Convective Storm types
James LaDue
FMI Severe Storms Workshop
June 2005
Outline
• Single cell convection
– Ordinary cell convection
– Sheared cell convection
• Multicell convection
Fundamental Concepts of
Convection
Ordinary cell convection
• Dominate when the
vertical shear is small
• Dominated by
buoyancy processes
Mogollon Rim, AZ
1999 James LaDue
Ordinary cell evolution
-10° C
TCU + 7 min
Ordinary cell evolution
-10° C
TCU + 14 min
Ordinary cell evolution
-10° C
TCU + 21 min
Ordinary cell evolution
-10° C
TCU + 28 min
Pulse storm downbursts
-10° C
TCU + 35 min
Radar and visual view of an
ordinary cell thunderstorm
• Look for onset of
elevated reflectivity
core as the updraft
reaches the freezing
level
• Note the time when the
intense reflectivity
reaches ground
• Note the time of
dissipation
Link to loop
What CAPE is the storm realizing?
CAPE = 1490 J/kg
Wmax = (2CAPE)1/2
= 54 m/s
Assume 50% or 27 m/s
EL temp = -60 C
But does this storm
appear to have a 27
m/s updraft and an EL
= -60 C?
What CAPE is the storm realizing?
A more realistic
parcel path is more
like the new curve
Causes?
•Dry air
entrainment
•Lower initial
parcel e
Influence of CAPE profiles
• Which sounding
is most likely to
produce a
stronger updraft?
• Sounding A
– Stronger initial
acceleration
– Less
precipitation
drag
CAPE (A) = CAPE (B)
Influence of CAPE profiles
• CAPE density = CAPE/depth
– When high, expect rapid upward parcel acceleration
– Occurs with steep lapse rates above and below the LFC
Influence of CAPE profiles
• Two temperature profiles
with the same moisture.
• Both yield 800 j/kg of CAPE
After McCaul and Weisman 2000 - MWR
Zb = 5.5 km
Zb = 2.5 km
• Lowering the
maximum buoyancy
level increases updraft
strength at low levels.
Downdrafts
• Commonly initiate in
the 3 – 5 km AGL
layer
• Initiated by
precipitation loading
• Evaporational
cooling adds
significant
contribution
Precipitation loading
becomes strong with
reflectivity > 55 dBZ
Downdraft buoyancy
• From evaporational
cooling
• Measured by
Downdraft CAPE
(DCAPE)
• Similar to CAPE
but in reverse
Average w of
the 700-500 mb
layer
w of the
updraft
Average w
of the
downdraft
Downdraft
initiation level
DCAPE
Downdraft buoyancy
• Larger DCAPE
mostly means
stronger
downdrafts
• However, stronger
CAPE can result in
stronger
precipitation
loading
Average w of
the 700-500 mb
layer
w of the
updraft
Average w
of the
downdraft
Downdraft
initiation level
DCAPE
Downdraft buoyancy
• DCAPE is never
fully utilized by the
downdraft
• Downdrafts are not
saturated and do
not follow the w to
the surface
Average w of
the 700-500 mb
layer
w of the
updraft
Average w
of the
downdraft
Downdraft
initiation level
DCAPE
DCAPE is still a good starting place to estimate downdraft
strength
Downdrafts in single cells
• Downdraft strength
– amount of DCAPE
– precipitation loading
– Nonhydrostatic vertical pressure profiles
Updraft/shear interactions
Shear interactions with updrafts
• Updraft tilt
• Causes separation
of precipitation and
updraft
• Precipitation
loading a lesser
threat to integrity of
the updraft
Shear interactions with updrafts
• Updraft tilt is a
function of its
strength
– Given same shear,
a weaker updraft
tilts more
• Updraft tilt also a
function of shear
strength
Origins of updraft rotation from
straight shear
• Incipient updraft
tilts horizontal
vorticity
• Result is a
counterrotating
twin vortex on
either side of an
updraft
Origins of updraft rotation from
straight shear
• Vortices generate
‘dynamic’ lows at
the points of
maximum rotation
(usually in
midlevels)
Origins of updraft rotation from
straight shear
• Dynamic midlevel
lows encourage new
updraft growth within
the rotation axis.
• Updraft appears to
move right and left of
the shear vector.
• Core initiates a
downdraft in the
middle.
• Result is a rotating
updraft.
Origins of updraft rotation from
straight shear
• The result is a
splitting storm
• The left and right
moving members
rotating in opposite
directions
Directional shear
H
High
L
L
Low
H
Directional shear
From COMET (1996)
Behavior of rotating storms
from curved shear
• Clockwise turning
shear with height
favors the cyclonically
rotating supercell
• Counter clockwise
turning shear with
height favors the
anticyclonically
rotating supercell
Estimating supercell motion
• The Internal Dynamics (ID) method
– Plot the 0-6 km mean wind
– Draw the 0-6 km shear vector
– Draw a line orthogonal to the shear vector through
the mean wind
– Plot the left (right) moving storm 7.5 m/s to the left
(right) of the mean wind along the orthogonal line.
Estimating supercell motion
• The Internal Dynamics (ID) method
– Plot the 0-6 km mean wind
– Draw the 0-6 km shear vector
– Draw a line orthogonal to the shear vector through
the mean wind
– Plot the left (right) moving storm 7.5 m/s to the left
(right) of the mean wind along the orthogonal line.
Estimating supercell motion
• The Internal Dynamics (ID) method
– Plot the 0-6 km mean wind
– Draw the 0-6 km shear vector
– Draw a line orthogonal to the shear vector through
the mean wind
– Plot the left (right) moving storm 7.5 m/s to the left
(right) of the mean wind along the orthogonal line.
Estimating supercell motion
• The Internal Dynamics (ID) method
– Plot the 0-6 km mean wind
– Draw the 0-6 km shear vector
– Draw a line orthogonal to the shear vector through
the mean wind
– Plot the left (right) moving storm 7.5 m/s to the left
(right) of the mean wind along the orthogonal line.
Bunkers et al.
(2000)
Other types of supercells
• High precipitation
• Classic
• Low Precipitation
LP supercells
• No official
definition
• Poorly efficient
precipitation
producers
• Generate outflows
too weak to
generate strong
low level
mesocyclones
LP supercells
• Tornado/wind
threat is small
• Large hail threat
is large
• Near zero fl flood
threat
Classic supercells
• No official definition
• More efficient
precipitation
producers
• Generate sufficient
outflows to generate
strong low level
mesocyclones
Classic supercells
• Tornado/wind
threat is large
• Large hail threat
is large
• Increasing fl
flood threat for
slow moving
cells
High Precipitation supercells
• No official definition
• Most common
• Moderate efficient
precipitation
producers
• Strong outflows
generate strong
low-level mesos but
mostly short-lived
HP supercells
• Tornado threat is
large
• Damaging wind
threat is larger
• Large hail threat
is large
• Most likely
responsible for
flash floods
30 Apr 2000 – Olney, TX
-
J. LaDue
HP Supercells
Adapted from Moller et al., 1990
Cold pool/shear interactions
• This is most
significant when
considering multicell
behavior
– Motion, longevity,
severity
Cold pool/shear interactions
This side is where
environmental and cold
pool vorticity inhibit
deep lifting.
This side is where
environmental and cold
pool vorticity enhance
deep lifting.
Based on theory by Rotunno, Klemp and Weisman, 1988
(RKW)
Cold pool shear interactions
• RKW theory shows how the shear/cold
pool interactions affect the depth of lifting
• Strength of surface convergence does
not indicate depth of lifting
Cold pool shear interactions
• According to RKW theory,
the shear component
perpendicular to the
orientation of the line
helps determine line
longevity
• Either of the top two
examples have good
component of shear
Other cold pool/shear
considerations
• RKW theory tested with idealized
model multicell initiation
• Other studies such as Coniglio and
Stensrud suggest shear layer deeper
than RKW is better for anticipating
long-lived multicell events
• Cold pool shear interactions only one
factor in determining convective
initiation potential in multicells
Multicell Motion
•
•
Determined by which side of the cold pool
initiates the most convection
Affected by
1)
2)
3)
4)
Shear-cold pool interactions
Instability gradients
Low-level convergence (SR sense)
3-D boundary interactions
Multicell Motion
2) Instability effects
– Can modulate
propagation of
multicells toward
areas of higher
instability
From Richardson (1999)
Multicell Motion
3) Low-level convergence effects
Use original MBE Vector (“Corfidi”) Technique
V cl
V prop = -VLLJ
VMBE
After Corfidi et al. (1996)
To see where low-level
convergence is located, and
help predict system motion
Multicell Motion
4) Boundary interactions
• Modulates/enhances development of new
convection
Blue = steering
layer flow
Green=triple pt
motion
Red = multicell
motion
(Weaver, 1979)
The Rear Inflow Jet
Squall lines w/ nondescending Rear Inflow Jets
(RIJs) live longer, but also consider deep-layered
shear .
Note the vorticity induced by the nondescending RIJ
can counteract that of the cold pool
Dynamics of a RIJ
• Strength of RIJ
depends on CAPE
(incr. temp excess)
and Shear (erect
updraft with more
direct heat into
anvil)
Three classes of multicells
SR line-perpendicular comp.
Speed
SR line-parallel wind
comp.
After Parker and Johnson (2000)
Bow echoes
• Bowing structure
accompanying
severe wind
events
• Often has similar
hodographs to
that of
supercells
SRH vs shear as a supercell
forecasting tool
• Shear can be used
without knowing storm
motion
• Once storm motion is
known, use SRH to
estimate supercell
strength
0 km
6
C
Vr
Summary
• Buoyancy considerations
– Convective Available Potential Energy (CAPE)
– Low vs. high CAPE density can alter updraft
speed by changing precipitation loading
– Different buoyancy profiles can alter strength of
low-level updraft even though CAPE is the same
– Downdraft CAPE, or DCAPE is a measure of
downdraft strength potential but does not include
precipitation loading
Summary
• Updraft/Shear considerations
– Causes updrafts to tilt lessoning precipitation
loading
– Results in updraft rotation
– Straight shear results in counterrotating
supercells
– Clockwise (counterclockwise) curved shear
enhances the cyclonically (anticyclonically)
rotating supercell
Summary
• Cold pool/Shear considerations
– Results in updraft rotation
– Straight shear results in counterrotating
supercells
– Clockwise (counterclockwise) curved shear
enhances the cyclonically (anticyclonically)
rotating supercell