Ordinary-cell convective storms - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 144

Figure 3.40. Idealized illustration of how a buoyant updraft may be influenced by vertical wind
shear or a cold pool or both. (Top left) Positive and negative vorticity are induced by horizontal
gradients in positive buoyancy near the right and left edges of the cloud. Positive and negative
signs refer to vorticity vectors pointing into and out of the page, respectively. With no en-
vironmental vertical shear and no cold pool, the axis of the updraft forced by the thermally
created, symmetric horizontal vorticity distribution is vertically oriented. (Top right) With a
cold pool, the distribution of horizontal vorticity is biased by the negative horizontal vorticity
induced at the leading edge of the underlying cold pool and causes the updraft to lean in the
downshear (with respect to shear produced by horizontal buoyancy gradients, not the environ-
mental shear, which in this case is zero; this is indicated by the vertical distribution of winds in
the environment at the upper right) direction. (Bottom left) With vertical shear in the environ-
ment at low levels as indicated at the lower right, but no cold pool, the distribution of
horizontal vorticity is biased toward positive vorticity (into the page) and the updraft leans
in the downshear (with respect to the environmental shear) direction. (Bottom right) With both
a cold pool and vertical shear, the two effects may negate each other, allowing the formation of
an erect updraft (fromRotunno et al., 1988). (The flow fields depicted in the left-hand panels do
not evolve into those at the right after precipitation develops; the left- and right-hand panels
are independent of each other and used for illustrative purposes only.)
magnitude of the CIN is very small. However, if the vertical wind shear vector
over the depth of the cold pool is oriented so that the horizontal vorticity vector
associated it with has a substantial component in the direction opposite to that of
the baroclinically generated horizontal vorticity at its leading edge (cf. the buoy-
ancy term in (2.51)), then the lift along the leading edge of the cold pool will be
more vertically oriented ( Figure 3.40 ) so there is a greater likelihood that air will
be lifted enough to reach its LCL and trigger a new convective cell.
This behavior can be understood in terms of the horizontal vorticity equation
(2.51) expressed in flux form and for a steady state as (3.26). Remember, also, we
are neglecting, for simplicity, Earth's rotation and friction. We integrate (3.26)
over a specific portion of the domain in the vertical plane ( Figure 3.41 ) : from