Quasi-Idealized Simulations of Training-Line/Adjoining-Strataform Mesoscale Convective Systems: Warm-Season Type Events

This study utilizes quasi-idealized numerical simulations to identify initiation and organizational mechanisms for the heavy-rain producing training-line/adjoining-stratiform (TL/AS) mesoscale convective systems (MCS) morphology. In an adjacent study, rotated principle component analysis was performed on atmospheric fields associated with TL/AS, revealing that they fall into two distinct synoptic categories: synoptic-type events featuring a pronounced upper level cyclonic trough and progressive cold/warm frontal structure, and warm-season type events, which are characterized by anticyclonically curved upper level flow and a quasi-stationary low-level warm front. This research focuses on the dynamics of warm-season type events.

Convection-permitting numerical simulations were conducted from composite atmospheric conditions for warm-season type events. Deep moist convection initiates along the nose of a low-level jet, where gradual isentropic up-glide results in an elevated moist-absolutely-unstable layer, and quickly grows upscale into an elevated TL/AS type MCS. Despite the presence of a shallow stable layer prior to convective initiation, a distinct cold pool develops at the surface. The trailing edge of this low-level cold pool, along with an array of gravity wave structures generated by latent heating remain quasi-stationary to the southwest of the MCS for several hours. Elevated conditionally unstable air arriving via the low-level jet is vertically agitated as it encounters these stationary features, resulting in moist absolutely unstable (MAUL) layers. Pre-existing convection within the MCS generates horizontal pressure perturbation forces, which modify the environmental wind environment to favor low-level convergence and continuous regeneration of convection over a fixed geographic region. Horizontal perturbation pressure gradient forces are subsequently maintained by newly developed convection, thus resulting in a reinforcing feedback loop wherein internal convective dynamics modify the surrounding environment to favor quasi-stationary convective behavior. This behavior is consistent with that observed in real-data simulations of TL/AS MCSs.

Another TL/AS MCS is produced in an otherwise identical simulation where evaporation is turned off and no cold pool is generated. Quasi-stationary convective behavior here is attributed to internal dynamics that are similar to those from the case with evaporation included. This simulation illustrates that a cold pool is not required for quasi-stationary behavior, MCS propagation, or linear convective organization.