Mechanisms for Upwind Propagation and Heavy Rainfall Production in Elevated Training Mesoscale Convective Systems
The synoptic environments in both the observed case and composite atmospheric fields provided a continuous supply of elevated moisture, conditional instability, and low-level lifting to the MCS. This maintained the first-order requirements for convection, and thus promoted the prolonged nature of the convective systems. The remainde espite the systems remaining predominantly quasi-stationary, progressive cold-pool-driven convective surges exited the region where heavy rainfall was produced at points during the simulated MCS evolutions. In each of these instances, new convection re-developed to the rear of the surging convective segment and well north of the surface outflow boundary laid out by previous convention (rearward off-boundary development, ROD). The ROD phenomena was explained by an inadequate balance between wind-shear and buoyancy gradients along the southwestern periphery of the surface boundary, and dynamics that led to maximized convergence within elevated southwesterly return flow in the wake of the surging MCS segment and north of the surface cold pool boundary. The second phenomena of interest constituted geographically fixed upstream backbuilding during periods of steady training convection (i.e. between cold pool driven convective surges). The analysis conducted thus far suggests that a sustained low-level low-pressure anomaly on the upstream end of the convective line maintained low-level convergence here and thus contributed to persistent upstream convective re-development.