The Impact of Cold Pool Strength on the Propagation Mechanisms of a Nocturnal Mesoscale Convective System

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Abstract In the first part of this study, a Weather Research and Forecasting (WRF) Model simulation of the 20 June 2015 Plains Elevated Convection at Night (PECAN) mesoscale convective system (MCS) was analyzed using a novel strategy for analyzing air parcel trajectories to show that the storm transitioned from surface-based to elevated as a response to the evolving nighttime environment, despite the surface cold pool remaining strong even after the system became elevated. To better understand the role of the cold pool in the propagation of an elevated MCS, three additional WRF simulations were carried out in which the magnitude of latent cooling due to evaporation was either doubled, halved, or removed entirely, effectively controlling the strength of the cold pool. The novel trajectory analysis developed in the first part of this study was then repeated for the additional simulations. It was found that in an environment where both surface-based and elevated instability were present, a stronger cold pool led to less surface-based convection, while a weaker cold pool led to more surface-based convection. During the second half of the simulation, when only elevated instability existed, all simulated storms remained elevated, but the mechanism by which the elevated convection propagated differed. A strong cold pool led to a bore-like feature developing in response to the forcing of the cold pool, which displaced the stable boundary layer and forced elevated, unstable air upward to its level of free convection. A weaker cold pool led to waves developing atop the stable boundary layer and propagating ahead of the surface outflow and initiating new convective updrafts. Significance Statement The mechanisms by which summertime nocturnal thunderstorms can remain long-lived are not as well understood compared to daytime thunderstorms, making them difficult to forecast. While significant progress has been made toward understanding these nocturnal storms, there are still questions about how they develop and are maintained. This paper investigates how a nocturnal storm’s ability to ingest air from different sources changes as a response to the strength of its cold pool. During periods that there were energy sources available near the surface and aloft, a stronger cold pool made the storm less likely to be fueled by surface air, while a weaker cold pool led to the storm being more likely to be fueled by surface air. These results counter conventional understanding of how these storms remain long-lived, and explanations for such counterintuitive behavior are given.

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