Analysis on Characteristics and Formation Mechanisms of Two Convective Rainstorms over the Loess Plateau

Qu Liwei; Liu Juju; Zhao Qiang; Liu Shengnan; Liang Mian; Pan Liujie

doi:10.16765/j.cnki.1673-7148.2025.02.005

Abstract：Based on the meteorological observation data,the ERA-5(0.25°×0.25°)reanalysis data of the European Centre for Medium-Range Weather Forecasts(ECMWF),the convective ambient conditions and trigger mechanism of two consecutive rainstorm processes over the Loess Plateau in north-ern Shaanxi Province in July 2019 are analyzed,and the similarities and differences of frontogenesis and dynamic factors of the rainstorms are discussed.The results are as follows:(1)The position of subtropical high was stable in both of the two rainstorm processes.The rainstorm on July 21 occurred in the warm zone in the south of the 700 hPa warm shear line.The transport of southerly warm and humid air in the lower layer was enhanced persistently,the water vapor convergence in the whole layer was strong,and the severe rainfall band moved from south to north,so the rainstorm belonged to warm-sector convective precipitation.During the July 28 rainstorm process,a cold shear line formed at 700 hPa,the water vapor convergence of the whole layer was weak,and the local humid conditions were even better.The high-level dry cold air intruded into the lower troposphere from north to south,forming a wet frontal zone.The local rain intensity was strong,and the severe rainfall band moved from north to south,so this was a frontal convective precipitation.(2)The CAPE of the two processes was of a narrow type.The lifting condensa-tion height in the July 21 process was low and the thickness of warm clouds was large.The July 28 process had a higher CAPE value,a much better energy condition and even stronger convection instability.(3)The trigger mechanisms of the two processes were different.The meso-scale convergence line formed by the intrusion of weak cold air from west triggered the July 21 rainstorm.The rainstorm process was dominated by the stratified mixed cloud precipitation echo,and the long-term maintenance of deep and persistent southerly jet created the train effect,resulting in a large amount of accumulated rainfall.The strong upward motion caused by frontogenesis in the July 28 process triggered the convection precipitati-on,combined with cyclonic convergence,forming a northeast-to-southwest banded cumulus cloud echo,so the single-site rainfall intensity became even stronger.(4)The convective vorticity vector and column cloud water in the two processes corresponded well with the evolution of severe rainfall falling areas.The July 21 process showed a trend of moving from southwest to northeast,while the July 28 process had a trend of moving from northwest to southeast.Thus,it can be seen that the convective vorticity vector can represent different characteristics of heavy rainfall in warm-sector convective precipitation and frontal con-vective precipitation,which can serve as a reference in the forecasting operation.