Nocturnal tornadoes and low-level static stability

Nocturnal tornadoes, while comprising only about a quarter of verified tornadoes, produce a disproportionate percentage of tornado fatalities. Despite their significance, there is little pre-existing literature on this subject. It seems possible that the dynamics of tornadogenesis would vary significantly over the diurnal cycle. For example, the depth and nature of storm inflow may change as the daytime boundary layer transitions into the nighttime stable layer.

A climatology of significant (F2 – F5) nocturnal (occurring between 3 UTC – 13 UTC) tornadoes in the contiguous United States from 2004 – 2006 reveals that the majority of cases occurred in squall line-embedded supercells or broken lines of supercells. Over half of the cases occurred during the cool season, mostly in Gulf Coast states. The location of the maximum number of cases shifted to the southern and central Plains during spring. Only one in 69 tornadoes in the climatology occurred during summer.

Several modeling studies have shown that increasing low-level vertical static stability impedes tornadogenesis. However, Rapid Update Cycle-2 (RUC-2) model proximity soundings were gathered for cases in the climatology, and suggest that over half of the tornadoes in the climatology formed within a stable boundary layer. We will simulate in a series of numerical modeling experiments mature supercells in environments of varying low-level vertical static stability, and examine the formation of tornado strength vortices.