• diurnal variation;
  • monsoon diurnal cycle;
  • precipitation;
  • moisture transport;
  • summer monsoon

[1] Diurnal variability of the summer monsoon over China, a key element affecting regional climate, is examined using the latest reanalysis dataset and satellite rain estimates. Diurnal variation of low-level wind is found to be pronounced over South China during active monsoon days. Mean wind speed attains a maximum (minimum) in early morning (afternoon), with a diurnal range of 2–3 m/s, double that of inactive monsoon days. The largest amplitude typically appears at 850 hPa, consistent with radiosonde observations. Such a monsoon diurnal cycle can strengthen low-level moisture transport at night by about 20% more than during the day. Nocturnal moisture fluxes converge toward Central China and lead to a meso-synoptic-scale moisture sink during the late night and morning, which plays a role in regulating the regional water budget on a diurnal time scale. Monsoon flow also helps provide substantial moisture over low-lying areas in the morning hours. Correspondingly, morning rainfall undergoes a remarkable increase and greatly contributes to the diurnal cycle of summer rainfall. The strongest response comes from meso-α-scale rain events that not only become prominent during active monsoon days but also possess a dominant morning peak. These morning events occur mainly on the basins and plains of Central China, where the monsoon diurnal cycle promotes nighttime mesoscale convection. These tend to shift northward from June to August, with the progress of the monsoon diurnal cycle, thereby producing the morning-peak summer rainband. The findings point to an efficiency of nocturnal monsoon flow influencing the warm-season weather and climate over eastern China.