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

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 a downdraft and updraft because horizontal vorticity tilted by an updraft alone acquires a vertical component only as it rises away from the surface. I will review the so-called barotropic and baroclinic mechanisms for how downdrafts lead to the development of near-surface rotation, which is a prerequisite for tornadogenesis. Baroclinity and downdrafts appear to be crucial for tornadogenesis, though excessive negative buoyancy inhibits tornadogenesis. I also will discuss the role of environmental vorticity in tornado formation; its role appears to be indirect (i.e., environmental horizontal vorticity does not appear to contribute to the low-level mesocyclone's vorticity budget in a major way). 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.