Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 229

Figure 4.44. Examples of crosswise (top) and streamwise (bottom) vorticity. The vorticity
vector is denoted by and the vertical shear vector by S (see also Figure 4.29, middle) (from !

Davies-Jones, 1984).

using an idealized, linear model (frictionless, Boussinesq, isentropic, dry, statically
unstable, basic state in which there is constant vertical shear), that vertical velocity
and vertical vorticity are positively correlated when there is streamwise vorticity.
What happens after a short period of time, however, depends on nonlinear
processes and one needs a numerical simulation to see how the storm will actually
behave.
A measure of the correlation between an updraft and vertical vorticity is given
by ''storm-relative environmental helicity (SREH)'', which is similar to (4.56),
except that v is replaced by the storm-relative value of v c, where c is the storm
motion vector. In practice, SREH is usually integrated over h, the depth of the
''inflow layer'' of the storm

ð h
0 ð v c Þ E ð JT

SREH ð c Þ¼ v Þ dz ð 4 : 57 Þ

When SREH is relatively high, then the environment is deemed to satisfy a
necessary condition for storm rotation intense enough for a mesocyclone to form.
Typical values of SREH for h ¼ 3 km (a height frequently used by forecasters)
associated with supercells are 150m 2 s 2 or higher. It is thought that the higher
the SREH, the more probable supercell tornadoes will occur.