Impacts of Low-Level Stability on MCS Propagation

The most basic ingredients of a nocturnal mesoscale convective system (MCS) are relatively well documented. The interactions between convection, an often-present nocturnal stable layer, and the resulting mechanisms that govern propagation and maintenance are less clear. Studies have indicated that MCSs can be maintained by cold pools, gravity waves, and various types of bores depending on the stability and shear regime present in the lower levels of the environment. These same mechanisms have also been shown to support back-building convection in some environments, but it’s less clear how exactly low level stability may impact the maintanence of this back-building. Further, 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 low-level shear between observed preconvective MCS environments. Recent work by Haghi et al. (2017) sought to classify flow regimes of convective outflows observed during IHOP using relationships between the Froude number and a ratio between the depth of the density current and the surface inversion layer. The goals of this work are to document the impacts that low level environmental stability can have on the organization, propagation, and maintenance of simulated MCSs, and to gain insight into potential mechanisms for the support of back-building convection, in a variety of boundary layer environments.

A series of idealized simulations using Cloud Model 1 (CM1) will be used to explore the relationships between stability and MCS propagation/maintenance mechanisms. A control simulation will be initialized with a base state sounding that has low level wind and thermodynamic characteristics representative of MCS environments with evidence of bore/gravity wave maintainance observed during PECAN. Experimental simulations will vary the intensity and depth of the stable layer in order to gain insight into the role it may play in the mode of propagation (e.g. density current/gravity wave/bore/intrusion) and the resulting MCS organization. Particular focus will be given to how stability may impact the processes that support back-building convection.