Progress in research of the July 2021 extreme precipitation event in Henan province，China

From 17 to 22 July 2021, an extremely heavy rainstorm occurred in Henan province, China, causing severe casualties and property damages. This rainstorm is named as "21.7" extreme rainstorm. The "21.7" extreme rainstorm has attracted great attention of many scholars, and significant progress was made with nearly 100 studies of this flood in less than two years. The present study reviews recent research progresses in characteristics of the "21.7" extreme rainstorm, including influencing weather systems and their mechanisms, the effects of underlying surface and climate warming and numerical forecast, etc. These progresses are compared with research progresses of the "75.8" heavy rainstorm. Results indicate that compared with the "75.8" heavy rainstorm, finer characteristics of rainfall and mesoscale and microscale systems in the "21.7" extreme rainstorm have been revealed due to the development of observation technology and the improvement of research methods, particularly in terms of microphysical processes. Among these findings, the dynamic process of coupling enhancement between mesoscale convective system and mesoscale convective vortex and the microphysical process of simultaneous growth of various scale particles promoted by melting of hail particles in the "21.7" extreme rainfall process are important findings. The "21.7" extreme rainstorm was more strongly influenced by tropical and oceanic factors. With global warming, the "21.7" extreme rainstorm showed a more pronounced response to tropical atmospheric circulation and ocean than the "75.8" heavy rainstorm, which may result in more extreme hourly rainfall during the "21.7" extreme rainstorm. However, urban impact on the "21.7" extreme rainstorm is complex. It is challenging to distinguish urban impact from global warming signals, which increases the uncertainty regarding the mechanism of the "21.7" extreme rainstorm. Finally, directions of future research are discussed.