Mei-yu frontal rainfall including jet-related rainstorms experiences pronounced daily variation (DV).Scientists tried to seek explanations from the DV of lower-level jet as a member of the mei-yu precipitation system.Based on case study and climate analysis,this work arrives at the central cause of DV of the mei-yu rainfall,which lies in the regional difference in DV of the lower-level temperature field of east Asia,and the cause relates essentially to 1)distinct topography in this region and 2) a particular cloud pattern during the wet season.Because of the existence of mei-yu precipitation cloud band the temperature DV is faint inside the mei-yu trough and on account of large-scale prominent terrain available,significant temperature DV occurs on both sides of the trough,leading to the reinforcement of the subtropical high and midlatitude transformed high,and the deepening and narrowing of the trough in the night with the result that the relative intensity and precipitation function of the mei-yu frontal rainfall system are changed at day and night,resulting in precipitation DV.And the DV of lower-level systems,e.g.,jet stream,is among the intermediate changes happening after the DV is displayed in the mei-yu pressure field.Regional difference in lower-level temperature DV is a climatic phenomenon that often emerges in this wet season of east Asia.