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East Asian atmospheric rivers (ARs) exhibit the most pronounced activity in summer with significant impacts on monsoon rainfall. However, their occurrence mechanisms are yet to be revealed in detail. In this study, we unravel the inherently complex nature of East Asian summer ARs by applying a multiscale index that quantifies the relative importance of high-frequency (HF) and low-frequency (LF) moisture transports in AR development. It is found that both HF and LF processes contribute to shaping the summertime ARs in East Asia, contrasting to the wintertime ARs dominated by HF processes. Stratification of ARs with the multiscale index reveals that HF-dominant ARs are driven by baroclinically deepening extratropical cyclones, analogous to the widely accepted definition of canonical ARs. In contrast, LF-dominant ARs result from an enhanced monsoon southwesterly between a quasi-stationary cyclone and an anticyclone with the latter being the anomalous expansion of the western North Pacific subtropical high. Such a pattern is reminiscent of the classical monsoon rainband. While HF-dominant ARs are transient, LF-dominant ARs are quasi-stationary with a higher potential for prolonged local impacts. The intermediate ARs, constituting a majority of East Asian summer ARs, exhibit synoptic conditions that combine HF- and LF-dominant ARs. Therefore, East Asian summer ARs cannot be explained by a single parent system but should be considered as a continuum of extratropical-cyclone-induced and fluctuating monsoon-flow-induced moisture plumes. This finding would serve as a base for the advanced understanding of hydrological impacts, variability, and projected change of East Asian ARs. Significance Statement: Despite the accumulation of studies on summertime atmospheric rivers (ARs) in East Asia, a comprehensive explanation for their occurrence mechanisms remains elusive. This study disentangles their complicated nature through case-level multiscale analyses. In contrast to wintertime ARs, summertime ARs are shaped by both high- and low-frequency moisture transports. The high-frequency moisture transport is associated with migratory extratropical cyclones which are suppressed but still active in summer, while the low-frequency moisture transport arises from the fluctuation of a quasi-stationary monsoon southwesterly along the periphery of the western North Pacific subtropical high. The varying relative contribution of high- and low-frequency components from one AR to another suggests that East Asian summer ARs represent a continuum of extratropical and monsoonal moisture plumes. [ABSTRACT FROM AUTHOR]

This study highlights the importance of the diabatic process in the heavy rainfall events (HREs) that are initiated on the eastern slope of the Tibetan Plateau and move to the lower reaches of the Yangtze River basin. These HREs, which cause significant socioeconomic losses in the Yangtze River basin, are typically maintained for 3 days. They develop when a large amount of moisture converges on the eastern slope of the Tibetan Plateau. By solving the quasigeostrophic (QG) omega equation, it is revealed that the vertical motion of HREs is organized by both dynamic and diabatic forcings, with the latter being dominant. The stationary boundary forcing on the eastern slope of the Tibetan Plateau also contributes to the initial organization of the HREs. While the dynamic vertical motion does not change much and the boundary forcing becomes negligible after the initial organization, diabatic vertical motion becomes more dominant in QG vertical motion (∼80%) as HREs develop and move downstream. The potential vorticity (PV) tendency budget analysis reveals that the development and eastward movement of the HRE-related surface cyclone is primarily associated with diabatic PV production to the east of the cyclone where a large amount of moisture converges. This result implies that the long-traveling HREs along the Yangtze River basin are highly self-maintaining in nature. [ABSTRACT FROM AUTHOR]