A Simplified Model of Processes Leading to Warm-Sector Deep-Convection Initiation

Previously published work on the US tornado outbreak of 26–27 April 2011 emphasized the environmental evolution in the warm sector of the parent extratropical cyclone. Analyzed data and mesoscale-model simulations of the event suggested the movement of a cold front aloft (CFA) into the warm sector prior to the onset of the supercell/tornado outbreak and its role in the flow modifications that established a highly favorable supercell environment. Specifically, the mesoscale-model simulations suggested the CFA initiated an elevated bore that propagated into the warm sector and produced sustained mesoscale lifting, abrupt reductions in temperature and stability within the lower to middle troposphere and concurrent deepening of the low-level moist layer. Moreover, this deepened low-level moist layer exhibited the character of an unsaturated PBL topped by a moist absolutely unstable layer (MAUL); instability within this PBL+MAUL layer, which had been confined to below the remnant midlevel inversion, was later (in the day) further augmented by low-level θe advection, surface heating and mesoscale ascent ahead of the CFA, producing deep convective initiation some 100-200 km ahead of the surface front. The present study reports our attempts at using a simplified mesoscale/cloud model to examine the physical processes underlying CFA-produced mesoscale lifting, in combination with PBL evolution, that produce deep-convection initiation far ahead of the surface front.