ABSTRACT
Many of the severe and tornado-producing thunderstorms in the U. S. incorporate strong rotation about a vertical axis and helicity. The formation processes are moderately well understood, as a result of numerical simulations compared with Doppler radar observations. Moderately strong vertical shear of the mean horizontal wind is required, which may be unidirectional or with strong directional changes with height, but the helicity maintenance mechanisms are apparently more efficient as hodograph curvature increases. The storms are strongly three dimensional, and show little or no tendence to be oriented downshear, as does convection between differentially moving paralleI"plates or in a sheared boundary layer. It is speculated that this difference is due to effects of water phase change, particularly conditional instability, even though these effects are not obviously required in the dynamic descriptions.
