Thunderstorm classification a brief review

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Thunderstorm classificationa brief review

• the distinction between the 3 storm types is
  largely controlled by ambient stability and wind
  shear

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Mesoscale organization

• Mesoscale convective systems are clusters
  containing thunderstorm cells
• The 3 types of MCSs are symmetric, asymmetric,
  and amorphous ones
• MCS organization and longevity are controlled
  mainly by low-level ambient wind shear, possibly
  also mid-level humidity
• MCSs may include squall lines or bow echoes
• Mesoscale convective complexes (MCCs) are a
  special category of MCSs, defined based on size,
  shape, and longevity

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Brief history of thunderstorm field research

• 48-49 Thunderstorm Project (Byers Braham)
• 55 creation of the NSSL to develop weather
  radars and other instruments to better observe
  thunderstorms (Kessler)
• 72-76 NHRE (hail, hail suppression)
• 78 NIMROD (microbursts) (Fujita)
• 79 SESAME
• 82 CCOPE
• 84 JAWS
• 87 PRESTORM (squall lines, MCSs)
• 90 COHMEX
• 95,97 VORTEX (tornadoes)
• 02 IHOP (convective initiation, low-level jet)

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The Thunderstorm Project

• Early field project summer 1946 in Florida, July
  1947 in Ohio
• Justified in part by need for wx information for
  the expanding aviation industry
• Ten military aircraft, P61C (Black Widow), five
  each mission, spaced at 5000 intervals
• Used new radar developments from WW-II (first use
  of 5 cm C-band radars)
• First meso-net (people recording wx at 5 min
  intervals during IOPs)
• In-flight data obtained from photographs of
  instrument panels
• focused on determining kinematic and thermal
  structure and evolution of thunderstorms

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Thunderstorm Stages

• References
• the project report The Thunderstorm
• Byers and Braham, 1948 Thunderstorm structure
  and circulation. J. Meteorol., 5, 71-86
• Thunderstorm described as composed of a number of
  relatively independent cells
• Each cell evolves through stages
• cumulus stage
• mature stage
• dissipating stage

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The Cumulus Stage
Wind, temperature, and hydrometeors

• Updrafts throughout, 5 m/s max (15 m/s peak)
  no downdrafts
• Cell sizes 2-6 km
• Updraft increases with height but diameter
  remains about constant (? entrainment).
• Horizontal convergence measured at low levels.
• Positively buoyant throughout
• Graupel and rain detected
• 15-30 min in duration

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Surface convergence pattern measured at the time
of first formation of cumulus clouds
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The Mature Stage

Wind, temperature, and hydrometeors

• Rain first reaches the ground heaviest rain and
  strongest turbu-lence in this stage
• Downdraft forms from above the FL, most intense
  near 10,000 ft
• Updrafts also remain strong, most intense higher
  in cell
• Strong surface divergence forms below the
  heaviest rain, and the cloud outflow forms a gust
  front at the surface
• Both positive and negative buoyancy is present
  (with magnitude of about 2 C)

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The Mature Stage

Wind, temperature, and hydrometeors

• Rain first reaches the ground heaviest rain and
  strongest turbu-lence in this stage
• Downdraft forms from above the FL, most intense
  near 10,000 ft
• Updrafts also remain strong, most intense higher
  in cell
• Strong surface divergence forms below the
  heaviest rain, and the cloud outflow forms a gust
  front at the surface
• Both positive and negative buoyancy is present
  (with magnitude of about 2 C)

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Surface wind measurements show outflow below the
region of radar echo
New convergence line ??
echo gt30 dB
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The Dissipating Stage

Wind, temperature, and hydrometeors

• Begins when there is no longer a low-level
  updraft
• Downdrafts weaken, turbulence becomes less
  intense, and precipitation decreases to light
  rain.
• Lasts about 30 min

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the Thunderstorm Project

• The 3 storm stages have since been interpreted as
  characteristic of airmass thunderstorms
• Byers and Braham recognize the importance of wind
  shear
• strong shear prolongs the mature stage by
  separating the precipitating region with
  downdrafts from the updraft region
• They also estimate entrainment
• estimated from mass balance 100 in 200 mb
• estimated from soundings around storms 100 in
  500 mb
• discrepancy probably arose from downward motion
  of mixtures after entrainment, making the former
  estimate more reliable

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Airmass Thunderstorms

• Scattered, small, short-lived, 3 stages
• Environment has little CAPE, but also little CIN,
  and little wind shear
• They are usually triggered along shallow
  convergence zones (BL forcing)
• Rarely produce extreme winds and/or hail, but may
  be vigorous with intense lightning

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Photo by NSSL
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Mature airmass thunderstorms over the Pacific
seen by the Space Shuttle
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Schematic of the evolution of an airmass storm,
as seen by a radar
Height (100s of ft)
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The reason why an airmass thunderstorms is so
shortlived is that there is little wind shear,
therefore the rainy downdraft quickly undercuts
and chokes off the updraft.
Photo by Moller
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Multicell Thunderstorms

• Life cycle of any one of the cells of a multicell
  thunderstorm is like any air-mass thunderstorm.
• Multicell storms can occur in a cluster, or be
  organized as one line.
• The life cycle of the multicell is much different
  due to the interaction of the cells one with
  another.
• The key to the long life of the multicellis the
  development of the gust front.

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Multicell storms were recognized by Byers and
Braham
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Roles of Cells in Multicellular Clouds

• Evidence since the Thunderstorm Project continues
  to support the relevance of individual cells in
  thunderstorm systems.
• The sequence on the right shows individual cells
  and their place in the evolution of a
  multicellular cloud.

Ludlam
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Shelf clouds above a gust front
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Multicell Thunderstorms

• Shelf Cloud often indicates rising air over the
  gust front.
• New cells develop in front of the storm.
• Gust front maintained by the cool downdrafts.
• Gust front is typically several miles in front of
  the thunderstorm
• Gust front appears like a mesoscale cold front.

• Outflow boundary is the remnant of a gust front.

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Multicell movement
old cell
Multicell storms move slightly to the right of
the upper-level wind
young cell
Photo by Doswell
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Multicell echo sequence
Photo by Moller
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Essential in the development of new cells, and
hence the longevity of multicell clusters, is the
interaction between the cold pool and low level
ambient shear
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Supercell Thunderstorms

• Supercell thunderstorms are defined as having a
  sustained deep-tropospheric updraft (more or
  less) coincident with a mid-level vorticity
  maximum
• They are typically severe (strong horizontal
  wind gusts, large hail, flash flood, and/or
  tornadoes)
• They are rare (lt1 in US, lt5 in Southern Plains
  in May), long-lived
• They are easily identifiable on radar
• Mesocyclone (sometimes TVS)
• elongated anvil (to the east), often with a V
  notch
• a hook-shaped flanking line (on the south side
  for right movers)
• bounded weak-echo region (WER)
• reflectivity often suggests hail presence

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Supercell Thunderstorms

• occur most frequently in the southern Great
  Plains in spring.
• compared to single cells, supercells are
• longer-lived
• larger
• organized with separate up- and downdrafts.

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anvil
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How does the (bounded) weak echo region (WER)
form ?
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weak echo region in a supercell storm

• As the storm intensifies, the updraft becomes
  stronger and more erect.
• The result are
• the development of mid-level echo overhang (WER)
• a tighter reflectivity gradient (hail is most
  common just north of the WER)
• a shift in cloud top position (right above the
  WER)
• these are strong indicators of a dangerously
  severe storm.

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Base scan (0.5) RHI
16.5 km echo tops
NW
SE
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Photo by Moller
This storm produced baseball hail east of
Carnegie, OK, as it was photographed looking east
from 30 miles. From right to left (south to
north), note the flanking line, the main storm
cell, and the downwind anvil above the
precipitation area.
Photo by Bill McCaul
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(above) a supercell with overshooting top, seen
from the SW (photo H. Bluestein) (right) a
Texas supercell seen from the NW note vertical
cloud wall and spreading anvil (photo by Moller)
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Thunderstorm evolution and shear

• no shear
• strong shear

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Storm classification summaryvariablesbuoyancy
and shear profiles
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Predicting supercell motion from wind profile
data (right movers)
v (m/s)

u (m/s)

• Justification
• nowcasting (just before supercell development)
• helicity is calculated in a storm-relative frame
  of reference
• (helicity determines
  tornado potential)

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• Physical concepts
• advection of the storm by the deep-layer mean
  wind
• interaction of the convective updraft with the
  sheared environment to promote rotation and
  propagation
• (does not account for propagation along
  boundaries, topographic ridges )

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Maddox 1976 30R75
Davies and Johns (1993) storm motion ranges from
30R75 to 20R85, depending on severity
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Colquhoun 1980 inflow equals outflow
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Bunkers, et al. (1998) ID method Based on the
internal dynamics of the supercell (hence called
the ID method) Galilean invariant and
shear-relative
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D7.5 m/s
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Australian supercell environment
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Where do we go from here?

• Storm cell organization COMET/METED
• Supercell dynamics (theory, transparencies based
  on Houze 1993)
• Anticipating convective storm structure (8-10 hr
  CD, Weisman, 1996)
• Case study 3 May 1999 (WAF paper, Thompson and
  Edwards 2002)
• Mesoscale organization
• Mesoscale Convective Systems Squall Lines and
  Bow Echoes (webcast)
• MCS matrix (11-14 hr CD, Weisman 2001)
• MCSs BAMEX Science Overview
• (time permitting)
• Tornado dynamics (transparencies)
• MCV dynamics (Fritsch 1996)
• Hurricane dynamics (numerous sources)