**4. Conclusions**

The "21.7" extreme rainfall event hit Zhengzhou, China on 20 July 2021, causing hundreds of fatalities and great economic losses. In particular, the 1 h precipitation at a national surface station in ZZ was 201.9 mm between 1600 and 1700 LST, breaking the record for hourly precipitation in mainland China. Our analyses show that the recordbreaking hourly precipitation was produced by a quasi-stationary, well-organized, deep convective storm in ZZ that was fed by abundant tropical moisture via an atmospheric river between the WPSH and Typhoon In-Fa. The Taihang Mountains northwest of ZZ played a vital role in turning the environmental low-level easterlies into mountain-parallel northeasterly flows in the north, helping to block the southerly flows from the south into ZZ. The storm that moved into ZZ to produce the extreme hourly rainfall was initiated on the southern slopes of the Song Mountains southwest of ZZ.

The low-level easterly flow into ZZ was steadily enhanced in the hours preceding the extreme rainfall, whereas the airflow out of the ZZ region on its west side weakened in the same period. The flow changes led to a net moisture flux into ZZ that quadrupled in 5 h preceding the extreme hourly rainfall in ZZ. The enhanced low-level easterly flow roughly balanced the relatively weak cold pool density current and kept the storm stationary over ZZ. The rainstorm contained unusually high concentrations of small raindrops with the presence of some very large drops (about 7 mm). The rain DSDs and the polarimetricradar-derived microphysical properties provided the first observational evidence that both oceanic (high number of raindrops, active warm rain processes) and continental (large raindrops, active ice processes) rain characteristics were active and very efficient in converting the abundant tropical moisture into the record-breaking hourly rainfall. These key dynamical and microphysical processes are summarized schematically in Figure 6.

**Figure 6.** The conceptual model of the maintenance and precipitation microphysics of the convective storm resulting in extreme hourly rainfall in ZZ. The blue line with triangles indicates the cold pool gust front. The red arrows represent the prevalent winds.

This study provides insights into how local extreme rainfall may be better predicted by including mesoscale and convective scale processes together with the well-forecasted favorable synoptic conditions for heavy rainfall. This study also points out the unique DSD differentiating this extreme rainfall event from most other documented precipitation events in different regions of the world. We admit that an MCS should consist of both convective core and anvil cloud regions, with the latter also likely to contribute to the production of extreme rainfall. In this study, our main purpose was to investigate the key factors causing the record-breaking hourly rainfall in a local ZZ region weather station. According to radar observations, this hourly extreme rainfall was directly caused by the convective cell over ZZ. More observational and modeling studies will be conducted to investigate whether there are optimal and synergetic combinations between dynamics and microphysics in producing the unique DSD identified in the "21.7" extreme rainfall event. To be able to accurately represent within numerical weather prediction models the unique microphysical characteristics of this event and all other important ingredients that act in synergy to produce such record-breaking extreme rainfall and to provide quantitatively accurate operational forecasting with sufficient lead time remain challenging. Gaining insights and understanding of the physical processes and mechanisms involved is critical and this study represents one of the first efforts toward this goal.

**Supplementary Materials:** The following supporting information can be downloaded at: https://www. mdpi.com/article/10.3390/rs15184511/s1, Figure S1: (Top) Terrain elevation (shading) and VDRAS analysis horizontal winds at 0.6 km MSL (vector) at 1200 LST on 20 July 2021. (Bottom) Potential temperature (shading) and VDRAS analysis wind field in the vertical plane along line AB at 1200 LST on 20 July 2021. Red contours indicate the speed of horizontal wind normal to the vertical plane (i.e., nearly parallel to the Taihang Mountain); Figure S2: (Top) Horizontal wind vectors and speed (shading) at 3 km MSL in the VDRAS analysis from 1000 to 1600 LST on 20 July 2021. (Bottom) Time evolution of the mean horizontal wind speed over ZZ at different heights. Reference [55] is cited in Supplementary Materials.

**Author Contributions:** Conceptualization, K.Z.; Methodology, X.X. and G.C.; Software, H.H., X.F., Q.L. and J.Y.; Formal analysis, A.Z., Q.Z. and F.Z.; Data curation, C.W.; Writing—review & editing, M.X., Z.-M.T., J.F. and W.-C.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was primarily supported by the National Natural Science Foundation of China (grants 42025501, 4212200198, 41875053, 61827901, and 42122036), the Open Grants of the State Key Laboratory of Severe Weather (2023LASW-A01), the National Key Research and Development Program of China (Grant 2017YFC1501601, 2018YFC1506404).

**Data Availability Statement:** All freely available data are mentioned in the section on Data and Methods.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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