Diurnal Variation Characteristics And Mechanisms Of The ZSL Inducing Extreme Precipitation Over The Southern Tibetan Plateau In Boreal Summer

Based on the ERA5 hourly reanalysis data with the spatial resolution of 1°×1°from June to August during 1980–2019,13 cases of the extreme-precipitation-inducing zonal shear line(ZSL)over the southern Tibetan Plateau(TP)with life cycles of more than 24 hours were selected and composited.The diurnal variation characteristics of the ZSL were explored from the dynamic,thermal,and water vapor conditions.Furthermore,the mechanisms of diurnal variation of the ZSL were explored through the diagnostic analysis of the potential vorticity(PV)budget equation.Results suggest that the frequency and intensity of the ZSL have significant diurnal variations,both reaching the daily peak around midnight.The diurnal variation of the ZSL can be influenced by both dynamic and thermal factors.The condensational latent heating has a significant contribution to diabatic heating.Moreover,the diagnostic analysis of the PV budget equation shows that the vertically non-uniform distribution of diabatic heating plays a primary role in PV variation.Therefore,the vertically nonuniform distribution of the condensational latent heating is a dominant factor influencing the diurnal variation of the ZSL.The conceptual model of the diurnal variation of the ZSL can be summarized as follows.Solar radiation and surface sensible heating increase rapidly from 12 to 21 LST.Both the temperature and humidity near the ZSL increase,causing the abnormal convergence of the wind and water vapor and enhancing ascending motion.The enhancement of the South Asia High and upper-level jet stream provides favorable conditions for the ZSL's development.The interaction between the high and low layers makes the ZSL enhance rapidly and reach its strongest at 00 LST.From 03 to 06 LST,the temperature and humidity near the ZSL weaken,the abnormal divergence of the wind and water vapor increase,and ascending motion turns into a weak descending motion.All these conditions are not conducive to the development of the ZSL.However,due to the precipitation that happened during this period,the feedback effect of condensational latent heating on the lower layers is enhanced,which counteracts some unfavorable factors,resulting in a secondary peak of the ZSL intensity at 06 LST.Then,since all conditions are not conducive to the development of the ZSL,the ZSL intensity gradually weakens and reaches its weakest at 17 LST.