Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 233

right and left mover, respectively. It is important to note that SREH is not
Galilean invariant, since it depends on storm motion. If the vertical shear vector
at all altitudes is normal to the storm-relative wind as it is when there is a
perfectly circular hodograph, then SREH is the highest it can be for a given
shear.
As noted earlier, the problem with the helicity approach to understanding
supercell dynamics is that storm motion must be known, but it is not yet predict-
able from theory. Storm motion is often influenced by the movement of a storm's
own gust front, which depends to some extent on cloud microphysics parameters
(through evaporative and sublimation cooling) and by factors external to the
storm such as the motion of outflow boundaries, fronts, the dryline, and
orography. An empirical technique for predicting storm motion which blends
theory with observations was presented by Matthew Bunkers and colleagues in an
oft-referenced paper published in 2000. This method makes use of motion with the
mean wind (calculated with respect to height—not to mass—for simplicity, and
without an apparent significant loss of accuracy) and normal-to-shear propaga-
tion. The contribution from the former is given by the 0-6 km height-weighted
mean wind; the contribution from the latter is given by a vector 7.5m s 1 in
magnitude, normal and to the right of the 0-0.5 to 5.5-6 km vertical shear vector.
It is useful for forecasting, in the absence of other factors such as gust front

propagation, etc.
The reader is cautioned that sometimes a supercell may become anchored with
respect to a location (e.g., when the motion of an outflow boundary is in the
direction opposite to that of propagation due to a storm's internal dynamics). In
this case, SREH may change when a storm interacts with the outflow boundary,
etc.

We gain some insight into the role of curved hodographs vs. straight
hodographs by considering the vertical component of (4.51), the pressure having
been separated into the dynamic part ( p d ) and the part due to buoyancy ( p b )

z ½ð u 2

2

þ w 2 p 0 d =@ p 0 b =@ @ w =@ t þ@=@ þ v Þ= 2 ð u vÞ¼ 0 @ z þ½ B 0 @ z

ð 4 : 64 Þ

where the components of horizontal vorticity are given by (4.37) and

(4.38). The second term on the LHS of (4.64) is the Bernoulli term and the third
term on the LHS is the Lamb term. Consider forcing in the vertical due to vertical
perturbation pressure gradients associated with the Bernoulli and Lamb terms. In
2000, Morris Weisman and Rich Rotunno demonstrated, using idealized numerical
simulations, that for supercells grown in environments of both straight and highly
curved hodographs, the upward-directed perturbation pressure gradient on the
right side of the shear vector, which is responsible for propagation of the updraft
in the normal-to-shear direction, is due mainly to the Lamb term. They therefore
concluded that the nonlinear pressure term, which does not depend upon hodograph
curvature, is of fundamental importance.

and