Analysis of Backbuilding of a Simulated MCS in an Environment with a Low-Level Stable Layer

The ‘arrow’ region of a ‘bow and arrow’ MCS (sometimes called rearward off-boundary development) can contribute to extreme rainfall, when heavy precipitation trains or redevelops over the same location for an extended period of time. Past studies have indicated the importance of gradual lifting of an inflow layer over a cold pool produced on the leading edge of convection in the maintenance of convection in this region. However, analysis of soundings collected during the Plains Elevated Convection at Night (PECAN) field campaign indicates substantial variability in the stability of the nocturnal boundary layer and in some cases, observed off-boundary back-building had little indication of a surface cold pool. Further, a simulated MCS with a strong low-level stable layer that produces this off-boundary back-building suggest that in some cases, it may be supported by mechanisms other than a density current. In the maintenance of back-building convection in environments with a low level stable layer, the specific roles of processes generated by the MCS itself, and those external to the MCS, have not been fully explained or quantified.

Idealized simulations were run with a homogeneous base state determined by an environmental sounding from a Plains Elevated Convection at Night (PECAN) field campaign case with a strong low-level stable layer and both progressive convection and offset back-building convection. Surprisingly, when initialized with a single warm bubble, the simulation produced an MCS that had regions of both progressive convection and offset-back-building convection. Additionally, the back-building in this simulation appears to be supported initially by an intrusion, and later by processes primarily within back-building convection itself, rather than by a cold pool produced by the leading line. The results of these simulations suggest that external forcing is not a requirement of back-building convection, and that it is possible to support offset back-building convection by mechanisms other than a cold pool.