CUMULONIMBUS COLLAPSE WITH
GRAVITY WAVE Text written by Mark Seltzer, all rights reserved.
Dartmoor/Exeter UK 17th June
This timelapse video demonstrates a collapsing cumulonimbus cloud, creating
a pool of cold air which radiated outwards along the surface, causing a small
propagating arcus "roll cloud" and a gravity wave which pushed the middle
atmosphere up. This push caused stratiform clouds to form. In visible satellite imagery, it was
evidenced that a radial gust front was generated from this event, and went backwards against
the prevailing wind. Sea breeze convergence and slight impedance of free
convection in the atmosphere were likely playing a key role here.
Arcus/roll clouds form as a
result of cold wells of air from downdraughts hitting the surface and
spreading out into the warm inflow. This usually means the storm cell is
either dying or has stable linear structure. Not to be confused with a
"Beaver Tail" in a Supercell.
Impedance of the updraught
usually means the updraught is forced into conditionally unstable air by
surface convergence, and is more likely to result in a sudden collapse
should the inflow be cut off or cease.
Stratiform clouds (flat stable
atmospheric clouds) can cohabit
with cumulonimbus clouds (unstable atmospheric clouds) as a result of gravity waves caused by collapsing
Environment: Impedance of updraught
Usually, when the inflow into a cumulonimbus cuts out, the storm stops
developing and the updraught column usually rises into the anvil, taking the
base with it. However when there is low vertical wind shear coupled with impedance (in
this case, updraught forced into
slightly stable or descending air), the updraught column is more likely to
collapse on itself. Surface forcing of the updraught is required to keep the
tower standing, usually in the form of sea breeze
convergence. Without this, the atmosphere will do
it's job of suppressing free-cell convection and no cumulonimbus towers can
Sea breeze surface convergence - air forced upwards
A quasi-stationary (apparently not moving) back-building cell grew over northeast
Dartmoor (Devon, UK), with the back-building side on show and the anvil
spreading away from the camera, thus giving a clear view of the updraught.
Surface wind convergence, caused
by a sea breeze front, forced air upwards (where wind currents meet, they
can't go down into the ground, so the have to go up) and hence in the
conditionally unstable airmass on this day, a cumulonimbus tower piled up.
CAPE was high on this day (4 figures) but where this cell developed,
vertical impedance was present in the upper atmosphere to counteract it.
There was enough
wind shear at tropopause height (~35,000ft) to take the very top of the tower away,
but not the bulk of it, meaning most of the tower was perpetually collapsing
on itself already due to the impedance. The sea breeze inflow was strong enough
keep it standing.
Collapse of the towers as inflow cuts off
The inflow from the sea breeze
convergence at the surface finally ceased, for whatever reason. With no updraught to
maintain the stance of the already-struggling cumulonimbus tower, the tower
completely collapsed on itself
to the ground. If there was no impedance in the atmosphere, the
tower would have likely continued upwards into anvil mode and the base would
have lifted. DCAPE (Downward Convective Available Potential Energy) was
evident here and used well.
Cold pool exhaust causes an Arcus "skirt" cloud
During the collapse, the invisible
cold plunge of air is revealed by an advancing (against the wind) Arcus/Roll cloud under the
parent base, sometimes mistakenly referred to as a wall cloud. This cloud
formation, which looks like a skirt, is caused by entrainment of the cooler,
damper air into the warmer boundary
layer it is pushing against, resulting in a lower cloud-base with rising or
These are often seen in linear
or dissipating severe thunderstorms, usually because they exhibit big DCAPE
values (strong heavy downdraughts). It's the same mechanism which creates a
"whales mouth" formation. So it is important to note that these cloud
formations are usually associated with an exhaust from, or will soon result
in, a dissipating part of a thunderstorm. Chasers of supercells know when
they see this, the (tornadic) show is over.
Similar but, subtly different,
is the "Beaver
Tail" seen in severe supercells, a sign of extreme development
(recycled outflow being sucked back up into the storm).
Gravity Wave pushes local atmosphere upwards
Back to this storm... the cold pool
also pushed the surrounding
environmental air upwards, rather like a tsunami, causing water vapour in
the middle atmosphere to condensate and clouds to
form again in stratiform layers.
Note that very little of this forced cloud resulted in towering cumuloform cloud,
showing the near-stable environmental conditions within the prevailing airmass itself.
This whole sequence shows how
the atmosphere behaves as a fluid, and how cumulonimbus
clouds can alter the surrounding environment in an unpredictable way. I
personally always wondered why you sometimes see Alto or Stratocumulus sheets around
cumulonimbus fields, and from observing this collapse I not better understand
how stratiform clouds can form in a seemingly unstable atmosphere.