Analysis of Back-Building Convection in Simulations with a Strong Low-Level Stable Layer

In a mesoscale convective system (MCS), convection that redevelops over (i.e. back-builds), and/or repeatedly passes over (i.e. trains) a region for an extended period of time can contribute to extreme rainfall and flash flooding. Past studies have indicated that both mesoscale ascent and lifting of the inflow layer by a cold pool or bore are important when this back-building/training convection is displaced from the leading line (sometimes called rearward off-boundary development or ROD). However, Plains Elevated Convection at Night (PECAN) field campaign observations suggest that stability of the nocturnal boundary layer is highly variable and some MCSs with ROD have little indication of a surface cold pool. The simulations presented in this work suggest that in an environment with strong boundary layer stability, ROD can be supported by mechanisms other than those mentioned above.
Simulations were initialized using an environmental sounding from a PECAN MCS with a strong stable layer and ROD. When a warm bubble was used to initiate convection, the simulation produced an MCS remarkably like that observed despite the homogeneous initial conditions. Some of the findings presented herein challenge existing understanding of nocturnal MCSs, and especially how downdrafts interact with a stable boundary layer. Notably, downdrafts can reach the surface, and different mechanisms support different regions of the MCS (even along the leading line), which influences the relevant inflow layer. Unlike previous studies of ROD, parcel lifting may be due to the intrusion of an elevated layer of downdraft air (or essentially, an elevated cold pool), favorably amplified gravity waves, or some combination of the two.