Geography Reference
In-Depth Information
Fig. 9.12
Development of rotation and splitting in a supercell storm with westerly mean wind shear
(shown by storm relative wind arrows in the upper left corner of each panel). Cylindrical
arrows show the direction of cloud relative air flow. Heavy solid lines show vortex lines
with a sense of rotation shown by circular arrows. Plus and minus signs indicate cyclonic
and anticyclonic rotation caused by vortex tube tilting. Shaded arrows represent updraft and
downdraft growth. Vertical dashed lines denote regions of precipitation. (a) In the initial
stage, the environmental shear vorticity is tilted and stretched into the vertical as it is swept
into the updraft. (b) In the splitting stage, downdraft forms between the new updraft cells.
Barbed line at surface indicates downdraft outflow at surface. (After Klemp, 1987.)
approximation the azimuthal velocity v
λ
(positive for cyclonic flow) is independent
of λ. In this system the storm relative horizontal motion and vertical component
of vorticity are given approximately by
r
−
1
∂ (rv
λ
)
∂r
U
≈
·
ω
=
ζ
≈
j
λ
v
λ
,
k
where
j
λ
is the unit vector in the azimuthal direction (positive counterclockwise)
and r is the distance from the axis of the vortex. Letting
i
λ
be the unit vector in the
