The precipitation in Sichuan Basin (SB), China, exhibits pronounced diurnal variation, including minimum rainfall in daytime and a prominent peak near midnight. This study investigates the primary mechanism of precipitation diurnal variation in SB using forecasts from three summer months of 2013 produced at a 4‐km grid spacing. The model forecasts reproduce the observed spatial distributions and diurnal cycles well, including the peak precipitation in SB at around 02 local solar time (LST). Contrary to the common belief that emphasizes the solenoidal effects associated with the Tibetan and Yunnan‐Guizhou Plateaus, prominent diurnal inertial oscillations of boundary layer south‐southwesterly low‐level jet into SB are shown to play more important roles in modulating the diurnal cycles of precipitation in SB. A basinwide moisture budget analysis is performed to reveal that the moisture flux from the southeast side of the basin dominates within the diurnal oscillations of the net moisture flux into the basin, and the much enhanced nocturnal low‐level jet plays a crucial role in the formation of nocturnal precipitation within the basin. The net moisture flux into SB reaches maximum at around 22 LST, the time boundary layer perturbation winds from the daily mean in the direction normal to the southeastern boundary of SB reach maximum, which is about 4 hr before precipitation peak at around 02 LST. Shallow thermally forced nighttime downslope flows and daytime upslope flows on the Tibetan Plateau and Yunnan‐Guizhou Plateau slopes contribute only a small portion of moisture fluxes through the basin boundaries.

Convection‐permitting simulations are used to investigate the key mechanism of Meiyu precipitation diurnal cycle over China. Six days from the 2014 Meiyu season are used to produce a “north” composite rainband over the Yangtze‐Huaihe River Basin and another 6 days used to produce a “south” composite band. Both rainbands have peak rainfall in the early morning, while the south band has a secondary peak in the afternoon. Low‐level ageostrophic winds (AGWs) are found to exhibit diurnal cycles with clockwise rotations and their directions, and magnitudes depend on the background geostrophic monsoon flows. Net moisture flux into a control volume enclosing each rainband is almost purely due to AGWs. For both rainbands, net flux reaches maximum at ~04 LST, about 3–4 hr before morning precipitation peak. For the north band, a prominent minimum occurs at ~19 LST, 4 hr before the precipitation minimum. The moisture fluxes through the southern control volume boundary make the largest contributions to the net flux and its diurnal variations. The diurnal variations of the AGWs and their relationship with the background monsoon flows agree very well with the prediction of Blackadar boundary layer inertial oscillation theory, and the convergence forcing by the AGWs resulting from the inertial oscillations plays a paramount role in modulating the diurnal cycles of Meiyu front precipitation, including the creation of early morning peak and evening minimum. Feedback of latent heat release plays only a secondary role. The commonly recognized diurnal monsoon variability can be explained by the Blackadar inertial oscillation theory.

Based on the hourly gauge-satellite merged precipitation data with the spatial resolution of 0.1° × 0.1° during 2013 ~ 2018, we have evaluated the performance of the convection-permitting Weather Research and Forecasting model at Nanjing University (WRF_NJU) in forecasting the precipitation diurnal variation and the associated atmospheric circulation over the eastern Tibetan Plateau and its surrounding regions during summer. Results indicate that WRF_NJU model can well reproduce the diurnal cycle of the summer precipitation in terms of the diurnal peak time, duration and magnitude. In addition, the eastward propagation of rainfall systems with long duration along the eastern Tibetan Plateau (ETP) and its adjacent areas can also be properly captured. The WRF_NJU model can reasonably reproduce the relevant atmospheric circulation during summer as well. However, the model tends to underestimate the summer precipitation amount (PA) and precipitation frequency (PF) over most time of a day. Relatively larger biases in the occurring time and magnitude of PA and PF diurnal peaks can be noted over the ETP and Sichuan Basin. Further analysis suggests that the underestimation of PA over the ETP is attributed to the much lower moisture supply, weaker low-level southwesterly winds and less convective available potential energy (CAPE) in the WRF_NJU model than in ERA5. Over Sichuan Basin, the underestimated PA is related to the weaker upward motion, which is corresponding to the cold biases of surface air temperature in WRF_NJU. Findings of this study provide the basic model biases and may be helpful to further improve the model physical processes.

The initiation and organization of a long‐duration hail‐producing mesoscale convective system (MCS) in eastern China are investigated using convection‐allowing simulations at 3‐km grid spacing with the Advanced Regional Prediction System. The lifecycle of this MCS is characterized by two stages. In the first stage, a series of convective storms are initiated along the northwest border of Jiangsu Provinces. These storms organize into a northwest southeast line as they moved southeastward, eventually organizing into an eastward moving bow‐echo structure during the second stage. Our analyses show that the storms initiate along a northwest‐southeast oriented convergence boundary set up between two low‐level mesoscale vortices, one to the northwest of Jiangsu Province and one over the East China Sea. Comparisons between the fake‐dry and control simulations show that the rearward (westward) spreading of a cold pool from another MCS proceeding the hail‐producing MCS plays a key role in generating and enhancing the vortex over Eastern China Sea. The tilting of baroclinically generated horizontal vorticity along the edge of the cold outflow creates a cyclonic and anticyclonic vortex couplet. The cyclonic vortex becomes the dominant East China Sea vortex as it is superposed onto the background cyclonic circulation. This cold outflow‐induced vortex also has strong impacts on the later organization of the hail‐producing MCS, leading to the eventual establishment of a bow‐echo structure. Finally, a three‐stage conceptual model for the initiation and organization of the long‐lasting multicellular MCS is proposed, and the understanding on the complex interactions between two MCSs will be helpful to operational forecasters.