Initiation Mechanisms of Nocturnal Convection Without Nearby Surface Boundaries Over the Great Plains

Nocturnal convection initiation accounts for nearly half of all CI events (daytime and nighttime) over the Great Plains over the warm season. There are three main types of CI events: those that occur on surface boundaries, those that occur on the cold side of surface boundaries, and those that occur with no nearby surface boundaries (NB [no boundary] CI mode). This study focuses on the poorly-understood NB nocturnal CI mode events. These storms typically occur in environments with weak forcing or occur far away from preexisting convection and away from surface boundaries. There are preferred characteristics to these events, however. For example, these events form later at night (around 0900 UTC), occur on the eastern side of the low-level jet (LLJ), and the linear systems tend to be oriented north/south. Three initiation mechanisms are explored here in detail: ascent due to atmospheric bores, ascent due to the nocturnal LLJ, and ascent associated with cyclonic potential vorticity (PV) anomalies. The gravity wave and LLJ mechanisms are explored using three-dimensional idealized numerical simulations, and PV inversion is used to explore the PV anomaly mechanism. A composite sounding was created based on 108 proximity soundings for NB CI mode events. Nine environments representing a range in winds and temperature are used to test each mechanism's effect on a range of nocturnal environments. In these simulations, the vertical velocity associated with atmospheric bores is between 0.5 and 1 m s-1, and the net displacement is 200 – 500 m. The vertical velocity associated with the LLJ is 1 cm s-1, but due to the sustained ascent throughout the night, the net displacements can be as much as 1 km. Finally, the ascent associated with PV anomalies is between 0.1 and 1 cm s-1, and the net displacement is typically between 50 and 100 m. Based on these displacements, gravity waves and the LLJ are likely CI mechanisms, while PV anomalies are not likely to cause CI