Sources of Momentum for Near-Surface Downdrafts in a Simulated Supercell Thunderstorm

Despite the importance of near-surface supercell downdrafts in modulating severe hazards like damaging straight-line winds and tornadogenesis, little has been done to quantify their sources of downward momentum. In a CM1 simulation of a supercell thunderstorm, analysis of Lagrangian trajectories revealed that reductions in the vertical pressure gradient (VPG) near cloud base height were the primary mechanism contributing to downdraft momentum. Surprisingly and unlike the VPG, the magnitude of negative buoyancy was found to be uncorrelated with near-surface downdraft intensity. A decomposition of the pressure field suggests that linear-dynamic high pressure increasing with height on the upshear side of the updraft is the primary source of the reduced VPG.