The Influence of Environmental Conditions and Cold Pool Generation on Surface-Based vs. Elevated Convection in a Nocturnal Mesoscale Convective System
Nocturnal mesoscale convective systems account for the majority of warm-season precipitation in the Great Plains region of the United States. Despite their prevalence, they are still represented poorly in numerical weather prediction models compared to their daytime equivalent. Nocturnal MCSs occur in environments often characterized by minimal surface-based convective available potential energy (CAPE), leading to the assumption that these systems are likely elevated. One of the goals of the Plains Elevated Convection at Night (PECAN) field project was to understand the environmental factors responsible for a nocturnal MCS exhibiting either surface-based or elevated convection, and how that affects the mechanisms for the MCS’ propagation.
This presentation will analyze the 20 June 2015 PECAN MCS case and document its progression into a stable environment during the nighttime transition. As it propagated eastward across the Great Plains it intercepted the nocturnal low-level jet while the boundary layer deepened and stabilized, presenting a unique opportunity to analyze the MCS’ behavior before and after this transition. Using a high resolution WRF simulation, the tendency of the MCS to exhibit surface-based or elevated convection was explored through a detailed series of parcel trajectory calculations, showing that once the system encountered the low-level jet and an elevated maximum in CAPE, updraft-bound parcels tended to more likely originate above the boundary layer than before. It was also found that the system did produce a strong cold pool that persisted over the duration of the simulation, even as the nocturnal transition set in, suggesting that the role of the cold pool in promoting surface-based convection may not be as straightforward as expected. To better understand this interaction, additional simulations were run in which the cold pool was either strengthened or weakened by altering the latent cooling produced by evaporation. The additional simulations revealed that a stronger (weaker) cold pool led to less (more) surface-based convection compared to the control simulation.
