The Trigger and Maintenance Mechanism of a Warm-Sector Heavy Rainfall in Southern Shandong
Based on data from regional automatic stations,ERA5 reanalysis,FY-2F cloud-top brightness temperature,and Doppler weather radar,this study conducted an extensive analysis of the circulation background,environmental conditions,mesoscale convective system(MCS)evolution characteristics,and trigger mechanism of heavy rainfall in southern Shandong province on August 5-6,2019.The key findings are summarized as follows:(1)The rainstorm process transpired under weak weather background forcing,occurring at the periphery of the subtropical high,and was characterized by unfavorable large-scale circulation patterns.(2)Favorable environmental conditions conducive to heavy rain development included a substantial and moisture-laden atmospheric layer,lower uplift condensation height(LCL),free convection height(LFC)and unstable stratification featuring upper dry layers and lower moist layers.(3)The heavy rainfall was concentrated in the warm sector transition zone spanning from the mountainous region of Mid-Shandong to the North Jiangsu Plain,presenting as a narrow,elongated band.Successive occurrences of quasi-steady Mesoscale β Convective Systems(MβCS)during the afternoon and night of August 5 were primarily responsible for the heavy precipitation centers,which predominantly clustered near the areas exhibiting maximum Top Brightness Temperature(TBB)gradients within the MβCS cloud system.(4)The trigger mechanisms for heavy precipitation in southern Shandong on the afternoon of August 5 and in Zaozhuang on the morning of August 6 were attributed to the surface mesoscale convergence line.Along this line,the MCS continuously emerged and underwent progressive development,displaying a"train effect"and consequently leading to heavy rainfall occurrences.Additionally,the heavy precipitation observed in the northwest of Linyi was initiated by the 850 hPa dew point front in the early hours of the 6th.The canyon wind effect and dynamic uplift on the windward slope within the mountainous region of Mid-Shandong significantly contributed to the amplification and intensification of the MCS.(5)The sustained occurrence of heavy precipitation was closely associated with the prolonged persistence of the 850 hPa dew point front and the surface convergence line,which were both driven by the reinforced cold pool and the influx of warm and humid air within the boundary layer.
warm-sector heavy rainfallMCSsurface convergence lineorographic effect
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