Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 224

Figure 4.39. Idealized illustration of how a new updraft is encouraged on the downshear
(vertical shear vector indicated by dashed vectors) side of an updraft and suppressed on the
upshear side of an updraft (cloud outline shown) by linear dynamic vertical perturbation
pressure gradient forces (red vectors) in unidirectional shear (wind profile indicated by vectors
at the left) in the presence of a buoyant updraft.
Figure 4.40. Illustration of half of a circle hodograph, for which the wind vector at any height
(red vector) points in the same direction as the horizontal vorticity vector (dashed vector) and
normal to the vertical shear vector (black vector).
The effect of nonlinear terms when the hodograph turns 180 with increasing
height is as follows ( Figure 4.42 ): Counter-rotating vortices are produced through
tilting in the normal-to-shear direction. Each vortex is associated with relatively
low pressure. Since the shear vector reverses direction aloft, there is relatively low
pressure both near the surface and at the top level in the normal-to-shear direc-
tion. Since the effects of tilting generally increase with height when the strength of
the updraft increases with height, the perturbation pressure is lower aloft than at
the surface. Thus, one would expect there to be a slight upward-directed perturba-
tion pressure gradient on both sides normal to the low-level and high-level shear
vectors. At middle levels, vortices are also produced from tilting in the normal-to-
shear direction; they are located along a line normal to the line joining the