Principles of Convection
BACKGROUND
 When vertical shear is weak,
the main influence on
convective updrafts &
downdrafts is bouyancy. As
the vertical wind shear
increases the environmental
winds are the main influence
on convective storm
organization and duration.
 This module shows the effects
of vertical wind shear on
convective storms. Using
hodographs we can assess
vertical wind shear.
COOL POOL INTERACTIONS
 A cool pool is the thunderstorm outflow, which can
trigger new cells if upward motion is present to lift
warm air to the LFC (level of free convection). A
strong vertical wind shear must be present for this to
occur. The enhanced lifting that results is described
by horizontal vorticity, which is similar to a paddle
wheel being pushed by vertical wind. Horizontal
buoyancy gradients generate horizontal vorticity.
(Using the shear as your finger curl & the thumb as
the vorticity vector, we can use the right hand rule to
find positive vorticity into the page or negative out of
the page).
COOL POOL INTERACTS W VORTICITY
 This would result in a strong jet of vertical motion in
between the regions.
 This would drag all the air to the stronger vortex.
As shown, the right side is favored for deep lifting.
On the left, because the environmental vorticity has
the same sign as the vorticity created by the cold
pool on its upshear side dragging air over the pool w
minimal lift. However, on the downshear side, to the
right, the environmental vorticity and the cold-pool
induced vorticity have opposite signs. This produces
a more vertical jet of air, resulting in deeper lifting.
The optimal state for deep lifting is when the cool pool & wind shear are balanced,
producing the greatest vertical jet. When cool pool > wind shear, the air will be lifted
to the height of the cold pool. When wind shear > cool pool, the air ahead of the cold
pool will be dragged up and then back downshear.
UPDRAFT INTERACTIONS
 Updraft tilt can be caused by an updraft
with weak vertical momentum which is
easily tilted by vertical wind shear. When
the vertical wind shear is deep, the
horizontal vorticity associated with the
deep shear layer adds to the horizontal
vorticity associated with the buoyancy
gradient in the updraft. This causes the
storm to tilt to the side with the same sign
of horizontal vorticity as the environment
• This updraft column blocks environmental flow creating
High pressure upshear & low pressure downshear. This
gradient also makes rising parcels lean downshear
(storm tilt).
ISOLATED STORMS &
SHEAR
 In ordinary cells, the lift produced by the gust front in weak shear
conditions is insufficient to consistently generate new cells on its
own so the cells move with the environmental wind.
 In multi-cell systems, the gust front still spreads outward, but bc of
high vertical wind shear new cell growth is favored on the
downshear side of the cold pool where the lift is the greatest. Strong,
low-level, vertical wind shear produces the strongest & longest
living multi-cell systems. Optimal condition for the generation of
new convective cells is when the cool-pool vorticity is equal to the
vertical wind shear vorticity.
 When strong shear is curved, mid-level rotation occurs and the
resulting pressure forcing can create new updrafts on the lateral end
of the storm. A sustained, rotating updraft is a primary
characteristic of supercell storms, which are often associated with
severe weather. Clockwise curvature=right moving storms. Counterclockwise curvature = left moving storms.
STORM SYSTEMS & SHEAR
 Vertical wind shear also influences evolution
of large storm systems as it interacts with the
cold pool to create the most organized
mesoscale convective systems.
 A squall line oriented perpendicular to the
low-level shear will be the strongest & longest
lived.
 Severe bow echoes (a small segment of a
strong isolated storm) are most often
observed in environments with moderate-tostrong low-level shear and very high CAPE.
Bow echo and supercell environments have
much overlap, bow echos form when there is
high vertical wind shear at low levels,
supercells at high vertical wind shear at deep
levels.