Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 240

Figure 4.52. Idealized illustration showing how streamwise vorticity associated with low-level
vertical shear could be advected toward an updraft and tilted to produce a mesocyclone (C) just
above the ground.

e ) zero, then vortex lines must always lie on

horizontal planes of constant

surfaces of constant e , as was shown earlier in this chapter. It follows that on

surfaces of constant e there can be no circulation, because there can be no com-

ponent to the vorticity vector normal to the surface of constant e . Therefore,

there can be no circulation around any curve lying entirely on a surface of con-
stant

e . Circulation analyses from numerically simulated data show how the area
of the material surface had been larger and the portion on the ''cold'' side had
been vertically oriented, while the portion on the ''warm'' side had been hori-
zontally oriented ( Figure 4.53 ). Based on (2.53), at the earlier time applied to the
material curve in Figure 4.53, the circulation tendency has zero contributions on
the horizontal side because B ¼ 0, but on the vertical side B k 0 (remember

that the convention for line integration is in the counterclockwise direction) to the
east, but only for a short distance, while B k

dl <

E

dl > 0 to the west, so that

E

DC 0. One concludes, then, that sinking motion on the cold side resulted in

the tilting of the surface downward onto the horizontal and that convergence
underneath the updraft shrank the area of the curve. This view is consistent with
the baroclinic generation of vorticity being tilted onto the vertical by a gradient in
sinking motion across the baroclinic zone and then stretched underneath the
updraft.
Low-precipitation supercells do not have strong surface cold pools, owing to
the lack of or relatively slow rate of evaporation of raindrops, or to only a slow
rate of melting of falling hailstones. It would not be expected, then, that they have
strong low-level mesocyclones unless there is strong, pre-existing horizontal vorti-
city in the boundary layer that is advected into the convergent area underneath
the updraft and then tilted. Alas, without a downdraft the boundary-layer
vorticity vector would become vertically oriented just above the ground—not
directly at the ground ( Figure 4.29, bottom panel; Figure 4.52 ). A downdraft is

= Dt >