorm matches well with the change of TBB value,and the maximum value of TBB corresponds to the strongest time of rainstorm.Before the occurrence of the rainstorm,the TBB has clearly shown that there are pairs of convective clouds in the southeast area of Guizhou,it shows that TBB has a certain role as a precursor to the rainstorm.(4)Using HYSPLIT model to analyze the water vapor transport of the sudden rainstorm,it is found that the water vapor in the rainstorm area has different sources at different altitudes.The lower layer is mainly dominated by local and Bay of Bengal water vapor;the middle layer of water vapor is mainly from the Bay of Bengal and the Caspian Sea;the upper layer of water vapor is divided into three paths: Bay of Bengal,Caspian Sea,and Atlantic Ocean.Overall,Bay of Bengal has significant water vapor transport to the rainstorm area in southeastern Guizhou at all heights,which is the main source of water vapor,and the mid-and-upper level water vapor transport from the Caspian Sea and the Atlantic Ocean also makes important contributions.(5)Using two reanalysis datasets(ERA5 and ERA-Interim)as the initial field to simulate the sudden rainstorm,the WRF numerical simulation of the sudden rainstorm selected in this paper is carried out.The results indicate that: ERA5 has a good effect on precipitation simulation in Southeast Guizhou,while ERA-Interim have a poor effect on this precipitation simulation.At the 850 h Pa wind field,only ERA5 simulates the low level shear line near Leigong mountain,ERA-Interim can not simulate this wind field situation.It shows that ERA5 has higher spatial-temporal resolution and better low-level wind field reproduction ability,which can better capture the medium and small scale information and improve the performance of the model in describing the precipitation.(6)The area and intensity of rainstorm are influenced by the terrain through modulating the dynamic and water vapor field of mountain.The water vapor and unstable energy is accumulated on the windward slope through the uplifting effect of the terrain,which enhances the instability of the stratification and triggers the unstable development of convection under the action of strong ascending motion.The topography of Mountain Leigong has a significant effect on sudden rainstorm: with the increasing of terrain height,the convergence intensity and ascending motion are strengthened and the development of convective clouds would be more active.Thus,the seeds of Seeder-feeder mechanism would be increased,the rain belt expanded and rainfall increased;on the contrary,the intensity of physical variables decreases,the rain belt moves west-ward and the rainfall decreases.