Supercells: Theory Richard Rotunno National Center for Atmospheric Research, USA Photo by Morris Weisman

Ordinary Cell Review

Mesoscale Convective System Review Lower Level Wind Shear: Multi-Cells Wind Shear

Mesoscale Convective System Review Mature System

Supercells

Tuscaloosa-Birmingham Storm, 27 April 2011

Physical Processes Depend on Buoyancy and Vertical Wind Shear Ordinary Cells  Occur in weak shear & have a 30-60 min life cycle. Multicells (MCSs)  Long-lived group of ordinary cells organized by the vertical wind shear to produce mesoscale convective systems. Supercells  quasi-steady, rotating, propagate to the right or left of the vertical wind shear vector

Hodograph

Vertical Wind Shear Affects Updrafts

Splitting Supercells, 3 May 1999

Supercell Across-Shear Propagation updraft shear vector cell motion updraft

Divergence of Momentum Equation

Divergence of Momentum Equation Does not produce propagation

Divergence of Momentum Equation Updraft in Shear Splat Spin Davies-Jones (2002 J Atmos Sci)

Divergence of Momentum Equation Spin Low pressure  Rotation

Horizontal vorticity associated with ambient vertical wind shear

Vortex Lines

Rotunno (1981 Mon Wea Rev)

Breaking of Left-Right Symmetry

Breaking of Left-Right Symmetry Updraft in Shear Splat Spin

2D Updraft in Sheared Environment Rotunno and Klemp (1982 Mon Wea Rev)

Supercell Thunderstorm with Tornado Adapted from Klemp (1987, Ann. Rev. Fluid Mech.)

Summary of Supercells Buoyancy and Vertical Wind Shear Ordinary Cells  Occur in weak shear & have a 30-60 min life cycle. Multicells (MCSs)  Long-lived group of ordinary cells organized by the vertical wind shear to produce mesoscale convective systems. Supercells  quasi-steady, rotating, propagate to the right or left of the vertical wind shear vector. Tomorrow: Tornadoes  Sources of rotation in supercells, tornado dynamics based on laboratory analogues