Synoptic Analysis And Prediction Of A Typical Warm-Sector Torrential Rainfall Event (5·7 Rainfall) In South China

Warm-sector torrential rainfall(WSTR)occurs in the warm sector at least 200 km ahead of a cold front or without a front during the pre-summer rainy season(April-June)in South China.Owing to the suddenness and heavy intensity of WSTR precipitation,such events can threaten public safety and cause damage to property.However,owing to their small spatial scale,WSTR predictability remains very low.This study aims at enhancing understanding of a typical WSTR event and improving related forecasting skills by initializing convective-scale numerical weather prediction models with the assimilation of high frequency convective-scale observations.First,to elucidate the climatic characteristics of WSTR events,16 WSTR and 16 squall line(SL)events that occurred during the 2013-2017 pre-summer rainy seasons were analyzed using ERA-Interim reanalysis data,3D radar reflectivity mosaics,and surface precipitation observations.Second,a typical WSTR case(i.e.,the "5·7" rainstorm)that set a new 24-h precipitation record of 542.7 mm in Guangzhou(China)was selected for more detailed analysis of the convective initiation(CI)process.To reproduce this CI process,an observational analysis dataset at 10-min intervals and 1-km grid spacing was generated by assimilating 6-min interval radar radial velocities and 5-min interval surface observations in the VDRAS 4DVAR system.Based on combination of these observational analyses and hourly ERA5 reanalysis data,the roles of local low topography and the urban heat island(UHI),as well as their interactions with the synoptic-scale dynamics,were analyzed.Third,after gaining better understanding of the key factors that led to the CI of this rainfall event,10-min interval Advanced Himawari Imager(AHI)radiance data in three water vapor channels were assimilated using the WRFDA 4DVAR system,and we examined their effects on both initiation/evolution of convection and subsequent precipitation forecasts.The reasons why assimilation of high-frequency satellite observations improved the forecasting skill for this WSTR event were also examined and analyzed.The principal findings of this study are as follows.The composite and statistical analyses revealed that WSTR and SL events were characterized by different climatic features.SL events occurred mostly in the afternoon,whereas WSTR events usually occurred at night or early morning.In comparison with SLs,WSTR events were characterized by higher precipitation intensity but lower radar echo centroids.For example,WSTR reflectivities increased in the downward direction and the 50 dBZ centroids were below the 0? level,whereas SL reflectivities increased in the upward direction and most 50 dBZ centroids were above the 0? level.The different echo structures indicate that WSTR events are led by warm-rain processes,whereas SL events are related more to ice-phase activities.The findings regarding the SL processes investigated in this study are consistent with those of previous investigations,i.e.,the SLs were associated with frontal processes where synoptic-scale ascent was caused by convergence of northerly and southerly winds.However,unlike SL processes,the studied WSTR events occurred with very shallow(below 950 hPa)synoptic-scale convergence and low-level convergence caused by differences in the speed of the southerly wind.Composite analyses of terrain height and water vapor fluxes indicated that the shallow convergence and lifting conditions of the WSTR events were associated with the low trumpet-shaped topography located near the north of the rainfall centers.Analysis of a typical case selected from the initial 16 WSTR events revealed that the CI process was a result from multiscale interactions of synoptic-scale dynamics,topography,and the UHI.Owing to variation of the synoptic-scale circulation,southerly winds over Guangzhou were prominently enhanced prior to CI,which introduced additional water vapor into the rainstorm area.This increase in water vapor increased the convective available potential energy and reduced both the convective inhibition and the level of free convection,producing conditions more conducive for CI.In such favorable synoptic-scale background and instability conditions,the coexistence of the local small-scale topography and the UHI directly provided lifting conditions for triggering the convection,which were generated from the low-level convergence.Although the low-level convergence was enhanced by the temperature gradients in the downstream area of the UHI,the dominant factor was the thermodynamic effect of the small-scale mountain to the north of the CI area.In addition to generating low-level convergence,the topography also acted as a barrier to low-level advection of water vapor from the south.Thus,the water vapor,which became concentrated in the boundary layer,was then transported upward by near-surface updrafts.With the vertical transport of water vapor,the atmosphere near 1-km altitude approached saturation and convection formed.It should be noted that the characteristics of this WSTR event,such as the weak synoptic scale forcing,increase of low-level southerly winds,and blocking effect by the low topography,are consistent with the conclusions drawn from the earlier climatic analysisBased on the above analysis,the principal factors that led to the CI of this rainstorm were the strengthening of the southerly winds,the increased advection of water vapor,and the local temperature gradient.In comparison with the 4DVAR experiment in which only conventional observations were assimilated,the addition of high-frequency(every 10 min)satellite radiances improved the model analyses and forecasts in terms of the above-mentioned southerly winds,moisture,and temperature.The initial moisture profile at the Qingyuan station was closer to the observed sounding,the 0-24-h forecasts of the atmospheric precipitable water were more consistent with the GPS observations,and the surface temperature gradient near the rainstorm area agreed better with the surface observations.In addition to correcting the temperature and humidity fields associated directly with the AHI brightness temperature,using the tangent linear model as the constraint in 4DVAR improved the wind analyses and thus the subsequent forecasts.Owing to the improved prediction of temperature,moisture,and wind fields before and after CI,the model forecasting error in terms of the initiation/evolution of convection was reduced and the fraction skill scores of the 20-h accumulated rainfall were improved significantly.