The Role of Descending Rear-Inflow Jets in Tornadogenesis in Quasi-Linear Convective Systems across Central Alabama

Previous literature has shown the role of a convectively generated rear-inflow jet (RIJ) in producing damaging winds in quasi-linear convective systems (QLCS). Studies have also hypothesized that a descending RIJ may also play a role in tornadogenesis in a QLCS. A research project performed at the National Weather Service in Birmingham, AL (BMX) investigated the QLCS tornado warnings that had been issued by BMX since the beginning of the storm-based warning era in October, 2007. The goal of this research project was to improve the quality of the tornado warnings issued by BMX.

An examination of the tornado database created at BMX indicated the previous storm mode classifications (supercell, QLCS, and tropical) were too broad to see specific areas in which BMX could reduce its FAR and increase its POD. New storm classifications were implemented which included non-supercells in a line (a more robust version of the original QLCS classification) and marginal supercells. These classifications led to a condensed radar analysis of NSL and marginal warnings which contributed a total of 26.4% of all false alarms since October, 2007.

The radar analysis yielded that marginal supercells had weak reflectivity (<55dBZ) in most cases and non-persistent rotation throughout their lifetimes. NSL tornadoes showed a strong correlation between a localized descent of a RIJ and tornadogenesis. 68 tornadoes (88 false alarms) were examined. 76.5% (1%) showed the presence of a descending RIJ, 1.5% (61.5%) showed no sign of a descending RIJ, and 22.1% (25%) were inconclusive due to radar limitations. Further examination of reconstructed RHIs near a preexisting mesovortex showed a rapid increase in rotational velocity and in mesovortex depth as the RIJ descended to the surface. Therefore, it will be concluded that the majority of NSL tornadoes in BMX's county warning area were produced as a consequence of a descending RIJ. The role of the RIJ in enhancing vorticity cannot be definitively concluded without high-resolution modeling, but it will be shown that the RIJ locally enhanced the convergence near mesovorticies prior to tornadogenesis, supporting the results of previous studies. Finally, it will be shown that the reclassification of the tornado database and the consequential radar analysis will provide new tools to BMX to aid in their warning decision process.