supercells are rotating thunderstorms
that can cause tornadoes and hail the
most widely accepted theory for storm
spin up produces a pair of counter
rotating supercells but such pairs are
rarely observed in the troposphere and a
theory is needed that produces a single
supercell a new mathematical model
called vertical rotating draft has been
developed that includes the gross
features of a supercell in highly
idealized form this new model produces a
single supercell principal ingredients
of the model include rotation about a
vertical axis horizontal convergence in
the middle troposphere caused by the
compression of sinking air parcels
buoyancy
and the Coriolis force
when these ingredients are used in fluid
dynamic equations the horizontal
pressure gradient is found to be zero
and air parcels move along inertial
circles projected on a horizontal
surface this circular motion gives the
model and inertial oscillation that
appears to have been overlooked in
previous supercell models the inertial
oscillation consists of a long quiescent
phase when the draft diameter is large
and rotates anti cyclonic Lee and a
short intense phase when the draft
diameter is small and rotates cyclonic
ly one cycle of mathematically driven
animation represents 13 hours of
quiescent phase and five hours of
intense phase during the intense phase
of the inertial oscillation the rotating
draft resembles a supercell rising
parcels of air expand causing the draft
to expand and rotate anti cyclonic Li
the updraft changes to a downdraft
contracts and rotation becomes cyclonic
downdraft is driven by evaporative
cooling and updraft by condensation
aletan expanding flow interacts with the
Coriolis force to produce anti cyclonic
rotation during the quiescent phase
contracting flow interacts with the
Coriolis force to produce cyclonic
rotation during the intense phase
additional physical ingredients of the
model include translation of the
rotating draft and vertical wind shear
the rotating draft moves at a constant
height above the surface of the earth
the intense phase has a small buoyant
draft cyclonic rotation marked by a
track that turns to the right this right
turn is a salient feature of actual
supercells the quiescent phase has a
large negatively buoyant draft with
antipsychotic rotation latent heating
changes the downdraft to an updraft in
the middle of the intense phase this
flow reversal interacts with the
vertical wind shear to cause the right
turn the latent energy required to
support this inertial oscillation is
greatest during the quiescent phase when
the draft diameter is large a supercell
storm that produced one and one half
centimeter hail is documented by radar
echoes spaced 30 minutes apart these
echoes become increasingly stronger as
the supercell bills the track turns to
the right hail is produced and strong
echoes are recorded the echoes then
weaken as the supercell dissipates
application of the mathematical model
starts in the middle of the quiescent
phase the first week radar echoes are in
theory created by a growing up draft in
a predominantly downward airflow after
the turning point the strong echoes and
hail are created in theory by a
predominantly upward airflow data from
the documented supercell storm generally
support the intense phase of the model
however the quiescent phase of the model
is not supported by the storm data
possibly because the quiescent phase
requires too much latent energy the new
mathematical model called vertical
rotating draft has an inertial
oscillation with an intense phase that
resembles a single supercell
this intense phase starts with a
predominantly downward flow that causes
the draft to contract and spin up partly
by action of the Coriolis force more
research remains to be done they
predominantly downward flow that
theoretically occurs before the turning
point in a supercell track needs to be
detected and measured this down flow if
confirmed by measurements would strongly
support this new theory for the spin up
of supercell storms