1 supercell thunderstorms adapted from materials by dr. frank gallagher iii and dr. kelvin...
TRANSCRIPT
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Supercell ThunderstormsSupercell Thunderstorms
Adapted from Materials by Dr. Frank Gallagher IIIAdapted from Materials by Dr. Frank Gallagher IIIand Dr. Kelvin Droegemeierand Dr. Kelvin Droegemeier
School of MeteorologySchool of MeteorologyUniversity of OklahomaUniversity of Oklahoma
Part III
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Importance of Storm-Relative Winds
Vorticity Vector
© 1990 *Aster Press -- From: Cotton, Storms
Play Movie
Storm-RelativeWinds
We obtain strong updraft rotation if the storm-relativewinds are parallel to the horizontal vorticity – or perpendicularto the environmental shear vector – this is easily determined
via a wind hodograph
Shear Vector
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Assessing S-R Winds
The degree of alignment between the storm-relative wind and the horizontal vorticity is critical for estimating the potential of an updraft to rotate
A particular tool – the wind hodograph – provides a simple graphical mechanism to assess this and other parameters
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Wind Hodograph A wind hodograph displays the change
of wind speed and direction with height (vertical wind shear) in a simple polar diagram.
Wind speed and direction are plotted as arrows (vectors) with their tails at the origin and the point in the direction toward which the wind is blowing. This is backward from our station model!!!
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Hodograph -- Example
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Hodograph
The length of the arrows is proportional to the wind speed. The larger the wind speed, the longer the arrow.
Normally only a dot is placed at the head of the arrow and the arrow itself is not drawn.
The hodograph is completed by connecting the dots!
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Hodograph -- Example
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SFC
500 m
1000 m
1500 m
2000 m
Hodograph -- Example
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Real Hodograph
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Hodograph Why Draw a Hodograph?
– Similar to a thermodynamic diagram – it makes life easier!
– We don’t have to look through a complex table of numbers to see what the wind is doing.
– By looking at the shape of the hodograph curve we can see, at a glance, what type of storms may form.
» Air Mass (garden variety) storms» Multicellular Storms» Supercell Storms» Tornadic Storms
– The shear on a hodograph is very simple to determine, as is the horizontal vorticity
– This allows us to assess helicity and streamwise vorticity
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Hodograph -- Example
Height (MSL) Direction Speed (kt)
250 m (SFC) 160 10
500 m 180 20
1000 m 200 35
1500 m 260 50
2000 m 280 75
Just by looking at this table, it is hard (without muchexperience) to see what the winds are doing and what
the wind shear is.
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Hodograph -- Example
Let us plot the winds using a station model diagram.
This is better but it is time consuming to draw and still is not that helpful. SFC
500 m
1000 m
1500 m
2000 m
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Hodograph -- Example Let us now draw the hodograph!
Let us draw thesurface
observation.160o at 10 kts
Since the windspeed is 10 kt,
the length of thearrow is only tothe 10 knot ring.
The directionpoints to 160o.
160
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Hodograph -- Example Let us now draw the 500 m observation.
Let us draw the500 m
observation:180o at 20 kts
Since the windspeed is 20 kt,
the length of thearrow is only tothe 20 knot ring.
The directionpoints to 180o.
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Hodograph -- Example We now place dots at the end of the arrows then erase the arrows.
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Hodograph -- Example We then connect the dots with a smooth curve and label the points.
SFC
500 m
1000 m
1500 m
2000 m
This is the finalhodograph!!!
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Hodograph -- Example What can we learn from this diagram?
– We see that the wind speeds increase with height.
» We know this since the plotted points get farther from the origin as we go up.
– We see that the winds change direction with height.
– In this example we see that the hodograph is curved and it is curved clockwise.
» If we start at the surface (SFC) and follow the hodograph curve, we go in a clockwise direction!
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Determining the Wind Shear– The wind shear vector at a given altitude is
tangent to the hodograph at that altitude and always points toward increasing altitudes
– The vector shear between two levels is simply the vector that connects the two levels
– Makes assessing the thermal wind vector (location of cold air) trivial!!
– The average shear throughout a layer is very useful in forecasting storm type
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SFC
1 km
2 km
3 km
Shear vectorat 2 km
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SFC
1 km
2 km
3 km
Shear vectorbetween
1 and 2 km
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SFC
1 km
2 km
3 km
Shear vectorbetween
sfc and 2 km
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Determining Horizontal Vorticity
– As shown earlier, the horizontal vorticity vector is oriented perpendicular and 90 degrees to the right of the wind shear vector
– This is very easily found on a hodograph!
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SFC
1 km
2 km
3 km
Horizontalvorticity vectors
Vertical windshear vectors
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Determining Storm-Relative Winds
– We can determine the S-R winds on a hodograph very easily given storm motion
– Storm motion is plotted as a single dot
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SFC
1 km
2 km
3 kmStorm Motion
(225 @ 30)
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SFC
1 km
2 km
3 km
Storm Motion Vector(225 @ 30)
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Determining Storm-Relative Winds
– We can determine the S-R winds on a hodograph very easily given storm motion
– Storm motion is plotted as a single dot– The S-R wind is found easily by drawing
vectors back to the hodograph from the tip of the storm motion vector
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SFC
1 km
2 km
3 km
Storm Motion Vector(225 @ 30)Storm-relative
winds can bedetermined at
any level, not just thosefor which observations
are available
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Use of Storm-Relative Winds
– Why do we care about the S-R winds?– Remember, only the S-R winds are
relevant to storm dynamics– In the case of supercell updraft rotation, we
want to see an alignment between the S-R winds and the horizontal vorticity vector
– This is easily determined on a hodograph
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SFC
1 km
2 km
3 km
HorizontalVorticity vectors
Storm-relativewinds can bedetermined at
any level, not just thosefor which observations
are available
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Importance of Storm-Relative Winds
Vorticity Vector
© 1990 *Aster Press -- From: Cotton, Storms
Play Movie
Storm-RelativeWinds
We obtain strong updraft rotation if the storm-relativewinds are parallel to the horizontal vorticity – or perpendicularto the environmental shear vector – this is easily determined
via a wind hodograph
Shear Vector
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Estimating the Potential For Updraft Rotation
– Ingredients» Strong storm-relative winds in the low-levels (at
least 10 m/s)» Strong turning of the wind shear vector with
height (90 degrees between the surface and 3 km)
» Strong alignment of the S-R winds and the horizontal vorticity – to develop rotating updrafts
» All of this can be quantified by a single quantity- the Storm-Relative Environmental Helicity
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Storm Relative Environmental Helicity
SREH -- A measure of the potential for a thunderstorm updraft to rotate.
SREH is typically measured over a depth in the atmosphere:– 1 to 3 km– 0 to 4 km
A good helicity estimate depends on accurate winds and storm motion data
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Storm Relative Environmental Helicity
SREH is the area swept out by the S-R winds between the surface and 3 km
It includes all of the key ingredients mentioned earlier
It is graphically easy to determine
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Storm Relative Helicity180
270
Storm MotionSFC
1 km
2 km
3 km4 km
5 km6 km
7 km
This area representsthe 1-3 km helicity
SREH Potential Tornado Strength
150 - 300 m2 s-2 Weak
300 - 500 m2 s-2 Strong
> 450 m2 s-2 Violent
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Typical Single-Cell Hodograph
Weak shear, weak winds
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Typical Multicell Hodograph
Somewhat stronger winds and shear, with S-R winds providing mechanism
Hodograph is essentially straight, especially at low levels
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Typical Supercell Hodograph
Strong wind, shear vector turns with height, strong S-R winds
Note curved shape of hodograph at low levels
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SFC
1 km
2 km
3 km
Storm Motion Vector(225 @ 30)
Which Storm Motion Produces a Strong, Cyclonically-Rotating Supercell?
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Estimating the Potential For Updraft Rotation
– Ingredients» Strong storm-relative winds in the low-levels (at
least 10 m/s)» Strong turning of the wind shear vector with
height (90 degrees between the surface and 3 km)
» Strong alignment of the S-R winds and the horizontal vorticity – to develop rotating updrafts
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SFC
1 km
2 km
3 km
HorizontalVorticity vectors
Storm-relativewinds
Speed of S-R WindsAlignment of S-R Winds and Vorticity
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SFC
1 km
2 km
3 km
HorizontalVorticity vectors
Storm-relativewinds
Storm-Relative Environmental Helicity
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SFC
1 km
2 km
3 km
HorizontalVorticity vectors
Storm-relativewinds
Speed of S-R WindsAlignment of S-R Winds and Vorticity
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SFC
1 km
2 km
3 km
HorizontalVorticity vectors
Storm-relativewinds
Speed of S-R WindsAlignment of S-R Winds and Vorticity
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SFC
1 km
2 km
3 km
HorizontalVorticity vectors
Storm-relativewinds
Storm-Relative Environmental Helicity
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SFC
1 km
2 km
3 km
HorizontalVorticity vectors
Storm-relativewinds
Speed of S-R WindsAlignment of S-R Winds and Vorticity
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SFC
1 km
2 km
3 km
HorizontalVorticity vectors
Storm-relativewinds
Storm-Relative Environmental Helicity
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SFC1 km
2 km3 km
Making an Optimal Hodograph
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SFC1 km
2 km3 km
Making an Optimal Hodograph
Note unidirectional shear, butGround-relative winds veer with height
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SFC1 km
2 km3 km
Making an Optimal Hodograph
HorizontalVorticity vectors
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SFC1 km
2 km3 km
Making an Optimal Hodograph
HorizontalVorticity vectors
Place the stormmotion to get a
strong cyclonic supercell
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SFC1 km
2 km3 km
Making an Optimal Hodograph
HorizontalVorticity vectors
Change the hodograph toobtain a strong supercellgiven this storm motion
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SFC1 km
2 km3 km
Making an Optimal Hodograph
HorizontalVorticity vectors
Change the hodograph toobtain a strong supercellgiven this storm motion
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SFC
1 km
2 km3 km
Making an Optimal Hodograph
HorizontalVorticity vectors
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SFC
1 km
2 km3 km
Making an Optimal Hodograph
HorizontalVorticity vectors
Storm-relativewinds
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Predicting Thunderstorm Type: The Bulk Richardson Number
Need sufficiently large CAPE (2000 J/kg) Denominator is really the storm-relative inflow kinetic energy (sometimes called the BRN Shear) BRN is thus a measure of the updraft potential versus the inflow potential
2S
CAPEBRN
25006000
2 )(2
1uuSwhere
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Results from Observations and Models
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General Guidelines for Use
40035
505
BRNforMulticells
BRNforSupercells
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