Mesoscale Convective Systems in Low Convective-Inhibition Environments

Numerical and dynamical models have previously quantified important factors associated with propagating mesoscale systems in sheared environments. The standard mesoscale convective system model has rearward-tilted ascent, mesoscale descent, and a density current circulation. Strong ascent at the downshear edge of the density current kicks off new convection that helps maintain the system and affects its propagation. But this model does not necessarily represent mesoscale systems in near-saturated environments where evaporation, cold pools and convective inhibition are characteristically weak. We utilize cloud-system resolving simulations and dynamical models to distinguish two categories of mesoscale system that contrast with the standard model. Firstly, in direct contrast, convection the rearward-tilted upshear-propagating system is initiated at the upshear edge of the density current. The system propagates in a nonlinear wave-like manner, and linear gravity wave dynamics feature in other respects. Secondly, the forward-tilted downshear-propagating system has an elevated overturning ascending branch with negligible CAPE and no mesoscale downdraft. Moreover, the density current is of secondary importance being replaced by a hydraulic-jump-like mesoscale ascending through-flow. The sources of energy for the downshear-propagating system are kinematic: work done by the horizontal pressure gradient and kinetic energy available from propagation and environmental shear.