*Article* **The Dynamics and Microphysical Characteristics of the Convection Producing the Record-Breaking Hourly Precipitation on 20 July 2021 in Zhengzhou, China**

**Kun Zhao 1,\*,†, Xin Xu 1,†, Ming Xue 1,2, Zhe-Min Tan 1, Hao Huang 1, Ang Zhou 1, Xueqi Fan 1, Qiqing Liu 1, Chenli Wang 1, Juan Fang 1, Wen-Chau Lee 3, Qinghong Zhang 4, Fan Zhang 5, Gang Chen <sup>6</sup> and Ji Yang <sup>6</sup>**


**Abstract:** An hourly rainfall of 201.9 mm fell in Zhengzhou on 20 July 2021, breaking the hourly rainfall record of mainland China and causing severe urban flooding and human casualties. This observation-based study investigates the associated convective-scale and mesoscale dynamics and microphysical processes using disdrometer and polarimetric radar observations aided by retrievals from the Variational Doppler Radar Analysis System. The synoptic flow forcing brought abundant moisture from the oceans and converged at Zhengzhou; then, the extreme rainfall was produced by a slow-moving convective storm that persisted throughout the hour over Zhengzhou. Unusually high concentrations of raindrops of all sizes (showing combined properties of maritime and continental convection) are revealed by the disdrometer data, whereas the polarimetric radar data suggest that both ice-based and warm rain processes were important contributors to the total rainfall. High precipitation efficiency was achieved with an erect updraft at the low levels, whereas enhanced easterly inflows kept the storm moving slowly. The interaction between convective-scale and mesoscale dynamics and microphysical processes within the favorable synoptic conditions led to this extremely heavy rainfall.

**Keywords:** extreme rainfall; dynamics and microphysics; slow-moving convection
