Transcript Supercell Storms - Kelvin K. Droegemeier

Supercell Storms
METR 4433: Mesoscale Meteorology
Spring 2016 Semester
Adapted from Materials by Drs. Kelvin Droegemeier, Frank Gallagher III
and Ming Xue
School of Meteorology
University of Oklahoma
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Supercell Thunderstorms


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
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Supercell Thunderstorms
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
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Supercell Thunderstorms

Form in an environment of strong winds
and high shear
– Provides a mechanism for separating the
updraft and downdraft
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Structure of a Supercell Storm
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Schematic Diagram of a Supercell Storm (C. Doswell)
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Structure of a Supercell Storm
Mesocyclone
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Supercell Structure
Forward Flank
Downdraft
Tornado
Rear Flank
Downdraft
Flanking Line/
Gust Front
Mesocyclone
Gustnado
Inflow
© 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic
Meteorology -- Volume II: Observations and Theory of Weather Systems
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Perturbation Pressure Field
Hydrostatic High
In Cold Pool
Inflow Low
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3D Flow in a supercell
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Animation of a Numerically
Simulated Supercell Storm

https://www.youtube.com/watch?v=Egu
mU0Ns1YI
R. Wilhelmson, University of Illinois at Urbana-Champaign
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A Supercell on NEXRAD Doppler Radar
Hook Echo
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A Supercell on NEXRAD Doppler Radar
Hook Echo
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Where is the Supercell?
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Where is the Supercell?
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Supercell Types
Classic
 Low-precipitation
 High-precipitation

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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

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LP Supercell
Side View Schematic
© 1993 American Geophysical Union -- From: Church et al., The Tornado
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LP Supercell
Top View Schematic
© 1993 American Geophysical Union -- From: Church et al., The Tornado
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LP Supercell
© 1995 Robert Prentice
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LP Supercell
© 1995 Robert Prentice
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Another LP Supercell
© 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic
Meteorology -- Volume II: Observations and Theory of Weather Systems
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A Tornadic LP Supercell
26 May 1994 -- Texas Panhandle
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© 1998 Prentice-Hall, Inc. -- From: Lutgens and Tarbuck, The Atmosphere, 7th Ed.
High Precipitation (HP)
Supercells

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



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
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HP Supercells
© 1993 American Geophysical Union -- From: Church et al., The Tornado
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HP Supercells
© 1993 American Geophysical Union -- From: Church et al., The Tornado
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HP Supercell
Heaviest
Precipitation
(core)
Kansas
Woods County,
Oklahoma
Oklahoma
4 OCT 1998
2120 UTC
KTLX
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Heaviest
Precipitation
(core)
Twenty
minutes
later …..
Kansas
Oklahoma
HP
Supercell
4 OCT 1998
2150 UTC
KTLX
Developing
Cells
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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
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Classic Supercells
© 1993 American Geophysical Union -- From: Church et al., The Tornado
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Classic Supercells
© 1993 American Geophysical Union -- From: Church et al., The Tornado
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Classic Supercell
Heaviest
Precipitation
(core)
Hook
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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.

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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
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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
– Key ingredients
» Conditional instability
» Source of lift and vertical motion
» Warm, moist air
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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
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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
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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
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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
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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
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Supercell Evolution -- Mature
Phase
Hook
Echo
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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
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Supercell Evolution -- Mature
Phase
Slice
4 km
Bounded Weak Echo Region
© 1990 *Aster Press -- From: Cotton, Storms
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Splitting Storms
If the shear is favorable, both
circulations may continue to exist.
 In this case the storm will split into two
new storms.
 We will look at this in greater detail
later.

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Splitting Storms
© 1990 *Aster Press -- From: Cotton, Storms
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Movie of Splitting
Courtesy NCAR
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Left
Mover
Splitting Storms
Split
Right
Mover
© 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic
Meteorology -- Volume II: Observations and Theory of Weather Systems
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Updraft
The updraft is the rising column of air in
the supercell
 It generally is 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

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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
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Structure of a Supercell Storm
MesoCyclone
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Mesocyclone
A cyclonic vortex marking the updraft of
a supercell storm
 Usually 2-10 km in diameter
 Vertically coherent for a few km,
sometimes extending throughout a
significant depth of the storm
 Vertical vorticity on the order of 10-2 s-1
 Visually manifest as the wall cloud
 Different mechanisms for mid-level and
low-level formation

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The Wall Cloud
MesoCyclone
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The Wall Cloud
MesoCyclone
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Wall Cloud
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Cyclonic rotation and strong rising motion often are
visible within the wall cloud
The squared-off lowering results from low pressure
inside of the rotating updraft: as air approaches the
vortex laterally, toward, it condenses – just like air
that rises vertically toward lower pressure condenses
to form clouds
L
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The Wall Cloud
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The Wall Cloud
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The Wall Cloud
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3D Storm Simulation Courtesy Lou Wicker, NSSL
http://kkd.ou.edu/METR_4803_Spring_2005/Wicker_Movie.mov
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Some Storms Produce Mesocyclone
Families: Cyclic Mesocyclogenesis
Burgess et al. 1982
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Cyclic Mesocyclogenesis: Conceptual Model from
Numerical Simulation
Adlerman, Droegemeier, and Davies-Jones 1999
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Cyclic Mesocyclogenesis: Conceptual Model from
Numerical Simulation
&
Adlerman, Droegemeier, and Davies-Jones 1999
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Comparison With Observations
Computer Simulation
Mobile Doppler Radar
Courtesy J. Wurman
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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
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Supercell Downdrafts

We shall examine two distinct
downdrafts associated with supercell
thunderstorms:
– Forward Flank Downdraft (FFD)
– Rear Flank Downdraft (RFD)
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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.

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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.

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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
74
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.

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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
76
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.

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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
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Formation of the RFD
This happens in the atmosphere also!
 The updraft acts a an obstruction to the
upper level flow.

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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
RFD
FFD
Mid-level
Flow
Gust
Front
Inflow
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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:

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Vorticity Dynamics
Must consider 3D equations of motion
 Can neglect Coriolis force

Vector Form
or
or
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Vorticity Dynamics
or
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Vorticity Dynamics

Recall the definition of vorticity as the
curl of the 3D velocity vector (del x V):
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Vorticity Dynamics

Taking del x momentum equation
gives
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Vorticity Dynamics
0
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Vorticity Dynamics

Rearranging gives
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Vorticity Dynamics

Rearranging gives
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Vorticity Dynamics

Tilting term can be written
 w v w u 

 


t
 x z y z 
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Vertical Wind Shear
Up
Westerly Winds
Increase in Speed
with height
North
East
90
Vorticity Dynamics

Tilting term can be written
0
 w v w u 

 


t
 x z y z 
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Vertical Wind Shear
Up
Westerly Winds
Increase in Speed
with height
North
 w u
 East
t y z
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Development of Mid-Level
Rotation
Up
North
East
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Tilting

In order to create vertical rotation from
horizontal rotation, we must tilt the
horizontal rotation into the vertical.
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Tilting

In thunderstorms, this tilting is achieved
by the updraft.
Updraft
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Development of Mid-Level Rotation
+ or
Cyclonic
Thunderstorm
Up
- or
Anti-Cyclonic
North
 w u
 East
t y z
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Tilting

Viewed from above, we see a pair of
counter-rotating vortices:
“Positive Rotation”
“Negative Rotation”
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Tilting
Vortex Tube
Updraft
Play Movie
© 1990 *Aster Press -- From: Cotton, Storms
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Development of Mid-Level
Rotation

In this simple example, the updraft has no
NET rotation because the vortex pair
straddles the updraft
+ w>0 
In most supercells, the updraft is dominantly
cyclonic. Why? The answer lies in the
STORM-RELATIVE winds.
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Storm-Relative Winds
Absolute velocity = Relative Velocity + Velocity of Coordinate System
40 mph
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Storm-Relative Winds
Absolute velocity = Relative Velocity + Velocity of Coordinate System
90 mph
40 mph
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Storm-Relative Winds
Absolute velocity = Relative Velocity + Velocity of Coordinate System
90 mph
130 mph
40 mph
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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
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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
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Storm-Relative Winds
Storm-Relative Winds = Environmental Wind – Storm Motion
Storm Motion = 20 mph
Environ = 40 mph
Storm-Relative = 20 mph
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Storm-Relative Winds
Storm-Relative Winds = Environmental Wind – Storm Motion
Storm Motion = 20 mph
Environ = 40 mph
Storm-Relative = -60 mph
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The Only Thing that
EVER Matters is the
Storm-Relative
Wind
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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
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Importance of Storm-Relative Winds
Want to intensify
the cyclonic vortex
on the south side
Vortex Tube
Updraft
Storm-Relative
Winds
Play Movie
© 1990 *Aster Press -- From: Cotton, Storms
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Importance of Storm-Relative Winds
Storm-Relative
Winds
Vortex Tube
Updraft
Play Movie
© 1990 *Aster Press -- From: Cotton, Storms
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Importance of Storm-Relative Winds
Vortex Tube
Storm-Relative
Winds
Updraft
Play Movie
© 1990 *Aster Press -- From: Cotton, Storms
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Importance of Storm-Relative Winds
Vortex Tube
Storm-Relative
Winds
Updraft
Play Movie
© 1990 *Aster Press -- From: Cotton, Storms
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Importance of Storm-Relative Winds
We obtain strong updraft rotation if the storm-relative
winds are parallel to the horizontal vorticity – or perpendicular
to the environmental shear vector
Vortex Tube
Storm-Relative
Winds
Updraft
Play Movie
© 1990 *Aster Press -- From: Cotton, Storms
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Vertical Wind Shear
Up
Westerly Winds
Increase in Speed
with height
North
East
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Vertical Wind Shear
Up
Shear = V(upper) – V(lower)
North
East
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Vertical Wind Shear
Up
Shear = V(upper) – V(lower)
North
East
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Vertical Wind Shear
Up
Shear = V(upper) – V(lower)
Shear Vector
East
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Development of Mid-Level
Rotation
Up
Note that the vorticity
vector points 90 deg to
the left of the shear vector
North
Shear Vector
East
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Importance of Storm-Relative Winds
We obtain strong updraft rotation if the storm-relative
winds are parallel to the horizontal vorticity – or perpendicular
to the environmental shear vector – because this leads to
immediate vortex stretching of the updraft
Shear Vector
Vorticity Vector
Storm-Relative
Winds
Play Movie
© 1990 *Aster Press -- From: Cotton, Storms
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Stretching (Convergence) Term
Becomes
u v
w
w

     ; here
 0 and   0 implies
0
x y
z
z
t
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Development of Mid-Level
Rotation
Updraft - Stretch
Up
North
East
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Development of Low-Level
Updraft Rotation
Cannot be explained by stretching at
mid-levels alone because of w=0
condition at ground
 Clear sequence of events precedes
rapid spin-up of vorticity at low levels:

– Decrease in updraft intensity
– Rear-flank downdraft (RFD)
– Cold outflow
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Supercell Structure
Forward Flank
Downdraft
Tornado
Rear Flank
Downdraft
Flanking Line/
Gust Front
Mesocyclone
Gustnado
Inflow
© 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic
Meteorology -- Volume II: Observations and Theory of Weather Systems
123
Recall Horizontal Vorticity Generation
Along Temperature Gradients

Air travelling along a frontal zone will develop
a horizontal rotation.
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Role of Forward Flank
Downdraft


Air flowing along
the cold boundary
of the FFD enters
the mesocyclone
This horizontal
vorticity is tilted at
very low levels
and stretched
125
3-D Depiction
From Klemp (1987)
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