How tornadoes develop: Ideas emerging from decades of theory, simulation, and field observations (INVITED)

I will examine the mechanisms of tornadogenesis within atmospheric convection, particularly supercell thunderstorms, which are responsible for virtually all strong tornadoes. It is well known that both environmental and storm-generated vorticity are important in the development of mesocyclones and tornadoes within supercells. Mesocyclones that develop at midlevels in a supercell updraft arise from the tilting of environmental vorticity. On the other hand, the development of rotation next to the ground occurs near the interface of downdraft and updraft because horizontal vorticity tilted by an updraft alone acquires a vertical component only as it rises away from the surface. As an air parcel passes through the left side of a downdraft and descends, the vorticity vector is initially tipped downward along with the trajectory, but the baroclinic generation (which is horizontal and streamwise) lifts the vorticity vector off of the trajectory. In other words, the baroclinity introduces "slippage" between the vortex lines and trajectories. Ultimately this allows the vorticity vector to have a vertical component next to the surface despite the trajectory turning horizontal there. I will also discuss the sensitivity of the above process to the buoyancy of the air parcels. Despite baroclinity and downdrafts being crucial to tornadogenesis, excessive negative buoyancy inhibits tornadogenesis. Lastly, I will discuss some of the ideas emerging from the recently completely Second Verification of the Origin of Rotation in Tornadoes Experiment (VORTEX2), and directions for future research.