Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 225

Figure 4.41. Illustration of how the half-circle hodograph in Figure 4.40, for which the hodo-
graph curves in a clockwise manner with height, promotes new updraft growth to the right and
suppresses new updraft growth to the left of the mean vertical shear, which is like the vertical
shear (vectors) at mid-levels, as a result of vertical perturbation pressure gradient forces
(dashed vectors) from the linear dynamic perturbation pressure gradient forces in the presence
of an updraft.
low-pressure areas produced at both low levels and upper levels; there is thus an
upward-directed perturbation pressure gradient force at low levels and a down-
ward-directed perturbation pressure gradient force aloft normal to the mid-level
shear vector. The effect of the nonlinear pressure field should therefore be rela-
tively small and the linear part of the perturbation pressure field more important.
In nature, hodographs frequently have both curved and straight sections.
Hodographs are often curved through some depth and straight elsewhere. For
example, in many tornado outbreaks there is clockwise curvature in the boundary
layer, above which the hodograph is straight (upper-left insets in Figure 4.14g ).
Enhanced curvature and lengthening of the hodograph at low levels in the Great
Plains of the U. S. is often associated with the development of the nocturnal
low-level jet. Sometimes hodographs exhibit curvature at low levels and reverse
curvature aloft ( Figure 4.43 ). The association of specific types of hodographs with