Download THUNDERSTOM PRIMER Air Mass Thunderstorms Mesoscale

THUNDERSTOM PRIMER
Air Mass Thunderstorms
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Thunderstorms with minimal interaction with the wind environment
Primarily convective, other sources of lift can contribute
Updraft typically vertical so it works against the rainfall
Lifetime 15 – 45 minutes
Can be severe (1” hail, winds >=58 mph) but mostly < limits
Mostly individual cells but can grow together to form a MCS
Mesoscale Convective Systems (MCS)
• Lines
◦ Squall Line
◦ Line Echo Wave Pattern (LEWP)
◦ Bow Echo
• Mesoscale Convective Complex (MCC - Circular or nearly circular clusters)
Squall Lines
Thunderstorm squall line 6JUN2008 on Grand Rapids, MI NEXRAD radar.
Thunderstorm squall line Iowa to west Texas 6JUN2008 NWS mosaic.
A Line of thunderstorms sharing a linear lifting mechanism (fronts,
convergence lines, outflow boundaries, trofs, gravity waves, isentropic lifting –
usually a warm front).
The squall line can sustain itself because of lift generated by cold outflow
which helps to lift the warm moist air flowing into the line.
Bow Echoes
Bow Echo 24May2003 eastern Oklahoma, KTLX NEXRAD
Line Echo Wave Patterns (LEWP)
LWEP 10JUN2003 Omaha, NE NEXRAD
A LEWP indicates the presence of small-scale low pressure systems along the
line of thunderstorms and a small-scale cold front/warm front couplet. A LEWP
can be thought of as conjoined bow echoes. The bulges in the lines form
because of strong mid-level winds, just like BOW ECHOES.
Rotation near the small-scale surface lows can cause tornadogenesis.
Mesoscale Convective Complexes
MCC 20JUN2007 06:16Z
MCC 12JUN2010 13:40z
MCC Definition Criteria:
• Either an area of cloud top >= 100k km² with temperature <= -32 °C,
• an area of cloud top of 50k km² with temperature <= -52 °C.
• Criteria must be met for 6 hours or greater.
MCCs are not well understood but generally form in two w ays:
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Afternoon thunderstorms merge into a cooperative cluster in an area of
strong warm air and moisture advection in the Great Plains. This often
occurs north of a warm front and ahead of (east of) an upper trof. The
MCC then propagates eastward/southeastward along the warm front
overnight. As dawn approaches the complex weakens quickly and
abruptly turns right and travels southward as it dissipates.
• The origin of some MCCs can be traced back to the high plains and
eastern slopes of the Rocky Mountains. Thunderstorms that form on the
slopes of the Rockies due to the convective and orographic lift become
organized into a cooperative, self sustaining cluster and travel like the
case above.
• Flooding rains, damaging winds, intense and frequent lightning and
tornadoes can result.
• Remnants of the MCC can persist and spawn additional thunderstorms
laeter in the day.
Supercell Thunderstorms
Like air mass thunderstorms the stereotypical super cell is a discrete cell, that
travels in a straight line, a few degrees to the right of the mean tropospheric
flow. It can travel hundreds of miles and last for 4 to 10 hours. Like all
classifications of natural phenomena there are many variations of supercell
form. Supercells have a strong, tilted and rotating updraft.
On March 2,2012 super cells formed in the Ohio River Valley and travelled for
oveer 400 miles dropping numerous tornadoes and at least 2 EF4s (see
picture).
2March2012 @ 20:23:48z from the Indianapolis NEXRAD radar. Twin tornadic
supercell thunderstorms just north of the northward pointing arrows and a
developing small supercell east of the northeast pointing arrow. The northern
cell caused the Holton, IN EF3 tornado. The westward most cell caused the
Moscow, OH EF3 tornado and the eastward most cell caused the CrittendenPiner EF4 tornado later in the day, pictured below.
Hook echo of the Crittenden-Piner-Morning View tornado about the time of the
photograph above.Note: At the time this image was made the NWS rated the
tornado as an EF3. Further investigation revealed EF4 damage and winds to
175 mph.
Schematic diagram of a typical supercell thunderstorm.
How a supercell would look in the Great Plains. This storm is a
composite of real clouds and a real supercell. The tornado and
wall cloud were created in photoshop.
Top Left: Doppler on Wheels and a rainshaft. Middle Right: Rainshaft, Top Row Middle Great
Plains tornado Top Row Right: Dimmit, TX. Bottom I-71 Carroll Co. KY Nov. 22, 1992
Tornadoes
Tornadoes are described by the Enhanced Fujita Scale. A complete description is beyond
this document but when National Weather Service investigators arrive on scene they:
• Determine the type of buildings, towers or trees involved in the damage. Strong
industrial buildings can with stand tornado winds much more than typical residential
frame houses and soft pine trees can snap more easily then old growth hardwood
trees. Damage patterns will be different for each.
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Determine the degree of damage and estimate the wind speed from damage
patterns.
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Determine the EF-Scale from the table below.
EF SCALE
EF Rating
3 Second Gust (mph)
0
65-85
1
86-110
2
111-135
3
136-165
4
166-200
5
Over 200
IMPORTANT NOTE ABOUT EF SCALE WINDS: The EF scale still is a set of wind estimates (not
measurements) based on damage. Its uses three-second gusts estimated at the point of
damage based on a judgment of 8 levels of damage to the 28 indicators (structures, towers
and trees). These estimates vary with height and exposure. Important: The 3 second gust is
not the same wind as in standard surface observations. Standard measurements are taken
by weather stations in open exposures, using a directly measured, "one minute mile" speed.
Tornado Formation In Thunderstorms
The big news making tornadoes, the EF3s, EF4s and EF5s, are caused by the tilted, rotating
updraft at the rear of a supercell thunderstorm. When there is sufficient rotation to
cause a tornado this is often called a mesocyclone. Mesocyclone tornadoes can also be
weak tornadoes in the EF0 – EF2 range.
Tornadoes also form along the leading edge of thunderstorms and squall lines. “Leading
Edge Vorticies” are usually EF0s, EF1s and EF2s. Only rarely will a leading edge vortex
spin up to EF3 strength. Leading edge vorticies are officially called tornadoes and sometimes “gustnadoes” because they often form on the cool outflow of thunderstorms.
At present (April 2012) there is no distinction between the two and the weak, sometimes
barely noticed leading edge vorticies get the same treatment as the big ones. This is bound
to change in the future as warning systems and technologies evolve.
Tornado Formation Mechanisms
Rotation, Rotation Intensification and Funnel Formation
As more is learned about tornadogenesis the more it becomes apparent that:
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Most of the rotation required for a tornado is imported into the storm from the storm
environment.
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Sources of rotation include:
◦ Vertical wind speed shear creating horizontal rotating tubes of air ahead of the
storm. The tubes are drawn into the storm by air flowing towards the low
pressure, tilted by the updraft and stretched vertically by the accelerating
updraft. In this way rotation is generated over many hundreds of square miles,
concentrated in the updraft. When stretched vertically the tube narrows and
conservation of angular momentum dictates the tube spin faster. This is called
spin up through vortex stretching
◦ Interaction of the cool air pool and the inflow followed by tilting and stretching.\
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Rotation is likely pushed to the tornado threshold when the rear flank downdraft
(RFD) develops and begins to wrap around the lower portion of the rotating updraft.
This helps confine and concentrate the rotation. A local intensification of the RFD,
called the occlusion downdraft (OD) in the vicinity of the tornado may concentrate
the rotation even more, increase the vertical pressure gradient and accelerate the
upward motion further stretching the vortex.
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The Dynamic Pipe Effect: When vortex stretching takes place and the rotation rate
increases cyclostrophic balance may be attained in the lower part of the
thunderstorm and just below cloud base. This means that the outward directed
centrifugal force is balanced by the inward directed pressure gradient force and
the movement of air into the vortex is restricted. With less air flowing into the
upper part of the developing tornado aloft, mass in the vortex just below cloud base
decreases because air continues to be removed by the updraft. As a result the
pressure drops in the higher part of the vortex increasing the vertical pressure
gradient and the updraft accelerates. When the updraft accelerates spin increases
because of increased vortex stretching and the faster rotation works downwards
twards the ground.
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The Surface Cool Pool of Air can be a source of rotation
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Swirl Ratio the ration of upward velocity in the funnel to tangential flow at the edge
of the vortex is termed swirl ratio. Technically this is a simplification but the rate of
rotation as indicated by the tangential velocity is a measure of the inflow of air the
the circulation of mass.
Swirl ratio = Tangential velocity / Updraft Velocity
If the upward vertical velocity is >> than the tangential velocity it is termed a low
swirl condition and the inflowing air is easily evacuated by the updraft. Well defined
funnels form in low swirl condition as long as the low swirl is not due to near zero
tangential velocity.
A large swirl ratio ( >1 sometimes 3 to 5 means) that mass is accumulating and the
updraft cannot keep up with the inflow. The funnel may fatten or break down into
smaller funnels rotating around one another as the central downdraft pushes
closer to the ground.
Tornado Occurrence
Lightning