Lift in the Vertical Shear of Southerly Jet: A Mechanism of Nocturnal Convection in the Absence of Boundaries

One-third of the observed severe nocturnal convection/thunderstorms in the U.S. Great Plains during the months of April-July in 1996-2015 were initiated in the absence of any nearby fronts or boundaries of contrasting air masses, which we label "elevated convection of the second kind." Even more intriguing is that nocturnal thunderstorms initiating in the absence of preexisting air mass boundaries were in low CAPE environments. This is in contrast to the expectation that thunderstorms are more likely to initiate in high-CAPE environments with abundant fuel for thunderstorm updrafts. This study addresses the mechanisms for the elevated convection of the second kind.

In the aforementioned nocturnal thunderstorm events, the mesoscale environment was generally characterized by westerly winds originating over the Rockies interacting with a southerly low-level jet from the Gulf of Mexico through the southern and central Great Plains. The inertial oscillation produces supergeostrophic winds in the nocturnal low-level jet, which veer overnight and develop a significant westerly component. The vertical shear arising from the westerly component creates a "northern vortex" in the lower half of the jet profile. Theories in fluid mechanics predict that the interaction of this vortex with strengthening westerly zonal flow will generate a lift. Such lift could be capable of initiating and sustaining nocturnal deep convection in the atmosphere.

Our initial experiments to examine and test this mechanism are being conducted with 2-D simulations in the Bryan Cloud Model (CM1). Preliminary results indicate that there is ascent on the east side of the jet core, where nocturnal convection typically initiates, and the magnitude of the ascent appears sufficient to initiate deep convection. Not only is there strong ascent east of the jet core, but parcels within the northern vortex are also lifted from their initial level, supporting the application of fluid mechanics principles to atmospheric processes of convection initiation.