Environmental Influences on the Elevated Nature of a Nocturnal Mesoscale Convective System

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Abstract The Plains Elevated Convection at Night (PECAN) project was designed to investigate the environmental factors responsible for nocturnal mesoscale convective systems (MCSs) exhibiting either surface-based or elevated convection and to identify the mechanisms by which these MCSs can propagate when conditions conducive to the propagation of similar daytime storms are absent. In this paper, we analyze the 20 June 2015 nocturnal MCS and its response to the evolving nighttime environment. Because the system matured before the nocturnal transition, this case provides a unique opportunity to explore the direct impact of the nocturnal environment on the MCS’s tendency to ingest either elevated or near-surface air. A high-resolution Weather Research and Forecasting (WRF) Model simulation of the event, together with a novel strategy for analyzing air parcel trajectories, is employed to determine the origin levels of air entering updrafts within the storm. The trajectory analysis demonstrated that, during the first half of the simulation, between 0300 and 0530 UTC, the MCS was ingesting over 80% of parcels originating from anywhere near the surface up to 2.5 km MSL. In the second half of the simulation after the nocturnal transition occurred, which happened simultaneously with the stabilization of the boundary layer and intensification of the low-level jet, parcels originating from within the boundary layer were no longer being ingested by the strongest updrafts, while parcels originating from above the boundary layer continued to be efficiently ingested by the MCS’s strongest updrafts, signifying a clear transition to elevated convection following the nocturnal transition. Evidence of bore-driven propagation was found during this elevated period. Significance Statement During the summer, nocturnal thunderstorms contribute a majority of the heavy precipitation that falls on the U.S. Great Plains region. Despite their prevalence, they are still relatively difficult to forecast, since the processes that allow them to remain long-lived differ significantly from those observed in similar daytime thunderstorms. In this paper, a numerical simulation of a nocturnal thunderstorm observed on 20 June 2015 is used to show that, after nightfall, the storm was no longer able to ingest air that originated near the surface, which is a typical fuel source for daytime thunderstorms, but rather ingested air that originated from well above the surface. We employ a novel strategy for analyzing air parcel trajectories to improve our understanding of how nocturnal thunderstorms draw energy from the evolving nocturnal environment.

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