Transcript Document

Deep Convection: Classification
Mesoscale
M. D. Eastin
Deep Convection: Classification
Types of Convective Storms:
• Single Cell Storms
• Multicell Storms
• Supercell Storms
Mesoscale
M. D. Eastin
Single Cell Storms
The Convective Cell:
• Ordinary deep convective cumulonimbus (Cb) clouds
• Have been studied and documented since the late 1800s
Mesoscale
M. D. Eastin
Single Cell Storms
The Convective Cell:
• First detailed documentation of “thunderstorms”
was by Horace Byers and Roscoe Braham
in the late 1940s
• Documented all convection that occurred
during a 3-month period in a ~100 square
mile area near Orlando, FL
• Data collection included 50 surface stations,
6 balloon launch sites, radar, and aircraft
simultaneously flying at 5 altitudes
• Results described the evolution of an ordinary
convective cell in three stages
Mesoscale
M. D. Eastin
Single Cell Storms
The Convective Cell:
Cumulus Stage: Developing cumulus cloud dominated by an updraft > 10 m/s
Minimal updraft tilt
No downdrafts
Precipitation develops aloft and is suspended by updraft
From Byers and
Braham (1949)
Mesoscale
M. D. Eastin
Single Cell Storms
The Convective Cell:
Mature Stage:
Cloud extends through depth of the atmosphere
Anvil cloud begins to spread out near tropopause
Downdraft develops due to precipitation loading and evaporational cooling
Precipitation reaches the ground
Leading edge of downdraft produces a gust front
From Byers and
Braham (1949)
Mesoscale
M. D. Eastin
Single Cell Storms
The Convective Cell:
Dissipating Stage: Precipitation core and downdraft wipe out the updraft
Cell becomes dominated by a weak downdraft
Light precipitation at the ground
From Byers and
Braham (1949)
Mesoscale
M. D. Eastin
Single Cell Storms
The Convective Cell:
• Basic building block of all
convective systems
• Lifespan is 30-60 minutes
 Occur in environments with:
weak vertical shear (< 10 m/s),
variable CAPE (500-2000 J/kg),
and small CIN ( > -50 J/kg)
• Motion is roughly the speed and
direction of the mean flow in the
0-6 km AGL layer
• Gust front spreads out equally in
all directions and rarely initiates
new convective cells
• Can produce rain, hail, strong
winds, but rarely tornadoes
Mesoscale
M. D. Eastin
Single Cell Storms
The Convective Cell:
• Basic building block of all
convective systems
• Lifespan is 30-60 minutes
 Occur in environments with:
weak vertical shear (< 10 m/s),
variable CAPE (500-2000 J/kg),
and small CIN ( > -50 J/kg)
• Motion is roughly the speed and
direction of the mean flow in the
0-6 km AGL layer
• Gust front spreads out equally in
all directions and rarely initiates
new convective cells
• Can produce rain, hail, strong
winds, but rarely tornadoes
Mesoscale
M. D. Eastin
Multicell Storms
The Multicell Storm:
• A collection of single-cell storms
at various stages in their lifecycle
• New cell development regularly
occurs on gust front flanks
Cell 3
Cell 2
Cell 4
Cell 1
Cell 5
Note: These images are qualitatively consistent with one another
Mesoscale
M. D. Eastin
Multicell Storms
The Multicell Storm:
• New cell development occurs on
the flanks of the gust front where
convergence with the ambient
storm-relative low-level flow is
maximized
• Individual cell motion (Vc) may be
different than the overall storm
motion (Vs)
• Individual cells continue to move
at the speed and direction of the
mean flow in the 0-6 km AGL layer
• The storm may move at a speed
slower or faster than the mean
wind (and in a different direction)
depending on which flank has the
maximum convergence
Mesoscale
M. D. Eastin
Multicell Storms
The Multicell Storm:
• Main inflow approaches the storm and
is lifted by the spreading gust front
• By the time the updraft has reached the
tropopause (anvil cloud), it is often well
behind the leading edge of the gust front
• Downdraft air originates at mid-levels from
precipitation loading and evaporational
cooling
 Updraft and downdraft are well separated,
allows the system to live for a much
long time than a single cell
Mesoscale
M. D. Eastin
Multicell Storms
The Multicell Storm:
• Common features include a shelf cloud,
overshooting tops, and an anvil cloud
• Lifespan 2-12 hours
 Occur in environments with:
• moderate vertical shear (10-20 m/s)
• variable CAPE (500-3000 J/kg)
• small CIN (> -50 J/kg)
• Can produce copious rainfall, hail, high
winds and some tornadoes along the
gust front
Shelf Cloud Example
Mesoscale
M. D. Eastin
Multicell Storms
The Multicell Storm:
Examples of Multicell Storms on Radar
• Often observed in a wide variety
of overall system structures
Examples include:
• Squall Lines (all varieties)
• Bow Echoes
• Mesoscale Convective Complexes
From Houze (1993)
Mesoscale
M. D. Eastin
Supercell Storms
The Supercell Storm:
• Single-cell storm that develops in isolation or
splits from a multicell storm
 Defining characteristic is a single, quasi-steady,
rotating updraft – often observed by radar as
a strong “mesocyclone” and with a “hook” echo
• Most rare, but most dangerous, storm type - can
produce large hail and strong, long-lived tornadoes
From Houze (1993)
Mesoscale
M. D. Eastin
Supercell Storms
The Supercell Storm:
• Life span up to 8 hours
 Motion is often slower than and
to the right of the mean flow
in the 0-6 km layer
 Occur in environments with:
• strong vertical shear (> 20 m/s)
• large CAPE (1000-4000 J/kg)
• small CIN ( > -50 J/kg)
Mesoscale
M. D. Eastin
Supercell Storms
The Supercell Storm:
 Early radar observations help identify many
common structural characteristics during
the mature stage of a supercell
Forward Flank Downdraft (FFD)
Storm
Motion
• Strongest and largest of the downdrafts
• Located below the primary anvil cloud
and separated from primary updraft
• Associated with the most intense
precipitation and gust front
Rear Flank Downdraft (RFD)
• Located adjacent to the primary updraft
• Associated with mid-level mesocyclone
• Collocated with the “hook” appendage
 Both downdrafts are driven by water loading
and evaporational cooling
 Note how their two gust fronts create
“meso-fronts” similar to an occluded low
Mesoscale
From Lemon and Doswell (1979)
M. D. Eastin
Supercell Storms
The Supercell Storm:
 Early radar observations help identify many
common structural characteristics during
the mature stage of a supercell
Primary Updraft (UD)
Storm
Motion
• Helical in structure
• Updraft speeds can reach 40-50 m/s
• Located at the occlusion point of the
two intersecting gust fronts
• Located within the “hook” structure
Hook Echo (see thick black contour)
• Distinct notch in the radar reflectivity
• Location of maximum inflow
• Location of primary updraft
• Location of any tornado
• Caused by the mid-level mesocyclone
advecting precipitation around itself
• Good evidence of a mesocyclone
• Also called an “inflow notch”
Mesoscale
Tornado (T)
From Lemon and Doswell (1979)
M. D. Eastin
Supercell Storms
The Supercell Storm:
 Modern Doppler radar observations continue to show
these common features as well as the strong rotation
associated with the mid-level mesocyclone
Mesocyclone
Doppler Velocity
Inbound
Flow
Outbound
Flow
Mesocyclone
Mesoscale
M. D. Eastin
Supercell Storms
The Supercell Storm:
Bounded Weak Echo Region (BWER)
• Distinct “gap” of low reflectivity
in radar cross-sections
• Location of the primary updraft
• Caused by a very strong ascent
lofting all precipitation and hail
(that normally fall through the
updraft) to the upper levels
• Updraft speeds must be
greater than 10 m/s
• Located within the hook echo
• Also called an “echo free vault”
 Presence of a BWER
and a hook echo is good
evidence of a very strong
and rotating updraft
(i.e. a supercell)
Mesoscale
M. D. Eastin
Supercell Storms
The Supercell Storm:
• Strong updrafts can produce
very large hailstones if the
updraft velocity is greater
than the fall velocity of the
hailstone (up to 20-30 m/s)
• The “overhang” of a BWER,
observed in radar reflectivity,
is often composed of small
hailstones that are initially
ejected from the updraft at
upper levels, but fall back
into the strong updraft at
lower levels
• This cycle can repeat itself
several times, allowing the
hailstone to grow larger
 A hail trajectory example
might follow 1 → 2 → 3
From Chisholm and Renick (1972)
Mesoscale
M. D. Eastin
Supercell Storms
The Supercell Storm:
• Often “split” into two separate storms
• After the split, the motion of the storm
on the right (left) is to the right (left) of
the mean 0-6 km environmental flow
• Called “right-movers” and “left-movers”
 The right-mover usually continues as a
long-lived supercell (thanks in part to
continued access to the warm, moist
low-level inflow from the southeast), and
often experiences a slower forward speed
Left
Mover
(LM)
LM
Mean
Wind
RM
 The left-mover usually begins to dissipate
(in part due to the right-mover blocking
access to the inflow), and often
experiences a faster forward speed
We will discuss the dynamical processes
involved with such events later……
Storm
Split
Right
Mover
(RM)
From Burgess (1974)
Mesoscale
M. D. Eastin
Supercell Varieties
A Spectrum of Supercell Types:
1.
2.
3.
4.
Classic Supercell
Classic supercells
High-precipitation (HP) supercells
Low-precipitation (LP) supercells
Shallow (miniature) supercells
Classic Supercells:
• Structure described on previous slides
• Tend to occur in the Central Great Plains
and Midwest (west of Mississippi River)
• Are capable of producing large hail, violent
tornadoes, and strong winds.
Mesoscale
M. D. Eastin
Supercell Varieties
High-Precipitation (HP) Supercells:
• Produce more rain than classic supercells
• Strongest RFDs and FFDs
• Tend to be less isolated – located at the
southern end of squall lines
• Often occur east of the Mississippi River
• Are capable of producing large hail, weak
tornadoes (rain-wrapped), downbursts,
and flash floods
Note the Elevation Angles
Mesoscale
M. D. Eastin
Supercell Varieties
Low-Precipitation (LP) Supercells:
• Produce less rain than classic supercells
• Weakest RFDs and FFDs
• Tend to be smaller in diameter
• Most often occur in the High Plains
along the dryline
• Still capable of producing large hail, but
tornadoes are less common
Note the Elevation Angles
Mesoscale
M. D. Eastin
Supercell Varieties
Miniature Supercells in Hurriance Ivan
Shallow (or Miniature) Supercells:
• Small diameter (<6 km) and shallow (<6 km)
compared to classic supercells
• Most often occur in tropical cyclones
• Small CAPE (<1000 J/kg) confined to
lower and middle levels
• Strong shear (up to 30 m/s) in lower 3 km
• Capable of producing weak tornadoes
From Eastin
and Link (2009)
Mesoscale
M. D. Eastin
Deep Convection: Classification
The following questions naturally arise….
 Given observations of the environment, which convective storm structure
should you anticipate?
• Single cells
• Multicells
• Supercells
 What environmental parameters should you look at?
• Vertical Instability (CAPE and CIN…more in next lecture)
• Vertical Shear (hodographs…more in next lecture)
 What physical processes are responsible for the aforementioned storm
structure and evolution? (more to come…)
Mesoscale
M. D. Eastin
Deep Convection: Classification
Summary
• Single Cell Storms
• History
• Three Stages (basic characteristics and structure)
• Significance
• Multicell Storms
• Basic Characteristics and Structure
• Motion and Propagation
• Varieties
• Supercell Storms
• Basic characteristics
• Defining structures
• Motion and storm-splitting
• Varieties (differences in structure and environment)
Mesoscale
M. D. Eastin
References
Atkins, N.T., J.M. Arnott, R.W. Przybylinski, R.A. Wolf, and B.D. Ketcham, 2004: Vortex structure and evolution within
bow echoes. Part I: Single-Doppler and damage analysis of the 29 June 1998 derecho. Mon. Wea. Rev.,
132, 2224-2242.
Byers, H. R., and R. R. Braham, Jr., 1949: The Thunderstorm. Supt. Of Documents, U.S. Government Printing Office,
Washington, D.C., 287 pp.
Burgess, D. W., 1974: Study of a right-moving thunderstorm utilizing new single Doppler radar evidence. Masters Thesis,
Dept. Meteorology, University of Oklahoma, 77 pp.
Chisholm, A. J. and J. H. Renick, 1972: The kinematics of multicell and supercell Alberta hailstorms. Alberta Hail Study,
Research Council of Alberta hail Studies, Rep. 72-2, Edmonton, Canada, 24-31.
Houze, R. A. Jr., 1993: Cloud Dynamics, Academic Press, New York, 573 pp.
Klemp, J. B., and R. Rotunno, 1983: A study of the tornadic region within a supercell thunderstorm. J. Atmos. Sci., 40,
359-377.
Lemon, L. R. , and C. A. Doswell, 1979: Severe thunderstorm evolution and mesocyclone structure as related to
tornadogenesis., Mon. Wea. Rev., 107, 1184–1197.
Weisman, M. L. , and J. B. Klemp, 1986: Characteristics of Isolated Convective Storms. Mesoscale Meteorology and
Forecasting, Ed: Peter S. Ray, American Meteorological Society, Boston, 331-358.
Wilhelmson, R. B., and J. B. Klemp, 1981: A three-dimensional numerical simulation of splitting severe storms
on 3 April 1964. J. Atmos. Sci., 38, 1581-1600.
Mesoscale
M. D. Eastin