The influence of environmental conditions and cold pool generation on surface-based vs. elevated convection in a nocturnal mesoscale convective system

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’ tendency to ingest either elevated or near-surface air. A high-resolution Weather Research and Forecasting Model (WRF) simulation of the event is used to analyze a detailed series of parcel trajectories launched from within the inflow region of the storm. The trajectory analysis demonstrated that, before the nocturnal transition, the MCS was ingesting over 80% of parcels originating from anywhere near the surface up to 2.5 km MSL. After the transition, occurring 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’ strongest updrafts, signifying a clear transition to elevated convection following the nocturnal transition. The surface cold pool of the MCS was shown to have persisted through the duration of the simulation, even when the MCS was most 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 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 bore-like features developing in response to the forcing of the cold pool, which displaced the stable boundary layer and forced elevated, conditionally unstable air upward to its level of free convection. A weaker cold pool led to waves developing on the stable boundary layer itself that propagated ahead of the weaker surface outflow, initiating new convective updrafts.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Atmospheric Sciences

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2024

Author dc:creator

• Adams, Alexander

Contributors dc:contributor

• Rauber, Robert
• Jewett, Brian
• McFarquhar, Greg
• Trapp, Robert

• Nocturnal Thunderstorms
• Mesoscale Convective Systems
• Elevated Convection
• Cold Pools
• Numerical Modeling

Statement dc:rights

• Copyright 2024 Alexander Adams

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/127149

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F127149