Statistical Characteristics Of Presummer Rainfall Over South China And Associated Synoptic Conditions

In this study,characteristics of presummer?April-to-June?rainfall over South China?SC?and the associated synoptic conditions are examined using 1980-2017 hourly rainfall observations at the national-level stations and reanalysis data,the high-resolution datasets collected at densely distributed automatic weather stations,weather maps,and radar network during 2011-17.Results show pronounced regional variations and substantial changes after the monsoon onset over South China Sea?SCS?,i.e.,between the pre-to postmonsoon-onset periods.Owing to more favorable thermodynamic conditions after the monsoon onset,rainfall intensifies generally over entire SC irrespective of the rainfall-event durations,with larger intensification magnitudes in the longer-duration?>6 h?events over the west-inland region with enhanced dynamic instability.Meanwhile the rainfall events occur more frequently over the west-inland and coastal regions,but less over the east-inland region.The inland-region rainfall is closely linked to dynamic lifting provided by subtropical synoptic systems?low pressure and associated front/shearline?,while the westward extension of the western North Pacific high and the intra-period intensification of the southwesterly monsoonal flows,respectively,play important roles in providing high-?e air to the west-and east-inland regions.The warm-sector,coastal rainfall is closely related to the speed reduction of the southerly boundary layer jet?BLJ?over northern SCS and associated convergence of BL high-?e air over the coast.Meanwhile the southwesterly,synoptic-system-related low-level jet?SLLJ?in the lower-to-middle troposphere to the south of the inland cold front contribute to the coastal rainfall occurrence by providing divergence above the BL convergence near the coast.The BLJs often simultaneously strengthen with the SLLJs during both pre-and postmonsoon-onset periods,suggesting a close association between the BLJs and the synoptic systems.The daily-averaged occurrence frequency of the 2011-17 extreme hourly rainfall(EXHR;>60 mm h^-1)increases by about 40%after the monsoon onset.The southeast-SC EXHRs mainly occur in the warm sector with weak?e-gradients,while the northwest-SC EXHRs are mainly produced along a surface front during both periods.The low-level shear line-and vortex-type EXHRs are mainly produced over GX province.These two types combined account for 18%and 31%of the total EXHR population during each period,i.e.,before and after the SCS monsoon onset,respectively.