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Multiscale processes from synoptic disturbances to diurnal cycles during the record-breaking heavy rainfall in summer 2020 were examined in this study. The heavy rainfall consisted of eight episodes, each lasting about 5 days, and were associated with two types of synoptic disturbances. The type-1 episodes featured a northwestward extending western Pacific subtropical high (WPSH), while the type-2 episodes had approaching midlatitude troughs with southward retreat in the WPSH. Each heavy rainfall episode had 2–3 occurrences of nocturnal low-level jets (NLLJs), in close association with intense rainfall in the early morning. The NLLJs formed partly due to the geostrophic wind by increased pressure gradients under both types of synoptic disturbances. The NLLJs were also driven by the ageostrophic wind that veered to maximum southerlies at late night due to the boundary layer inertial oscillation. The diurnal amplitudes of low-level southerlies increased remarkably after the onset of type-1 episodes, in which the extending WPSH provided strong daytime heating from solar radiation. By contrast, the wind diurnal amplitudes were less changed after the onset of type-2 episodes. The NLLJs strengthened the mesoscale low-level ascent, net moisture flux convergence, and convective instability in elevated warm moist air, which led to the upscale growth of MCSs at the northern terminus of the LLJ after midnight. The MCSs-induced mei-yu rainband was reestablished in Central China during the type-1 episodes with the increased diurnal variations. The findings highlight that the regional diurnal cycles of low-level winds in response to synoptic disturbances can strongly regulate mesoscale convective activities in a downscaling manner, and thus produce heavy rainfall. [ABSTRACT FROM AUTHOR]

Monsoon southerlies can be particularly active for days and produce substantial rainfall over East Asia. These multiday episodes of southerly monsoon surge may exhibit distinct diurnal variations due to regional forcings under given large-scale conditions. This study categorizes the southerly surges into two types with different wind diurnal variations to clarify their influence on rainfall over East Asia. In the summers of 1998–2019, there are 63 episodes of southerly surges with large wind diurnal cycles and 55 episodes with small diurnal cycles. The first type of southerly surges usually occurs with anomalous low-level warming over southeastern China related to the westward extension of the western Pacific subtropical high. The second type of southerly surges instead occurs with anomalous cooling due to the deepened midlatitude trough. They thus represent the different mechanisms downscaling from large-scale conditions to regional diurnal forcings. After the onset of the first type, the intensified monsoon southerlies at night lead to the northward displacement of large-scale ascent and northward water vapor transport with warm moist energy. The monsoon rainband tends to move to the north of 35°N with a robust response in precipitation systems, especially in the meso-α-scale rain events from midnight to morning. As a comparison, the rainband stays at 30°–35°N after the onset of the second type when the strengthened large-scale ascent and water vapor convergence are located relatively south. These differences between the two types of southerly monsoon surges highlight that the multiday large-scale conditions interact with subdaily regional forcings and greatly regulate the detailed evolution of summer rainband over East Asia. [ABSTRACT FROM AUTHOR]