LABORATORY SIMULATION OF THE DOWNBURST IN SUPERCELL THUNDERSTORMS
Westmoore High School
12613 South Western Ave.
Oklahoma City, OK 73170
Supercell Thunderstorm, Updraft, Convection Bubble (Thermal), Convection
Jet (Plume), Forward Flank Downdraft (FFD), Rear Flank Downdraft (RFD),
Mesocyclone (MC), Anticyclonic Couplet (ACC), Tornadic Vortex Signature (TVS),
Tornado, Wall Cloud, Funnel Cloud, Downburst, Microburst, Travelling Microburst
(TMB), Gust Front, Hook Echo (HE), Bow Echo, Flanking Line, Tornadogenesis,
Doppler Radar, Reflectivity, Bounded Weak Echo Region (BWER), Conservation of
Angular Momentum (CAM), Dynamic Pipe Effect (DPE), Pressure Gradient Force
(PGF), Thermal Gradient Field (TGF), Buoyancy Force, Evaporative Cooling,
Shearing Deformation (SDF), Tilting, Stretching, Shearing Instability
(SI),Convergence (CON), Divergence (DIV), Horizontal Vortex Tube (HVT), Inflow
(IN), Windshear (WS), Veering Wind, Jetstream, Fujita (F) Scale
It has long been known that
most tornadoes (and all strong ones) develop between the updraft and the
intense precipitation shaft or downdraft beneath a thunderstorm-scale,
low-pressure system called the mesocyclone (1)(2). Commonly located in the right (southern) half of the supercell
thunderstorm (3), the clockwise circulation of the mesocyclone is created by
the updraft penetrating a vertical windshear that increases in speed and
changes direction (clockwise or veer) with height (4)(5)(6). What has eluded
meteorologist so far is the connection between the mesocyclone and
tornadogenesis, and what role, if any, the updraft and downdraft play in this
process.
Many meteorologists believe the downdraft, occurring along
the right rear-flank of the supercell thunderstorm, might be the actual trigger
or at least plays an integral part in the tornadogenesis process (1)(7)(8).
This is based on field observation (9) by tornado chase teams, radar
observation (1)(10), downburst damage swaths near tornado damage tracks (see
figure 1) (11)(12)(13), computer simulations of
the supercell thunderstorm (14)(15), and laboratory simulations of the supercell
thunderstorm (8)(16).
One tornadogenesis model (17)
suggests this rear-flank downdraft (RFD) pulls the rotation occurring in the
mesocyclone downward towards the ground by a vertical, shearing deformation
field (SDF) (see figure 2a).
This pulling or stretching causes the radius of rotation to decrease,
thereby increasing the spin of the descending column of air through the conservation
of angular momentum.
Another model (Shearing Instability) (11)(18)(19) suggests
the outflow of the RFD causes the air at the surface to rotate in a cyclonic
direction (see figure 2b).
This rotation is created by horizontal, directional windshear that
forms along a gust front between the RFD outflow and the supercell thunderstorm’s
inflow. Upward stretching, by updrafts located inside cumulus towers along
this gust front or flanking line, intensify the rotation. According to the model, when this cyclonic rotation drifts into
and lines up vertically with the principle updraft and mesocyclone, tornadogenesis
can occur. By converging against the
storm’s inflow, the RFD outflow can increase the cyclonic rotation created
by the shearing instability. This
model may also explain why multiple tornadoes (not suction vortices) form
simultaneously from the same thunderstorm.
Figure 1– Path of Bossier City, Louisiana tornado. Note occurrence of downburst (microburst)
with tornadogenesis (12).
A laboratory model (Tornado Machine) (8) that simulates the
flow pattern of the supercell thunderstorm (3) has shown a high correlation
between the occurrence of the RFD and tornadogenesis (see figure
3). By simulating and altering the flow pattern
inside the simulated supercell thunderstorm (16), vortexgenesis in the Tornado Machine did not occur until a downdraft
of sufficient strength was created along the simulated rear-flank.
Even with a very intense updraft (including updraft pulse) (14) and
the presence of a simulator mesocyclone, vortexgenesis did not occur until
the simulator RFD was created in sufficient strength.
The above observation does
not undermine the importance of having a relatively strong updraft and
associated cyclonic rotation occurring within and below the mesocyclone. In fact, during the Decaying Stage
simulation of the Tornado Machine
experiment, the intense vortex of the simulated Mature Stage dissipated when
the updraft propeller was decreased to a low setting while keeping the RFD
propeller at a moderate to high setting.
During vortexgenesis in the Tornado Machine, a vertical SDF existed
between the updraft and downdraft propellers. From figure 4 (Mature Stage
simulation), low-level convergence, as evidence from the possible bow echo like
pattern in front of the vortex (11), was also present. These observations in the Tornado Machine strongly support the
theory that the vertical SDF created between the RFD and the updraft and
shearing instability, created beneath the mesocyclone between the RFD outflow
and supercell thunderstorm’s inflow (11)(18), contribute to
tornadogenesis.
Figure 4 – Observed airflow
in the Tornado Machine set to simulate the Third Stage of tornadogenesis. An intense vortex is observed.
Notice possible bow echo in front (south) of the vortex, with fog streamlines
flowing (front to back) ahead (east) of the bow echo (16).
Experiments in the Tornado Machine (16) also showed a high
correlation between the strength of the RFD and the power of the tornado,
providing the updraft remained relatively strong and the RFD outflow did not
choke off the low-level, cyclonic rotation occurring below the principle
updraft. This experimental result, along
with downburst damage swaths observed near tornado damage tracks, suggests the
downburst, imbedded in the RFD and hook echo, could also be involved in the
triggering process or at least, intensify an already existing tornado.
The downburst is defined
(11) as a very strong, penetrative downdraft, which can produce an outburst of
damaging winds up to F3 strength. The
microburst is a smaller scale version of the downburst, but can be just as
strong and violent.
The question presented in
this paper and science project is what triggers the RFD and downburst. The answer to this question may reveal the
trigger for tornadogenesis or tornado intensification. There have been several models presented in
the past (11) regarding the origination or trigger of downbursts. According to
Fujita (11), further attempts (research, observations and experiments) are
necessary to find the mechanisms by which some selected downdrafts descend to
the ground as damaging downbursts.
HYPOTHESIS
A new downburst origination
model (Decreasing Updraft) is
presented in this paper and science project.
It is based on radar observations by Lemon and Doswell (1), in which the
Bounded Weak Echo Region (BWER) around the updraft in supercell thunderstorms
disappeared just prior to tornadogenesis.
According to Lemon and Doswell, the BWER is created by the uploading of
water (raindrops) and ice (hail) by the very strong updraft, and its
disappearance is due to the release of the water and ice by the weakening
updraft. This weakening updraft has
been observed during tornadogenesis.
It is also based on
accidental observations during experiments with the Tornado Machine. When hot
water was poured over the dry ice to create a thick fog to visualize the
vortex, a thermal of carbon dioxide and water was frequently observed to rise
from the dry ice and water mixture.
Once the thermal cooled and weakened (20), a downward rush of air was
soon afterwards observed to fall back onto the surface of the Tornado Machine. It was believed that this downward rush of
air was carbon dioxide, since at the same temperature it is heavier than air.
It is proposed that the down
rush of water and ice released by the decreasing updraft could possibly create
a downburst in supercell thunderstorms.
It is also proposed that the Decreasing
Updraft downburst can trigger or strengthen the tornado by increasing the
strength of the RFD and the SDF occurring between the RFD and updraft, and its
outflow and shearing instability occurring below the mesocyclone between the
outflow and the thunderstorms inflow.
The purpose of this science project is to
test the Decreasing Updraft model as
the trigger for downbursts in supercell thunderstorms. A laboratory apparatus using a convective
jet (plume) (20) was developed and used to simulate the decreasing updraft in
this project.
A review of supercell thunderstorm
models (1)(21)(22)(23) (see figure 5) presented
over the past forty years show many similarities in their basic structure. All models contain an intense updraft penetrating
a vertical windshear that increases in speed and veers with height.
This windshear (WS), associated with the jetstream, creates a north-to-south,
horizontal vortex tube (HVT) (2)(4)(5)(6)(7).
As shown in figure 6, a mesocyclone is formed on the right (south)
flank of the thunderstorm when the intense updraft pushes upward through this
vertical windshear, tilting the axis of the WS-HVT into the vertical (2)(4)(5)(6).
An anticyclonic couplet (ACC) is formed on the left (north) flank (2)(4)(5)(23)(24).
The veering wind with height
(5), the rightward movement of the thunderstorm relative to the environmental
wind (6)(24), and the Coriolis effect (5) favor the development of the
mesocyclone over the ACC. Computer
models of the supercell thunderstorm (5)(14) also created a dominant
mesocyclone over the ACC with environmental winds that veer with height.
Figure 6 - Schematic diagram
of how a typical HVT (created by a vertical windshear (WS) associated with
the jetstream) changes its orientation by interaction with an updraft (4).
Viewing into the page (east) the mesocyclone would be on the right (south)
and the ACC would be on the left (north).
A similar configuration was
achieved by Connell (25) using an upward jet through a strong crossflow (see
figure 7). Mid-level, counter-rotating
vortices formed immediately downstream or in the wake of the penetrating
updraft or jet. The cyclonic vortex (mesocyclone) formed on the right (south)
side with the anticyclonic vortex (ACC) forming on the left (north) side. An opposing, converging flow was created
between the two vortices and a downdraft was created on the rear-flank, because
of the blocking updraft.
Most models listed above contain
two downdrafts, one downstream (east to north) of the updraft (forward-flank
downdraft or FFD) and the RFD located upwind (west to south) of the updraft
(see figure 8).
All of the models above
agree that the FFD originates with mid-level, outside air circulating around
the storm along the right-flank and south of the updraft and mesocyclone. The counter-clockwise (cyclonic) circulation
associated with the mesocyclone pulls this outside air into the
thunderstorm. With precipitation
falling downstream (26)(27) into this relatively drier outside air, the air
becomes cooler and denser (due to evaporative cooling) than the air inside the
thunderstorm and sinks toward the ground with the falling precipitation. Once the FFD reaches the ground, it spreads
outward, creating the forward gust front, associated self-cloud, and a thermal
gradient field (TGF) with the warmer air south and east of the FFD (26).
Figure 7 - Upward
Jet through a Crossflow model (25). (a)
Side view of a supercell thunderstorm updraft imbedded in a crossflow.
The plus and minus signs denote pressure excess and deficiency regions,
respectively. (b) Top view shows generated, low-pressure
vortex pairs. Bottom vortex would
be the mesocyclone and the top vortex the ACC.
(c) Top view of the vortex pair with the resulting streamline flow
occurring within the thunderstorm.
With a low-level pressure
gradient force (PGF) created below the rotating updraft and mesocyclone (6)(7),
a low-level inflow (IN) is created south and east of the updraft. As the inflow moves toward the updraft and
parallel or streamwise (2) to the TGF, the inflow (since warmer air rises and
cooler air sinks) begins rotating about a horizontal axis (right hand rule (6)
with thumb pointed toward the updraft), creating a new HVT (7)(15)(24). When the updraft moves over the IN-HVT, the
updraft tilts (right hand thumb tilted upward) and stretches the IN-HVT upward
(2) (see figure 2c). This process is
similar to the upward tilting and stretching of the WS-HVT. The added cyclonic spin of the tilted and
stretched IN-HVT causes the rotation of the mesocyclone to increase even
more. This increase spin causes the
vertical velocity of the updraft to increase due to an increase in the upward,
pressure gradient force (PGF) (14), creating the Dynamic Pipe Effect (DPE)
(28)(29) (see figure 2d), associated Tornadic Vortex Signature (TVS) (30), and
the wall cloud. If the upward vertical
velocity, cyclonic rotation or spin, and upward PGF is strong enough, the DPE
could possibly reach the surface with the required intensity for tornadogenesis
(28)(29).
One disparity in the thunderstorm
models is the origination of the RFD. In Browning (22) and Klemp (21) models (see figure
5a and 5d), the FFD is eventually wrapped around the north side of the
updraft by a strengthening mesocyclone. It
eventually reaches the rear flank of the thunderstorm creating the hook echo.
In Lemon and Doswell (1) model (see figure 5c),
the RFD originates as outside air impinges the upper, rear flank side of the
updraft. With the updraft acting as a barrier to this
flow, it is forced downward (31) until it reaches the ground along the rear
flank. In Heymsfield (23) model (see figure 5d),
the RFD originates as mid-level, outside air circulating around the storm’s
left (north) flank. It is pulled into the thunderstorm by the ACC,
then rotated counter-clockwise (cyclonic) around to the rear flank by the
mesocyclone.
Figure 8- Schematic plan
view of a typical tornadic, supercell thunderstorm (1).
The thick line encompasses radar echo or reflectivity.
An encircled “T” shows the location of the tornado.
As stated earlier in the
Introduction, the RFD can pull the rotation occurring in the mesocyclone
downward toward the ground by a vertical SDF.
This pulling or stretching increases the cyclonic rotation within the mesocyclone,
intensifying and lowering further the TVS.
The ensuing RFD helps create and intensify the shearing instability (SI)
occurring below the mesocyclone and between the RFD outflow and the
thunderstorm’s inflow.
The downburst or
microburst (smaller scale) is an intense, penetrative downdraft, which spreads
outward in all directions upon reaching the surface (see figure
9) (11)(32). The lateral scale
of the downburst is approximately 2-3 miles with the microburst one half to
one mile. The life cycle of a downburst,
as it descends within the rain shaft, can be as long as 10-15 minutes with
the maximum surface winds lasting 2-4 minutes.
The vertical speed of the downdraft can be as strong as 6000 feet per
minute (approx. 60 knots) with surface winds reaching 45 knots for stationary
storms (32) or F-3 strength for travelling microbursts (TMB) (11)(32) (see
figure 10).
Except for the Barrier Downburst model, all of the
downburst origination models (11) presented below point to the area east to
north of the updraft as the possible origination point. They also agree that this origination occurs
in the middle to upper levels of the supercell thunderstorm. Similar to the
RFD, the downburst would be circulated by the mesocyclone counter-clockwise
from the origination point to the rear flank of the thunderstorm. Here, it interacts with drier outside air
impinging the storm from the west. Due
to evaporative cooling, the downburst becomes more negatively buoyant,
increasing its downward momentum.
Figure 9 - Vertical cross-section
of the evolution of the downburst or microburst (32).
T is the time of initial divergence at the surface.
(a)
(b)
Figure 10 - Schematic diagram of the Traveling Microburst (TMB) accompanied by an extreme wind (up to F3 strength) near the ground (11). (a) Outburst Stage. (b) Foreburst Stage. Note movement of extreme wind from point A in Outburst Stage to point B in Foreburst Stage.
Four downburst origination
models (11) are presented below (see figure 11):
This is similar to the RFD
model by Lemon and Doswell (1) (see figure 5c and
11a). The thunderstorm’s principle updraft (acting
as a barrier to the flow) forces outside air (impinging the storm from the
west) downward to the surface along the rear flank (31). The downburst is created from evaporative cooling
when precipitation, falling from the rear (upwind) part of the anvil, mixes
with the downdraft, thereby increasing its negative buoyancy and downward
momentum. It should be noted that
with the formation of the DPE around the updraft, the updraft could block
any radial flow impinging on it (29). This
is due to the cyclostrophic balance between the air rushing in (pressure gradient
force) and the rotating air pushing outward (centrifugal force). The DPE could possibly be a blocking updraft.
Barnes (6) also showed how an excess of hydrostatic pressure associated
with the detrainment layer in the upper half of the thunderstorm could also
act as a blocking updraft.
Converging
Downflow Model
This is similar to the RFD
model by Heymsfield (24) (see figure 5d and 11b).
The downburst originates as mid-level, outside air circulating around the
storm’s right and left flank. Outside
air along the left (north) flank is pulled into the thunderstorm by the ACC
where it collides with outside air being pulled in from the right (south)
flank by the mesocyclone. This collision, occurring in the middle to
upper parts of the thunderstorm, forces the air downward. The primary mechanism forcing the outside air
into the thunderstorm is counter-rotating vortices created in the wake of
a rigid updraft (possibly DPE, excess hydrostatic pressure or both), which
is penetrating the strong crossflow of the jetstream (25). The mesocyclone
pulls the descending air around the north side of the updraft to the rear
flank.
Both models can be explained using Connell’s Upward Jet through a Crossflow model (25)
(see figure 7).
For the Barrier model, the excess pressure created
against the updraft by the collision with the impinging southwest airflow
would force the colliding air downward along the rear-flank (31). For the Converging
Downflow model, the rigid updraft would also force the air around it,
creating the two vortex pair as illustrated in figure
7. This intensifies the mesocyclone further, but
more importantly, the ACC, since it is initially suppressed by the veering
environmental winds, Coriolis effect, and the rightward movement of the thunderstorm
relative to the environmental winds.
Cumuli-form type clouds have been frequently observed overshooting the
anvil of severe thunderstorms (33)(34). These
overshooting towers (see figure 11c) were associated
with very intense updrafts that penetrate the very stable and dry, stratospheric
air located above the thunderstorm. Eventually
the rising air of the overshooting updraft or tower becomes cooler
(adiabatic cooling) than the surrounding air above the thunderstorm
and descends. As the tower collapses into the thunderstorm, it pulls the drier,
stratospheric air into the thunderstorm.
If the collapsing overshooting top is to reach the surface as a powerful
downburst, it must entrain and evaporate cloud droplets and hydrometeors from
inside the thunderstorm, or otherwise the descent would stop (adiabatic warming)
near the top of the thunderstorm. According
to Squires (35), the returning downdraft from the overshooting top could penetrate
the “cumulus” (i.e. several km) by mixing in the cloud droplets and hydrometeors.
Interaction with the updraft will subject both to attrition, but it
is possible the downdraft could reach the surface, especially if it remained
unsaturated. Even after saturation, the descending parcel
or downdraft would continue to subside or descend to the surface, especially
in regions of relatively drier air. Near
saturated RFD’s have been observed in supercell thunderstorms (36).
One observation
by Lemon and Doswell (1) showed that prior to tornadogenesis the updraft increases,
thereby increasing the water (raindrop) and ice (hail) content surrounding
the updraft in the upper region of the thunderstorm (see figure 11d).
This is seen on radar as a Bounded Weak Echo Region (BWER).
During tornadogenesis, the updraft weakens and the surrounding water
and ice disappear (on radar) with an increase in the RFD.
This disappearance is believed due to the rapid downward rush, from
the pull of gravity, of the rain and hail released by the weakened updraft.
Once released, the mesocyclone swings the downward rush of rain and hail to
the rear-flank of the thunderstorm.
Parts of the
released water and ice would fall to the north and east of the updraft in the area
of the FFD. This should strengthen the
FFD, but since the air in this area of the thunderstorm is already near
saturation, a downburst would not be created here. This is due to the lack of evaporative cooling, which is needed
to increase the negative buoyancy of the descending air (34)(35)(37). This is not to be confused with the initial
formation of the FFD. As discussed
earlier, drier outside air is pulled into the thunderstorm along the south
flank by the mesocyclone. This drier
outside air mixes with the more moist air inside the thunderstorm and descends
(due to evaporative cooling) creating the FFD.
In all four
models, the contribution of dry air impinging the supercell thunderstorm along
the rear-flank is vital to the downward acceleration (from evaporative cooling)
of the downburst toward the surface (34)(35)(37). This is especially true if the downburst is near saturation upon
reaching and mixing with this drier air (35).
Perhaps this would explain the frequent occurrence of tornadoes and severe
weather events in supercell thunderstorms developing along drylines and frontal
dryslots (38)(39)(40).
With the cyclonic
circulation of the mesocyclone, the downburst or microburst reaches the surface
along the right-rear flank within or near the hook echo. With the supercell
thunderstorm moving with or to the right of the environmental winds, the downburst
would be similar in structure to the Traveling Microburst model or TMB (11) (see figure
10). The resulting eastward outflow of the downburst
creates a bow echo to the right (south) of the mesocyclone and updraft (see
figure 12). This
outflow increases the shearing instability occurring below the mesocyclone,
which could possibly trigger the tornado or strengthen an already existing
one. Overtime this stronger outflow would eventually choke off the thunderstorm’s
inflow. This choking weakens the updraft
in the lower part of the supercell thunderstorm, thus shutting down the generation
of rotation (DPE and upward tilting and stretching of the IN-HVT and WS-HVT)
needed for tornadogenesis.
A Plume Apparatus has been developed to test the Decreasing Updraft model for downburst origination. Even though there is still some discussion
(41) whether the updraft is a mass of rising bubbles (thermal) or an upward jet
(plume), all of the Supercell models
(1)((21)(22)(23), the Upward Jet through
a Crossflow model. (25), and the Dynamic
Pressure Distribution model (6) in figure 13 agree the updraft in supercell
thunderstorms has a jet-like or plume
characteristic instead of thermal one.
As a result, the simulated updraft for this experiment should be a
plume.
The Plume Apparatus was designed after observing a down rush of water
from decreasing plumes created in a water organ during the Dancing Waters show. The Plume Apparatus was similarly built by
projecting a water hose upward using a wooden base. A down rush or downburst of water should be created in the
apparatus by momentarily pinching the water hose. This pinching restricts the flow of water and its upward
pressure, thereby releasing the excess water supported upward by the previously
strong water plume due to gravity.
Again, this down rush of water should simulate the downward rush of water
and ice released by the decreasing updraft (upward jet).
If downbursts can be created
in the Plume Apparatus, it is
believed this would validate the radar observation by Lemon and Doswell (1) and
support the Decreasing Updraft model
as a possible mechanism for downburst origination in supercell thunderstorms.
Figure 13 – Dynamic
Pressure Distribution model. Used
to estimate dynamic pressure distribution associated with jet-like updraft
(6). This model neglects the effects
of rotation and interactions with ambient flow.
The bottom-shaded area is the surface, while the top is the level at
which the updraft is arrested due to negative buoyancy.
METHODOLOGY
The Plume
Apparatus, using a water hose oriented vertically, was used to simulate the
Decreasing Updraft model for downburst
origination. The method or procedures
used to create the decreasing updraft and resulting downburst are given below:
1.
Conduct
experiment with no ventilation or air currents.
2.
Turn
water on to create a three-foot plume.
3.
Momentarily
(less than 0.5 seconds) pinch the water hose supplying the plume.
4.
Observe
and note results.
All of the above steps for this experiment were tested four times (trials) to support the results of this paper and science project.
RESULTS
In each of the four trials during the experiment, a plume of water
was observed to upload water at the top of the plume (see figure 14a). Once the upward flow of water (plume or jet)
was momentarily restricted and weakened by pinching the water supply hose
and cutting it from its source, a rush of water, released by the once stronger
water plume, was observed to fall to the ground (see figure 14b).
The results of this experiment validate the radar
observation by Lemon and Doswell (1) and support the Decreasing Updraft model as the trigger of the downburst in
supercell thunderstorms.
DISCUSSION
The above results suggest that an intense updraft (possibly updraft
pulse) (14), uploading water and ice in the upper regions of the supercell
thunderstorm, initially sets the stage for downburst development. This uploading is seen on radar as a
BWER. As the updraft weakens, the extra
water and ice being held aloft by the previously strong updraft is released and
the BWER disappears. The ensuing down
rush of water and ice follows the same path as the RFD and gains momentum from
evaporative cooling along the rear flank.
The mesocyclone continues to
pull the downburst around to the right-rear flank of the thunderstorm where
it reaches the surface near or within the hook echo. With the forward motion of the thunderstorm, the downburst takes
on a TMB configuration (see figure 10), creating
a bow echo along the right (south) flank of the mesocyclone.
TVS strengthening and lowering may occur within the mesocyclone as
the stronger downburst strengthens the vertical SDF occurring between the
principle updraft and RFD.
Tornadogenesis or tornado
strengthening may occur as the outflow or bow echo intensifies further the
cyclonic, directional windshear and low-level convergence (shearing
instability) occurring below the mesocyclone between the bow echo and the
thunderstorm’s inflow.
POSSIBLE TRIGGERS FOR THE DECREASING UPDRAFT
Overshooting Top
The Overshooting Top model (figure 11c) may
be the cause for the decreasing updraft in the upper levels of the supercell
thunderstorm. As the overshooting top or tower penetrates
the very stable and dry air located above the thunderstorm’s anvil, the air
inside the tower becomes cooler than the surrounding air and descends. The descent or return of the air inside the
tower now opposes the air rising upward within the updraft (35). This weakens the updraft in the upper part
of the thunderstorm. Any extra water
and ice being held upward by the previously strong updraft is now released,
and the downburst is created. This
may explain the frequent observation of severe weather events including tornadogenesis
occurring with the collapse of overshooting tops and descending cloud tops
(42)(43)(44).
Dynamic Pipe Effect
The DPE may be the cause for the decreasing updraft in the
upper level of the supercell thunderstorm.
Close examination of the DPE
model (see figure 11d) shows it resembles the
description of the converging updraft by Scorer and Ludlam (20).
As the air in the jet or updraft accelerates upward, it converges,
thereby aiding the formation of the DPE.
For the updraft above the DPE, a diverging jet (20) more closely fits
the computer results of the DPE
model (29). This would also agree with the Dynamic Pressure Distribution model (6)
given in figure 13. According to Scorer
and Ludlam (20), the vertical velocity in a diverging jet increases toward
the center of the jet or updraft, but the velocity decreases with height. As the upward velocity increases, it eventually
becomes so large that the laminar flow of the jet disintegrates into “meandering
lumps” or eddies. These “meandering
lumps” decrease the lifting capacity of the updraft above a strengthening
DPE. The now weakened updraft releases
any extra water and ice being held upward by the previously strong, laminar
flowing updraft, and the downburst is created. This model agrees with measurements of penetrating
aircraft (41), where the updraft was observed to have a smooth and uniform
appearance near the bottom or cloud base but a turbulent, bubbly structure
near the top or anvil.
Overshooting Top/DPE
It is quite possible that a combination of the Overshooting Top and DPE models could create the Decreasing Updraft downburst. With the formation of the DPE, the upward velocity
of the updraft will increase, creating the overshooting top and the uploading
of water and ice (BWER) in the top part of the thunderstorm (see figure 15a).
As the DPE intensifies and updraft increases, the critical speed is
finally reach that causes the laminar flow of the updraft to disintegrate
into eddies or “meandering lumps”. This weakens updraft in the upper part of the
thunderstorm, with the resulting collapsing top weakening the updraft further
(see figure 15b).
The speed and laminar flow of the updraft in the lower part of the
thunderstorm would not be affected (20).
This could explain observations by aircraft flights that indicated
the updraft near cloud base was smooth and uniform, but turbulent and bubbly
in the upper portions of the thunderstorm (41).
This Overshooting Top/DPE combination would
increase the amount of water and ice released by the decreasing updraft, and
add to the power and violence of the downburst and possibly the tornado.
CONCLUSION
It has long been known that
most tornadoes develop between the updraft and the intense downdraft beneath
the mesocyclone. The role of the
updraft in this process is the generation of rotation (mesocyclone and TVS)
through the upward tilting and stretching of horizontal vortex tubes (HVT),
created by the jetstream or vertical windshear (WS) and the thermal gradient
field lying streamwise with the storm’s inflow (IN). As the rotation of the mesocyclone increases, the upward PGF and
updraft velocity are increased, which increases the spin even more, creating
the DPE and TVS.
With the rotating updraft
setting the stage for tornadogenesis, field observations by tornado chase teams
and frequent observations of downburst or microburst damage patterns near
tornado damage tracks point to the RFD (including downburst) as the possible
trigger for tornadogenesis. Experiments with the Tornado Machine support these observations. In that science project, the role of the RFD
and the downburst was shown to be two-fold. First, the RFD concentrates and
intensifies rotation occurring with the mesocyclone (TVS strengthening and
lowering) through the vertical SDF.
Second (through shearing instability), it creates and intensifies
low-level cyclonic rotation through a directional windshear and convergence
occurring near and below the mesocyclone between the RFD outflow or bow echo
and the thunderstorm’s inflow. Through upward stretching, the principle updraft
increases this cyclonic rotation further.
Experimental results in the Tornado Machine also showed a direct
correlation between the strength of the RFD and the power of the tornado with
the downburst creating the most powerful tornadoes. This providing the updraft remains relatively strong and the RFD
outflow does not choke off the cyclonic rotation being created by the updraft.
The question presented in
this paper and science project was what triggers the downburst and the
RFD. By using the Plume Apparatus, a downburst was created by restricting the upward
force of the jet or plume. The results
of this experiment support the Decreasing
Updraft model as a possible trigger for downbursts in supercell
thunderstorms and possibly the trigger for powerful tornadoes.
It was proposed that a
descending Overshooting Top, formation of eddies or “meandering lumps” above a
strengthening DPE, or a combination of both could be the cause of the Decreasing Updraft in the upper part of
the supercell thunderstorm.
I extend my sincere appreciation to my father, Lloyd
Tidwell, who as my Qualified Scientist (MS in Meteorology, College of
Engineering, University of Oklahoma) and Designated Supervisor guided me
through this research project and edited this manuscript. As my dad, he helped me build the two
Laboratory Apparatus and the display board for my project. I also extend a special thanks to Mr.
Bradley Brauser for being my Science Seminar Teacher and Adult Sponsor.
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