Supercell Thunderstorms

Supercell Thunderstorms
METR 1004: Introduction to Meteorology Adapted from Materials by Dr. Frank Gallagher III and Dr. Kelvin Droegemeier School of Meteorology University of Oklahoma

A very large storm with one principal updraft Quasi-steady in physical structure Continuous updraft Continuous downdraft Persistent updraft/downdraft couplet Rotating Updraft --- Mesocyclone Lifetime of several hours Highly three-dimensional in structure

Potentially the most dangerous of all the convective types of storms Potpourri of severe and dangerous weather High winds Large and damaging hail Frequent lightning Large and long-lived tornadoes

Form in an environment of strong winds and high shear Provides a mechanism for separating the updraft and downdraft

Structure of a Supercell Storm
Updraft Downdraft

Initial storm development is essentially identical to the single cell thunderstorm Conditional instability Source of lift and vertical motion Warm, moist air

Schematic Diagram of a Supercell Storm (C. Doswell)

Mesocyclone

Supercell Structure Inflow
Forward Flank Downdraft Tornado Mesocyclone Rear Flank Downdraft Flanking Line/ Gust Front Inflow Gustnado © 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology -- Volume II: Observations and Theory of Weather Systems

Storm-Relative Winds Absolute velocity = Relative Velocity + Velocity of Coordinate System 40 mph

Storm-Relative Winds Absolute velocity = Relative Velocity + Velocity of Coordinate System 90 mph 40 mph

Storm-Relative Winds Absolute velocity = Relative Velocity + Velocity of Coordinate System 90 mph 130 mph 40 mph

Storm-Relative Winds Absolute velocity = Relative Velocity + Velocity of Coordinate System Relative Velocity = 90 mph Absolute Velocity = 130 mph Velocity of Coordinate System= 40 mph

Storm-Relative Winds Absolute velocity = Relative Velocity + Velocity of Coordinate System Environmental Wind = Storm-Relative Winds + Storm Motion Storm-Relative Winds = Environmental Wind – Storm Motion Storm Motion = 30 mph Environ = 20 mph Storm-Relative = -10 mph

Storm-Relative Winds Storm-Relative Winds = Environmental Wind – Storm Motion Storm Motion = 20 mph Environ = 40 mph Storm-Relative = 20 mph

Storm-Relative Winds Storm-Relative Winds = Environmental Wind – Storm Motion Storm Motion = 20 mph Environ = 40 mph Storm-Relative = -60 mph

The Only Thing that EVER Matters is the Storm-Relative Wind

A Supercell on NEXRAD Doppler Radar
Hook Echo

Where is the Supercell?

Supercell Types Classic Low-precipitation High-precipitation

Low Precipitation (LP) Supercells
Little or no visible precipitation Clearly show rotation Cloud base is easily seen and is often small in diameter Radar may not indicate rotation in the storm although they may have a persistent rotation LP storms are frequently non-tornadic LP storms are frequently non-severe

A Tornadic LP Supercell
26 May Texas Panhandle © 1998 Prentice-Hall, Inc. -- From: Lutgens and Tarbuck, The Atmosphere, 7th Ed.

High Precipitation (HP) Supercells
Substantial precipitation in mesocyclone May have a recognizable hook echo on radar (many do not, however) Reflectivities in the hook are comperable to those in the core Most common form of supercell May produce torrential, flood-producing rain Visible sign of rotation may be difficult to detect -- Easily detected by radar

HP Supercell Heaviest Precipitation (core) 4 OCT 1998 2120 UTC KTLX
Kansas Woods County, Oklahoma Oklahoma 4 OCT 1998 2120 UTC KTLX

Twenty minutes later ….. HP Supercell Heaviest Precipitation (core)
Kansas Oklahoma HP Supercell 4 OCT 1998 2150 UTC KTLX Developing Cells

Classic Supercells Traditional conceptual model of supercells
Usually some precipitation but not usually torrential Reflectivities in the hook are usually less than those in the core Rotation is usually seen both visually and on radar

Classic Supercell Heaviest Precipitation (core) Hook

Hybrids Class distinctions are much less obvious in the real world!
Visibly a storm may look different on radar than it does in person -- makes storms difficult to classify Supercells often evolve from LP  Classic  HP. There is a continuous spectrum of storm types.

Supercell Evolution Early Phase
Initial cell development is essentially identical to that of a short-lived single cell storm. Radar reflectivity is vertically stacked Motion of the storm is generally in the direction of the mean wind Storm shape is circular (from above) and symmetrical

Supercell Evolution -- Early Phase
Side View Top View Heaviest Precipitation © 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology -- Volume II: Observations and Theory of Weather Systems

Supercell Evolution Middle Phase
As the storm develops, the strong wind shear alters the storm characteristics from that of a single cell The reflectivity pattern is elongated down wind -- the stronger winds aloft blow the precipitation The strongest reflectivity gradient is usually along the SW corner of the storm Instead of being vertical, the updraft and downdraft become separated

Supercell Evolution Middle Phase
After about an hour, the radar pattern indicates a “weak echo region” (WER) This tells us that the updraft is strong and scours out precipitation from the updraft Precipitation aloft “overhangs” a rain free region at the bottom of the storm. The storm starts to turn to the right of the mean wind into the supply of warm, moist air

Supercell Evolution -- Middle Phase
Side View Top View Heaviest Precipitation © 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology -- Volume II: Observations and Theory of Weather Systems

Supercell Evolution Mature Phase
After about 90 minutes, the storm has reached a quasi-steady mature phase Rotation is now evident and a mesocyclone (the rotating updraft) has started This rotation (usually CCW) creates a hook-like appendage on the southwest flank of the storm

Supercell Evolution -- Mature Phase
Side View Top View Hook Heaviest Precipitation © 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology -- Volume II: Observations and Theory of Weather Systems

Hook Echo

Supercell Evolution Mature Phase
The updraft increases in strength and more precipitation, including hail, is held aloft and scoured out of the updraft As the storm produces more precipitation, the weak echo region, at some midlevels, becomes “bounded” This bounded weak echo region (BWER), or “vault,” resembles (on radar) a hole of no precipitation surrounded by a ring of precipitation

Splitting Storms If the shear is favorable (often a straight line hodograph), both circulations may continue to exist. In this case the storm will split into two new storms. If the hodograph is curved CW, the southern storm is favored. If the hodograph is curved CCW, the northern storm is favored.

Splitting Storms Split Left Mover Right Mover
© 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology -- Volume II: Observations and Theory of Weather Systems

Updraft The updraft is the rising column of air in the supercell
They are generally located on the front or right side of the storm Entrainment is small in the core of the updraft Updraft speeds may reach 50 m s-1!!! Radar indicates that the strongest updrafts occur in the middle and upper parts of the storm

Updraft Factors affecting the updraft speed
Vertical pressure gradients Small effect but locally important Regions of local convergence can result in local areas of increased pressure gradients Turbulence Buoyancy The more unstable the air, the larger the buoyancy of the parcel as they rise in the atmosphere The larger the temperature difference between the parcel and the environment, the greater the buoyancy and the faster the updraft

Meso- Cyclone

The Wall Cloud Meso- Cyclone

The Wall Cloud

Supercell Downdrafts The same forces that affect updrafts also help to initiate, maintain, or dissipate downdrafts: Vertical PGF Buoyancy (including precipitation loading) Turbulence Downdraft wind speeds may exceed 40 m s-1

Supercell Downdrafts We shall examine two distinct downdrafts associated with supercell thunderstorms: Forward Flank Downdraft (FFD) Rear Flank Downdraft (RFD)

Forward Flank Downdraft
Associated with the heavy precipitation core of supercells. Air in the downdraft originates within the column of precipitation as well as below the cloud base where evaporational cooling is important. Forms in the forward flank (with respect to storm motion) of the storm. FFD air spreads out when it hits the ground and forms a gust front.

Rear Flank Downdraft Forms at the rear, or upshear, side of the storm.
Result of the storm “blocking” the flow of ambient air. Maintained and enhanced by the evaporation of anvil precipitation. Enhanced by mid-level dry air entrainment and associated evaporational cooling. Located adjacent to the updraft.

Supercell Downdrafts Forward Flank Downdraft Rear Flank Downdraft
Inflow © 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology -- Volume II: Observations and Theory of Weather Systems

Rear Flank Downdraft Forms at the rear, or upshear, side of the storm.
Result of the storm “blocking” the flow of ambient air. Maintained and enhanced by the evaporation of anvil precipitation. Enhanced by mid-level dry air entrainment and associated evaporational cooling. Located adjacent to the updraft.

Supercell Downdrafts Forward Flank Downdraft Rear Flank Downdraft
Inflow © 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology -- Volume II: Observations and Theory of Weather Systems

Formation of the RFD Imagine a river flowing straight in a smooth channel. The water down the center flows smoothly at essentially a constant speed. The pressure down the center of the channel is constant along the channel.

Formation of the RFD Let us now place a large rock in the center of the channel. The water must flow around the rock. A region of high pressure forms at the front edge of the rock -- Here the water moves slowly -- Stagnation Point

Formation of the RFD This happens in the atmosphere also!
The updraft acts a an obstruction to the upper level flow.

Formation of the RFD The RFD descends, with the help of evaporatively cooled air, to the ground. When it hits the ground, it forms a gust front. Upper-level Flow Updraft FFD RFD Mid-level Flow Gust Front Inflow

Supercell Updraft Rotation
In order for supercells to rotate, there must be some type of rotation already available in the environment. We shall consider several different ways of creating vertical vorticity or rotation about a vertical axis:

Convergence Consider your sink. Initially there is some weak rotation, but as the water converges toward the drain, the speed of the rotation increases. This is similar to the way an ice skater speeds up when their arms are pulled in.

Horizontal Temperature Differences
Air travelling along a frontal zone will develop a horizontal rotation.

Recall the Cold-Air Outflow

Vertical Wind Shear Another method of creating horizontal rotation is by vertical wind shear. Fast Wind Slower Wind

Vertical Wind Shear Up North East Westerly Winds Increase in Speed
with height North East

Development of Rotation
Up North East

Tilting In order to create vertical rotation from horizontal rotation, we must tilt the horizontal rotation into the vertical.

Up Thunderstorm North East

Up Updraft - Stretch North East

Tilting In thunderstorms, this tilting is achieved by the updraft.

Tilting Viewed from above, we see a pair of counter-rotating vortices:
“Positive Rotation” “Negative Rotation”

Supercell Rotation In supercells, the updraft usually spins only in one direction (usually CCW). Because of the environmental shear, the updraft is enhanced on the southern flank of the storm. The CCW rotation is typically found on the southern flank and is favored if the storm moves toward the south (inflow along vortex lines) The northern flank rotation is not favored and usually is weak.

Importance of Storm-Relative Winds
Want to intensify the cyclonic vortex on the south side Vortex Tube Updraft Play Movie © 1990 *Aster Press -- From: Cotton, Storms

We obtain strong updraft rotation if the storm-relative winds are parallel to the horizontal vorticity – or perpendicular to the environmental shear vector – this is easily determined via a wind hodograph Vortex Tube Updraft Storm-Relative Winds Play Movie © 1990 *Aster Press -- From: Cotton, Storms

Vertical Wind Shear Up North East Westerly Winds Increase in Speed
with height North East

Shear = V(upper) – V(lower)
Vertical Wind Shear Up Shear = V(upper) – V(lower) North East

Shear = V(upper) – V(lower)
Vertical Wind Shear Up Shear = V(upper) – V(lower) Shear Vector East

Up Note that the vorticity vector points 90 deg to the left of the shear vector North Shear Vector East

We obtain strong updraft rotation if the storm-relative winds are parallel to the horizontal vorticity – or perpendicular to the environmental shear vector – this is easily determined via a wind hodograph Shear Vector Vorticity Vector Storm-Relative Winds Play Movie © 1990 *Aster Press -- From: Cotton, Storms

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

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!!!

Hodograph -- Example

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!

Hodograph -- Example

Hodograph -- Example 1000 m 500 m 1500 m SFC 2000 m

Real Hodograph

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

Hodograph -- Example Just by looking at this table, it is hard (without much experience) to see what the winds are doing and what the wind shear is.

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. 2000 m 1500 m 1000 m 500 m SFC

Hodograph -- Example Let us now draw the hodograph! 160
Let us draw the surface observation. 160o at 10 kts Since the wind speed is 10 kt, the length of the arrow is only to the 10 knot ring. The direction points to 160o.

Hodograph -- Example Let us now draw the 500 m observation.
Let us draw the 500 m observation: 180o at 20 kts Since the wind speed is 20 kt, the length of the arrow is only to the 20 knot ring. The direction points to 180o.

Hodograph -- Example We now place dots at the end of the arrows then erase the arrows.

Hodograph -- Example We then connect the dots with a smooth curve and label the points. This is the final hodograph!!! 1000 m 500 m 1500 m SFC 2000 m

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!

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

Shear vector at 2 km 2 km 1 km 3 km SFC

Shear vector between 1 and 2 km
SFC

Shear vector between sfc and 2 km

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!

Horizontal vorticity vectors Vertical wind shear vectors
2 km 1 km 3 km SFC Vertical wind shear vectors

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

2 km 1 km 3 km SFC Storm Motion 30)

Storm Motion Vector (225 @ 30)
2 km 1 km 3 km SFC Storm Motion Vector 30)

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

any level, not just those for which observations
2 km 1 km 3 km SFC Storm Motion Vector 30) Storm-relative winds can be determined at any level, not just those for which observations are available

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

any level, not just those for which observations
Horizontal Vorticity vectors 2 km 1 km 3 km SFC Storm-relative winds can be determined at any level, not just those for which observations are available

We obtain strong updraft rotation if the storm-relative winds are parallel to the horizontal vorticity – or perpendicular to the environmental shear vector – this is easily determined via a wind hodograph Shear Vector Vorticity Vector Storm-Relative Winds Play Movie © 1990 *Aster Press -- From: Cotton, Storms

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

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

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

Storm Relative Helicity
180 This area represents the 1-3 km helicity 3 km 4 km 2 km 5 km 7 km 6 km 1 km SFC Storm Motion 270 SREH Potential Tornado Strength m2 s-2 Weak m2 s-2 Strong > 450 m2 s-2 Violent

Typical Single-Cell Hodograph
Weak shear, weak winds

Typical Multicell Hodograph
Somewhat stronger winds and shear, with S-R winds providing mechanism Hodograph is essentially straight, especially at low levels

Typical Supercell Hodograph
Strong wind, shear vector turns with height, strong S-R winds Note curved shape of hodograph at low levels

Which Storm Motion Produces a Strong, Cyclonically-Rotating Supercell?
2 km 1 km 3 km SFC Storm Motion Vector 30)

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

Alignment of S-R Winds and Vorticity
Speed of S-R Winds Alignment of S-R Winds and Vorticity Horizontal Vorticity vectors 2 km 1 km 3 km SFC Storm-relative winds

Storm-Relative Environmental Helicity
Horizontal Vorticity vectors 2 km 1 km 3 km SFC Storm-relative winds

Making an Optimal Hodograph
3 km 2 km 1 km SFC

Note unidirectional shear, but Ground-relative winds veer with height
Making an Optimal Hodograph 3 km 2 km 1 km SFC Note unidirectional shear, but Ground-relative winds veer with height

Horizontal Vorticity vectors 3 km 2 km 1 km SFC

strong cyclonic supercell
Making an Optimal Hodograph Horizontal Vorticity vectors 3 km 2 km 1 km SFC Place the storm motion to get a strong cyclonic supercell

Horizontal Vorticity vectors 3 km 2 km 1 km SFC Change the hodograph to obtain a strong supercell given this storm motion

Horizontal Vorticity vectors 3 km 2 km 1 km SFC

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

Results from Observations and Models

General Guidelines for Use

Supercell Thunderstorms

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