Download Write Up on the Double Vortex Model

The Double Vortex Model
The double vortex model was developed by Joe Eagleman at the University of Kansas to explain
the internal structure and dynamics of tornado producing thunderstorms.
The basic idea is that supercell thunderstorms consist of two large rotating cylinders. Perhaps
only 2% of thunderstorms are large enough and strong enough to be called supercells. They
must have a highly organized internal structure to spawn tornados. Remember too that a
thunderstorm is just a cloud – air (well, moving air) and water vapor. The question Eagleman
asked was, how can these large thunderstorms maintain their structural integrity when the wind
sheer forces should tear them apart?
Air rushes into a severe thunderstorm from many different directions and different speeds. For
example the air at the base may come in from the east at 30 knots and at the top from the west at
150 knots. In the middle the wind might come in from the south or southwest at 60 knots. All
these winds should tear the cloud apart. However, Eagleman noticed that the strongest tornadic
thunderstorms moved most slowly; the winds were very fast, but the tornado funnel and parent
thunderstorm moved slowly along its track relative to the ground. He also noted that a fast track
tornado makes a straight line, but a slow track might leave a loop-de-loop damage pattern. He
concluded that the parent thunderstorm must have some kind of internal structure which would
translate the winds at various levels and their sheer forces in to rotary motion and perhaps even
provide a kind of gyroscopic stability according to the laws of conservation of angular
momentum.
These super cell thunderstorms contain two large, rotating cylinders. The one on the northeast is
called the meso-anticyclone and is larger, and slower rotating. This is where the rain falls. The
more interesting cylinder is located to the southwest, and is called the mesocyclone. It rotates
faster and is the connection between the whole thunderstorm and the tornado funnel. Both of
these large rotating cylinders not only resist the wind sheet forces acting on them but can
translate these to a series of rotary motions within the storm. For instance, if the jet stream aloft
splits and goes around the sides of the storm, then the jet pulls on each side of the storm causing
one cylinder to spin clockwise (meso-anticyclone) and one to spin counter clockwise (meso
cyclone). Note too that if the jet is faster, then the cylinder rotates faster, and if the cylinder is
smaller, it also rotates faster. Eagleman went so far as to argue that there is a progressive
transfer of angular momentum form the (grand parent) cyclonic storm system to the (parent)
double vortex thunderstorm to the (child) tornado.
There are several updrafts within the double vortex system. The thermal updraft is the flow of
maritime tropical air into the thunderstorm. However their occurs at fairly low levels in the
atmosphere because that is where the heat and humidity are. Both represent energy sources
(latent heat) to help drive the storm. Remember too that even ordinary thunderstorms liberate
energy equivalent to 3 atomic bombs. T-storms are literally conduits through which energy
flows from the earth’s surface to high levels of the atmosphere.
The dynamic updraft is the flow of air (suction) into the tornado funnel. There is also a rear
flank downdraft (RFD) in this part of the storm. Some kind of interaction occurs between the
dynamic updraft and the RFD that helps tornadoes to form.
In Texas, most tornadic thunderstorms occur along the dry line. This is the boundary between
maritime tropical air to the east and continental tropical air to the west. The dry line can provide
extra lifting because it acts like a front. The dry desert air is more dense than the maritime
tropical air. So there is a difference in density across a dry line just like a front. Many times too
the continental tropical air higher up in the atmosphere forms an inversion layer called a cap.
Usually inversions dampen out convective currents. However, a thin inversion layer will dampen
out convective currents but allow a more vigorous current to break through. When this happens,
this particular cell can suck in lower level maritime tropical air for many miles around and
develop into a supercell.
This model also explains the three major zones affected by the storm. Zone 1 to the northeast
has rain. Zone 2 in the center of the storm has hail. Zone 3 in the southwest has the tornado.
In detailed surveys of tornado damage, Eagleman found that the south and southwest corner of a
house is the worst place to be. The best place is underground in a basement or storm shelter. On
the first floor, an interior closet or bathroom offers the best protection. Opening windows will
not help and is a dangerous waste of time when a tornado is approaching. He found that many
roofs are not attached to the walls very well and especially that many houses are not bolted to the
foundation. Finally, a modern ranch type house is really an airfoil in disguise. A house with a
steeper pitched roof disrupts the air flow. A shallow pitch roof can be lifted off the house by the
flow of air over the top.