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Colloquium: Our Current Understanding of Tornadic Storms

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Yvette P. Richardson, Pennsylvania State University
15 January 2015 from 4:00 PM to 5:00 PM
117 Osmond Laboratory
Contact Name
Mose Chan
Contact Phone
(814) 863-2622
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Decades of observational, numerical, and theoretical studies

have revealed many of the essential processes governing tornado

formation and eventual demise.  Perhaps our clearest understanding

regards the development of the parent supercell thunderstorm,

characterized by a rotating updraft at midlevels (e.g., several km above

the ground) in the atmosphere.  This midlevel rotation (i.e., vertical

vorticity) is found to result from the reorientation of horizontal

environmental vortex lines by the storm's updraft as the storm develops

in a region where winds change direction and/or speed dramatically with

height.  However, the development of rotation very near the surface, as

required for tornado formation, relies on the reorientation of vorticity

generated within storm-induced gradients of buoyancy, and this

reorientation must be accomplished by a downdraft.  The final step in

tornado formation involves contracting this rotating air to a small

radius beneath an updraft, resulting in tornado-strength tangential

winds as angular momentum is conserved.  The climatological dependency

of tornado formation on environmental parameters such as the level of

cloud base and the change in environmental winds over the lowest

kilometer will be explained in the context of our theoretical

understanding.  Observations of tornado formation and demise from a

recent field project, the second Verification of the Origins of Rotation

in Tornadoes Experiment (VORTEX2), will be highlighted.