Deep Convection: Classification

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Deep Convection Classification
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Deep Convection Classification

• Types of Convective Storms
• Single Cell Storms
• Multicell Storms
• Supercell Storms

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Single Cell Storms

• The Convective Cell
• Ordinary deep convective cumulonimbus (Cb)
  clouds
• Have been studied and documented since the late
  1800s

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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

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Single Cell Storms
The Convective Cell Cumulus
Stage Developing cumulus cloud dominated by an
updraft gt 10 m/s Minimal updraft tilt No
downdrafts Precipitation develops aloft and is
suspended by updraft
From Byers and Braham (1949)
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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)
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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)
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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 (lt 10 m/s),
• variable CAPE (500-2000 J/kg),
• and small CIN ( gt -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

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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 (lt 10 m/s),
• variable CAPE (500-2000 J/kg),
• and small CIN ( gt -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

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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
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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)

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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

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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 (gt -50 J/kg)
• Can produce copious rainfall, hail, high
• winds and some tornadoes along the
• gust front

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Multicell Storms

• The Multicell Storm
• Often observed in a wide variety
• of overall system structures
• Examples include
• Squall Lines (all varieties)
• Bow Echoes
• Mesoscale Convective Complexes

Examples of Multicell Storms on Radar
From Houze (1993)
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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)
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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 (gt 20 m/s)
• large CAPE (1000-4000 J/kg)
• small CIN ( gt -50 J/kg)

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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)
• 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

Storm Motion
From Lemon and Doswell (1979)
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Supercell Storms

• The Supercell Storm
• Early radar observations help identify many
• common structural characteristics during
• the mature stage of a supercell
• Primary Updraft (UD)
• 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

Storm Motion
Tornado (T)
From Lemon and Doswell (1979)
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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

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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

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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

From Chisholm and Renick (1972)
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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
• The left-mover usually begins to dissipate
• (in part due to the right-mover blocking
• access to the inflow), and often

Left Mover (LM)
Storm Split
Right Mover (RM)
From Burgess (1974)
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Supercell Varieties
Classic Supercell

• A Spectrum of Supercell Types
• 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.

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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
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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
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Supercell Varieties

• Shallow (or Miniature) Supercells
• Small diameter (lt6 km) and shallow (lt6 km)
• compared to classic supercells
• Most often occur in tropical cyclones
• Small CAPE (lt1000 J/kg) confined to
• lower and middle levels
• Strong shear (up to 30 m/s) in lower 3 km
• Capable of producing weak tornadoes

Miniature Supercells in Hurriance Ivan
From Eastin and Link (2009)
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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 CINmore in next
  lecture)
• Vertical Shear (hodographsmore in next lecture)
• What physical processes are responsible for the
  aforementioned storm
• structure and evolution? (more to come)

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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

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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, 11841197. 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.