The Influence of Cell Mergers on Supercell Characteristics and Tornado Evolution on 27–28 April 2011

Abstract Numerous questions remain regarding the influence of environmental inhomogeneities on supercell evolution. Motivated by this topic, this study associates cell-merger occurrence with supercell evolution and tornado production during the prolific 27–28 April 2011 outbreak in the U.S. Southeast. This event included 29 discrete supercells that produced 102 tornadoes and featured 300 cell mergers. Cell-merger frequency increased for supercells that initiated farthest east, possibly owing to changes in overall convective coverage over time. There is some signal for stronger mesocyclones to be associated with more mergers in the primary supercell’s forward flank. There is also a slight tendency for supercells that encounter more cell mergers to produce tornadoes more quickly, especially for those that formed away from a significant zonal boundary. However, there is a slight tendency for supercells spawning the longest-lived tornadoes (especially those with durations over 60 min) to be associated with fewer cell mergers during the 15-min window preceding tornadogenesis. Of particular importance, a significant inverse relationship exists between premerger mesocyclone strength and the subsequent change in mesocyclone strength during the merger (i.e., weaker mesocyclones tended to strengthen as a result of the merger, and vice versa). These findings highlight the influence that cell mergers can have on supercell evolution and tornado production—even within an incredibly volatile environment—and motivate future work exploring the physical processes involved and ways to translate these findings into experimental techniques or guidance for operational forecasters. Significance Statement The prolific 27–28 April 2011 supercell tornado outbreak in the U.S. Southeast featured 29 supercells that produced 102 tornadoes. This study analyzes mergers between these tornadic supercells and 300 weaker cells to determine if the mergers corresponded with important supercell characteristics. This appeared to be the case during this event; cell-merger events tended to be associated with tornadic periods of the supercells’ life cycles and influenced low- and midlevel mesocyclone strength, supercell evolutionary time scales, and subsequent tornado duration. These results are important for (i) better understanding of different supercell evolutionary paths in similar background environments and (ii) motivating future work in investigating experimental products related to these findings.