How to Build a Multi-Mode Mesoscale Convective System: Requirements of an MCS with Simultaneous Regions of Forward and Backward Propagation

The basic ingredients of a nocturnal mesoscale convective system (MCS) are relatively well documented. How exactly these ingredients must come together to produce an MCS that has both forward propagating and back-building components is less clear. Keene and Schumacher (2013) explored a number of cases and suggest that enhanced convergence and horizontal wind shear may play a role in maintaining a region of back-building convection behind a bow echo. Using a composite environment, Peters and Schumacher (2015) were able to create an idealized MCS with both forward propagating and backbuilding components, and found a cold pool produced by the leading line was critical to maintain the quasi-stationary region of the MCS. Motivating this work, simulations initialized with a homogeneous environment using environmental soundings from a case that produced an MCS with both forward propagating and back-building components, fail to capture the type of back-building seen in the more complex simulations of Peters and Schumacher (2015). Give the importance of environmental heterogeneity in other modes of convection (e.g. Richardson 2007), it’s likely that environmental heterogeneity also plays an important role in maintaining the back-building convective region.  This work seeks to identify the minimum basic requirements necessary to simulate a nocturnal MCS that has simultaneous forward propagating and back-building convective modes. First, dependence on moisture gradients, temperature gradients, and low level wind shear gradients is tested independently, and then increasingly more complex background environments are used until the desired idealized MCS is successfully produced. Sensitivity to gradient orientation and intensity is also explored.