Diurnal variation of precipitation over southeastern China: 2. Impact of the diurnal monsoon variability

The monsoon flow influence on the diurnal variation of precipitation over southeastern China (SEC) is examined using satellite rainfall data, reanalysis data, and data sets from field experiments. The low-level wind averaged over southern China is strongest at late night or in early morning, which is inferred as diurnal monsoon variability (DMV). Such nocturnal acceleration occurs frequently from presummer, when monsoon flow becomes active. It usually develops in the warm region south of frontal zone; it is related closely to the ageostrophic component. In response to the DMV, rainfall over SEC increases remarkably, especially during early morning when the wind maximum of southwesterlies occurs. Prolonged organized mesoscale convection is the main cause of this rainfall enhancement. On a diurnal timescale, the DMV may regulate the warm, moist inflow that feeds the frontal zone, thereby enhancing large-scale frontogenesis from late night to morning. On a regional scale, the DMV, when coupling with local circulation diurnally, enhances the southerlies on the east flanks of plateaus and over the western SEC, generating convective instability successively there through the early morning. Consequently, the organized mesoscale convection is reinforced to produce more morning rainfall during presummer over SEC area, especially over its western region.