Observed Relationships between Supercell Mesocyclone Intensity and Evolution, Background Environmental Characteristics, and Cell Mergers

Abstract Cell mergers with supercells are relatively common, but much remains unknown about how they may influence subsequent supercell hazards. Furthermore, many outstanding questions regarding mesocyclone evolution exist despite numerous studies linking supercell hazards with the background environments in which they occur. In this study, we analyze the Multi-Year Reanalysis of Remotely Sensed Storms dataset along with hundreds of observed supercell tracks to begin addressing these ideas. In line with recent studies, the outcome of a supercell–cell merger (specifically the final strength of the low-level supercell mesocyclone) is not strongly related to the background environment. Of the parameters that we tested, mixed-layer (ML) LCL exhibited the largest correlation, but the very small coefficient of determination suggests limited operational use. More significantly, the incorporation of Storm Prediction Center objective analyses yields novel quantification of observed mesocyclone strengths in different environments. Of the environmental characteristics tested, kinematic parameters like 0–3-km storm-relative helicity (SRH) and 0–3-km bulk wind difference are more correlated with peak mesocyclone intensity than thermodynamic variables like CAPE and CIN. 0–3-km SRH exhibits the largest correlation, and its variability explains roughly one-third of the variance of peak azimuthal shear. We show trends in peak mesocyclone intensity across notable environmental parameter spaces, as well as how low-level mesocyclone strength fluctuates as background environmental characteristics evolve. Environmental trends during and preceding the times of peak mesocyclone strength are quantified. These analyses may be useful for predicting short-term mesocyclone intensity changes in real time. Significance Statement This study addresses open research questions related to how storms merging with supercell thunderstorms may influence supercell evolution and how supercells tend to evolve in different background environments. We find that an environmental measure of cloud-base height is statistically correlated with whether a supercell–cell merger will yield a strengthening or weakening supercell, but the strength of this correlation is quite weak. We find stronger correlations between peak supercell strength across storm lifetimes and some environmental characteristics, particularly parameters related to the change in the wind with height in the lowest few kilometers above ground level. These relationships may be useful for predicting short-term changes in supercell strength in real time.