Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 227

Figure 4.43. Hodograph at Norman, OK at 12:00 utc on May 23, 2011, showing clockwise
curvature from the surface to 2 km (heights shown in red) and counterclockwise curvature from
2 km to 7 km AGL (wind speed in m s 1 ). Black numbers plotted are pressure (hPa).

4.5.3 Straight vs. curved hodograph dynamics: two paradigms

In the past two decades, the dynamics of supercells have been explained using two
paradigms: (a) the ''vertical shear perspective'' and (b) the ''helicity perspective''.
In the former, the main idea is that the storm propagates because it is rotating;
in the latter, the main idea is that the storm generates rotation because it is
propagating. According to the vertical shear perspective, the basic physical pro-
cesses responsible for supercell behavior are the tilting of environmental
horizontal vorticity and the subsequent propagation owing to the rotation pro-
duced (nonlinear effect) and turning of the hodograph with height (linear effect).
This is the perspective that we have taken so far. The problem is that one needs a
theory that explains both propagation and rotation.
We will now consider the ''helicity'' perspective to see what its strengths are.
The helicity approach was originally sparked by work by Doug Lilly in the early
1980s in which it was hypothesized that rotation in a convective storm reduces
turbulent dissipation and increases its stability so that it can be longer lived; the
seed idea came from turbulence theory. The equation of motion (4.11), with the
restriction of steady state removed, may be expressed as

2 v E v Þþ½ð JT v Þ T v ¼ 0 r p 0 þ Bk

1

Dv = Dt ¼ @ v =@ t þ J ð ð 4 : 51 Þ

Now, v ! , so when ! T V ¼ 0 a part of the advective term in the equation of

JT