Mechanisms for Organization and Echo Training in a Flash-Flood-Producing Mesoscale Convective System

In this research, a numerical simulation of an observed training line – adjoining stratiform (TL/AS) type mesoscale convective system (MCS) was used to investigate processes leading to upwind propagation of convection and quasi-stationary behavior. The studied event produced a broad swath of 150-280 mm of rainfall between 00 UTC and 12 UTC on 28-July 2011, with localized totals exceeding 380 mm over Dubuque, Iowa, resulting in damaging flash flooding.

Sustained southwesterly low-level flow provided persistent elevated convective available potential energy and low-level baroclinic lifting to the event location, and thus maintained the essential requirements for convection. Despite the system remaining predominantly quasi-stationary, there were two cold-pool-driven convective surges that exited the region where heavy rainfall was produced. In each of these instances, low-level low-pressure perturbations associated with mid-tropospheric latent heating along the forward propagating convective flank generated low-level horizontal convergence over the cold pool left in the systems wake. This upstream convergence facilitated redeveloping convection away from the upstream cold pool periphery, where the vertical wind shear orientation was unfavorable for new convective initiation.

During episodes of geographically fixed upstream back-building, a low-pressure anomaly persisted at low levels within the backbuilding region. This feature sustained low-level flow convergence, facilitated continued re-development of new convection, and potentially served as a source for localized layer lifting. Back trajectory analysis from the upstream MCS region provided supporting evidence of this layer-lifting prior to the entry of parcels into upstream convective updrafts.