Mesoscale Processes Leading to Warm Sector Convection Initiation During the 27 April 2011 Afternoon Supercell Outbreak

One of the most prolific tornado outbreaks ever documented occurred on 26–27 April 2011 and comprised three successive episodes of tornadic convection that primarily impacted the southeastern United States, including two quasi-linear convective systems (QLCSs) that preceded the notorious outbreak of long-track, violent tornadoes spawned by numerous supercells over the warm sector during the afternoon and evening of 27 April. Our previously published work on this outbreak has emphasized the environmental evolution during the three convective episodes, including the movement of a cold front aloft (CFA) into the Southeast prior to the onset of the supercell outbreak, and the importance of flow modifications from the antecedent QLCSs in establishing highly favorable shear profiles over the warm sector.

Here, we use convection-permitting WRF ensemble simulations (Δx = 3 km) to elucidate the dynamical significance of the interaction between the CFA and the prefrontal moist layer and how this interaction led to 1) rapid mesoscale destabilization over the Southeast during the morning of 27 April and 2) unique thermodynamic profiles that supported the formation of mesoscale cloud bands and ultimately deep convection initiation (CI) over the warm sector. Specifically, we demonstrate that the CFA was preceded by an elevated bore that propagated into the warm sector and produced sustained mesoscale lifting, abrupt reductions in temperature and stability within the lower to middle troposphere (i.e., substantial weakening of a midlevel capping inversion and accompanying decreases in convection inhibition), and concurrent deepening of the low-level moist layer. Moreover, this deepened low-level moist layer exhibited the character of an unsaturated PBL that was topped by a moist absolutely unstable layer (MAUL), and instability within this combined layer (hereafter the PBL*)—which was confined beneath the remnant midlevel inversion—was sustained and further augmented by low-level θe advection, surface heating, mesoscale ascent ahead of the CFA. Convective overturning was restricted to the PBL* until the resultant updrafts were able to penetrate through the overlying inversion layer to yield deep CI, and this overturning manifested as longitudinal roll circulations that were at least somewhat analogous to traditional horizontal convective rolls (HCRs) that form within a sheared PBL, although they exhibited a horizontal spacing of ~20–30 km—much larger than the typical spacing of HCRs. We further investigate the characteristics and dynamics of these longitudinal cloud bands using idealized large-eddy simulations conducted with CM1, and results from this effort will also be discussed.