The Response of a Long-Lived Mesoscale Convective System to Changes in Lower-Tropospheric Conditions

Most continental regions experience an afternoon or evening maximum of convective activity, which is dominated by convection driven by surface heating. However, the Great Plains of North America experiences a nocturnal maximum in convective precipitation during the warm season, which is associated with the frequent occurrence of elevated mesoscale convective systems (MCSs). These elevated convective systems are sustained by inflow above a near-surface stable layer, and their existence is often attributed to the presence of a nocturnal low-level jet (LLJ), which transports warm, moist air poleward during the nighttime. While the presence of a strong LLJ and a stable layer are well known to exist in nocturnal convective environments, there is a lack of understanding on what environmental factors control the longevity of these nocturnal systems. While the majority of nocturnal MCSs tend to weaken or dissipate by late morning, many continue into the afternoon. Additionally, few studies have examined how convection evolves as it encounters environments with changing boundary layer characteristics throughout the diurnal cycle.

Herein we present observations and WRF-ARW simulations of a poorly forecast, long-lived MCS that initiated at approximately 0300 UTC (2100 LST) in a nocturnal environment thought to be unsupportive of convection. This MCS produced numerous severe reports, including a nocturnal tornado. Surface observations suggested that two mesoscale boundaries originating from a weak cold front were responsible for triggering this initially-elevated convection. WRF simulations depicted a rapidly evolving environment owing to a nocturnal LLJ, which was critical for the initiation and persistence of this MCS. Additionally, the structure of the LLJ led to considerable spatial variability in the low-level convective instability field owing to moisture advection, downwelling radiation, and turbulence.

The simulated system was initially elevated and maintained by a bore-like disturbance, but the development of a strong cold pool resulted in the transition to a cold-pool-driven system despite the fact that the highest instability existed aloft. As the system moved farther south into an increasingly unstable air mass, the system eventually became surface-based and persisted until moving off the Gulf Coast. The evolution of the convective structure and lifting mechanism during the system’s lifetime was examined in terms of vorticity balance (i.e., RKW theory) and wave analysis frameworks with similar results to past studies. The ability for the MCS to become maintained by a strong cold pool without the presence of a bore allowed the system to persist into a destabilizing environment, which is unfavorable for the maintenance of bores. After the MCS cold pool became the dominant forcing for ascent, the system behaved in a similar manner to that expected from RKW theory.

This study is one of the first to examine this persistence and transition in terms of both wave dynamics and vorticity balance frameworks. Because elevated, nocturnal systems are commonly maintained by bores, the ability for the primary convective region of the system to reduce its dependence on a bore for maintenance suggests that it was able to persist through a changing environment. Future studies, especially those utilizing high-resolution observations, are needed to determine whether or not this process is found in other long-lived, persistent MCSs.