Diagnosis of the Kinetic Energy of the "21<middle dot>7" Extreme Torrential Rainfall Event in Henan Province, China

An extreme torrential rain (ETR) event occurred in Henan Province, China, during 18-21 July 2021. Based on hourly rain-gauge observations and ERA5 reanalysis data, the ETR was studied from the perspective of kinetic energy (K), which can be divided into rotational wind (V-R) kinetic energy (K-R), divergent wind kinetic energy (K-D), and the kinetic energy of the interaction between the divergent and rotational winds (K-RD). According to the hourly precipitation intensity variability, the ETR process was divided into an initial stage, a rapid increase stage, and maintenance stage. Results showed that the intensification and maintenance of ETR were closely related to the upper-level K, and most closely related to the upper-level K-R, with a correlation coefficient of up to 0.9. In particular, the peak value of hourly rainfall intensity lagged behind the K-R by 8 h. Furthermore, diagnosis showed that K transformation from unresolvable to resolvable scales made the ETR increase slowly. The meridional rotational wind (u(R)) and meridional gradient of the geopotential (phi) jointly determined the conversion of available potential energy (APE) to K-R through the barotropic process, which dominated the rapid enhancement of K-R and then caused the rapid increase in ETR. The transportation of K by rotational wind consumed K-R, and basically offset the K-R produced by the barotropic process, which basically kept K-R stable at a high value, thus maintaining the ETR.