Environments of tornadic and non‐tornadic supercells in China and optimized significant tornado parameter for China region

Abstract With a sample of 273 supercells spanning 20 years, inflow environment differences between tornadic and non‐tornadic supercells in China and its three subregions (northern, central and southern China; CNN, CNC, and CNS) are examined using sounding‐derived parameters. Proximity soundings are extracted from the hourly ERA5 reanalysis data. The supercells are categorized as significantly tornadic [rated (E)F2+], weakly tornadic [rated (E)F1], and non‐tornadic. Thermodynamic parameters, such as convective available potential energy (CAPE), lifting condensation level (LCL), low‐level relative humidity (RH) and convective inhibition (CIN), cannot discriminate between tornadic and non‐tornadic supercells effectively. In addition, thermodynamic parameters based on mixed‐layer (ML) lifted parcels show worse skill than those for surface‐based (SB) or most unstable (MU) lifted parcels. Storm‐relative helicity (SRH300) in the range 0–300 m and 0–300‐m bulk shear (SHR300) demonstrate greater forecasting skills compared to SRH and shear over deeper depths. Based on predictive skills and distributions of individual parameters, a new significant tornado parameter (STP) formulation, STP300cn, using MUCAPE, MULCL, MUCIN, SRH300, and SHR300 is composed. True skill score (TSS) is used to measure the capability of the individual or combined parameters in discriminating significantly tornadic from non‐tornadic supercells. The thresholds and normalization factors for terms in STP are calibrated to the China cases to obtain optimal predictive TSS scores. The calibrated STP parameter, called STP300cn, achieves a TSS of 0.51 in China overall, compared to the 0.14 and 0.29 of the two original versions of STP. It achieves a TSS of 0.37, 0.66, 0.42 for CNN, CNC and CNS, respectively, all much higher than those of the original STP parameters.