Severe Storm Structures

1
Severe Storm Structures

• Clark Evans
• Current Weather Discussion
• 26 March 2008

2
Goals and Outline

• To be able to identify conditions favoring linear
  versus discrete storms
• To be able to identify and analyze a multitude of
  linear convective systems
• To better understand supercells and supercellular
  structures, including left- and right-moving
  storms

3
Linear and Discrete Modes
(image courtesy NWS Southern Region)

• Linear an organized set of strong to severe
  storms often taking a linear shape
• Examples squall line, derecho, mesoscale
  convective system (MCS), etc.

4
Linear and Discrete Modes
(image courtesy Univ. of Oklahoma)

• Discrete an individual, often supercellular
  storm. Has a uniquely identifiable updraft and
  storm structure, potentially including forward
  and rear flanking downdrafts.
• Associated with the most prolific tornado
  outbreaks

5
Linear and Discrete Modes

• What atmospheric characteristics distinguish
  between conditions favorable for discrete versus
  linear storms?
• What atmospheric characteristics promote the
  upscale development from discrete storms to
  linear features?

6
Linear and Discrete Modes

• Factors that are important for determining storm
  mode include
• Type of focusing mechanism
• Strength of upper-level forcing
• Strength of capping, if any
• Orientation of wind fields with respect to
  large-scale boundaries
• Time of day?
• and perhaps more.

7
Linear and Discrete Modes

• Which of these boundary types favor discrete
  storm development?
• Which of these boundary types favor linear storm
  development?

8
Linear and Discrete Modes

• Generally, it is tied to the other environmental
  factors at play for each type of boundary
• Capping
• Strength of large-scale forcing
• Upper level wind fields
• That said, each boundary type does have
  distinguishing characteristics for storm mode

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Linear and Discrete Modes

• Cold frontsoften linear
• Sharply sloped
• Often near strongest upper level forcing
• Warm frontsdiscrete potential
• Not sharply sloped
• Usually weaker upper level forcing
• Dry lines and pre-frontal troughsdepends
• Capping often plays a role
• If capping is broken or weak, magnitude of upper
  level forcing plays a role

10
Linear and Discrete Modes

• A closer proximity to upper level forcing often
  leads to a greater linear storm threat
• Associated with very strong height falls,
  vorticity advection and/or upper level
  divergence/diffluence
• Found with very strong low level convergence
  along a boundary
• Subsequent large-scale lift often completely
  erodes any cap that was in place
• Colder mid-level temperatures enhance instability
• Vertical wind profile tends to become more
  unidirectional, leading to straight-line
  hodographs and favoring lines
• Often found in conjunction with sharp focusing
  mechanism, leading to large-scale storm and
  updraft development

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Forcing and Focusing Example

• Left 500 hPa analysis
• Right Surface analysis
• Both from 0000 UTC 6 February 2008, courtesy SPC

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Forcing and Focusing Example

• Closer to the stronger upper-level forcing and
  sharper surface boundary linear
• Further away in free warm sector along
  pre-frontal wind shift discrete

Radar mosaic at 2234 UTC 5 February 2008 Courtesy
Storm Prediction Center
13
Linear and Discrete Modes

• Discrete modes are favored when there is some
  capping in place
• What is too much capping? Somewhat subjective and
  depends on the situation.
• Need sufficient surface-based heating coupled
  with synoptic-scale and mesoscale lift to erode
  cap locally
• If synoptic-scale lift and forcing are too
  strong, cap may be broken everywhere all at once,
  thus favoring linear development

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Capping Erosion Example

• Lake Charles, LA soundings, courtesy SPC
• Left 1200 UTC 5 Feb Right 0000 UTC 6 Feb. 2008

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Capping Erosion Example

• Combination of surface-based heating plus
  large-scale lift helped erode the cap during the
  day on 5 February 2008
• Coupled with subtle pre-frontal wind shift
  focusing mechanism and distance from upper level
  forcing, discrete activity took place!
• Do recall, though, that wind shear and
  instability parameters for severe convection must
  still be met despite distance from upper level
  forcing!

16
Capping Erosion Example

• Del Rio, TX soundings, courtesy SPC
• Left 1200 UTC 4 May Right 0000 UTC 5 May 2007

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Capping Erosion Example

• Strong surface-based heating helped put a dent in
  the cap
• but, weak synoptic-scale forcing and lift only
  minimally lifted the cap
• Thus, the cap remained in place and storm
  development did not occur (independent of extreme
  instability and weak shear)

18
Linear and Discrete Modes

• Structure of the deep layer wind field can be
  important in determining storm mode.
• Primarily arises out of consideration for
  hodograph structure and orientation to any
  focusing mechanisms.
• There is no set rule of thumb that works for
  all situations and types of boundaries, however.

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Linear and Discrete Modes

• Generally speaking, greater directional shear
  leads to a greater discrete storm potential
  (left).
• Weaker directional shear straight-line
  hodographs leads to a greater linear storm
  potential (right).

20
Linear and Discrete Modes

• Storms that can ride the warm front have the
  potential to be long-tracked tornado producers
  (left)
• Storms that are directed across the warm front
  become elevated and occasionally linear, negating
  the tornado threat (right)

21
Linear and Discrete Modes

• Time of day plays a minor role in determining
  storm mode.
• Generally speaking, if storms form before the
  boundary layer decouples, discrete activity is
  more likely.
• Before sunset or generally shortly thereafter
• Decoupling favored under relatively light winds
  or strong radiational cooling conditions
• Storms forming or lasting after this are more
  linear.

22
Linear and Discrete Modes

• As instability weakens after sunset, storms can
  have a harder time maintaining their updrafts.
• Downdrafts thus dominate
• Cold pool generation leads to storms becoming
  outflow dominant
• New storms can form along outflow boundary,
  leading to upscale growth
• If atmospheric conditions are ripe, this upscale
  growth can lead to an MCS, squall line, or
  derecho
• Need sufficient shear, low-level instability, and
  forcing moving in conjunction with convective
  system

23
Linear and Discrete Modes Summary

• Dependant upon the interaction of many factors.
• Discrete modes are favored
• During the daytime away from the strongest
  upper-level forcing
• With greater directional shear values
• Away from the sharpest focusing mechanisms
• In regions of some capping
• Linear modes are favored
• Closer to well-defined focusing mechanisms
• Closer to stronger upper level forcing and weaker
  directional shear
• In regions of weak to no capping
• They are also favored at night as discrete
  convection grows upscale.

24
Linear Convective Systems

• Linear convective systems come in all shapes and
  sizes and are referenced with numerous terms
• Mesoscale convective systems
• Squall lines and bow echoes
• Derechos
• Quasi-linear convective systems (QLCS)
• Line echo wave pattern (LEWP)

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Mesoscale Convective Systems

• Generic term for any organized convective feature
  on the mesoscale
• Characterized by a descending rear inflow jet
  (notch) on velocity (reflectivity) radar imagery
• Driven primarily by buoyancy and strong
  upper-level divergence

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Mesoscale Convective Systems

• Often characterized by circular appearances on
  satellite imagery with cold cloud tops
• Can lead to the formation of a mesoscale
  convective vortex (MCV) at mid-levels

(image courtesy NWS WFO No. Indiana)
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Mesoscale Convective Systems

• What are the factors used to diagnose MCS
  longevity?
• Maximum shear in 0-1 and 6-10 km layers
• Mid-level lapse rates
• Most unstable CAPE
• Mean winds above the boundary layer
• In general, higher is better for each parameter.
• Resource Coniglio and Corfidi (2006)

28
Mesoscale Convective Systems

• MCS features both development and longevity
  are not well-handled by models
• Large-scale models cannot represent cold pool
  generation and propagation
• Mesoscale models often either way too strong and
  fast or dont show any development at all
• Non-horizontal resolution problems include
• Convective parameterizations
• Boundary layer parameterizations
• Microphysics parameterizations
• And more

29
Squall Lines and Bow Echoes

• Multicell complex of strong to severe storms
• Leading edge buoyancy driven, strongest
  convection
• Back edge trailing stratiform region, where
  storms go to die
• Generally strongest on the southern edge of its
  forward bulge
• Tail-end Charlie structures as well

(image courtesy theweatherprediction.com)
30
Squall Lines and Bow Echoes

• Four primary types of squall lines
• Broken line generally occurs along some focusing
  mechanism
• Back building training storms gradually grow
  into a line, often with mean winds parallel to
  the focusing mechanism
• Broken areal upscale growth of discrete
  convection
• Embedded areal linear growth as a result of
  elevated instability, either upright or slantwise

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Squall Lines and Bow Echoes

• Longevity and severity dependent upon
• Instability, surface-based or elevated
• Proximity and magnitude of upper-level forcing
  (closer or stronger is better)
• Ability to generate or transfer momentum to the
  surface
• Mid-level dry air generation
• Strong upper-level winds transport
• Can last well beyond the point the severe threat
  ends/diminishes

(image courtesy theweatherprediction.com)
32
Squall Lines and Bow Echoes

• Common features of bow echoes are book end
  vortices
• Mesocyclone vortices that form along curving ends
  of the bow echo
• Northern one can lead to tornado formation with
  cyclonic rotation

(image courtesy NWS Western Region)
33
Derechos

• Intense, often long-lived squall lines
• Technical definition a widespread and long
  lived windstorm that is associated with a band of
  rapidly moving showers or thunderstorms.
• Come in two types
• Serial
• Progressive

(image courtesy NWS WFO Louisville, KY)
34
Derechos

• Serial derecho extensive squall line with
  multiple embedded bow echoes
• Progressive derecho smaller-scale, often
  associated with just one bow echo

(images courtesy NWS WFO Louisville, KY)
35
Derechos

• Most prevalent during summer under ring of fire
  scenarios
• Shortwave disturbances around periphery of
  subtropical ridge ignite convection
• Moderate shear plus extreme instability leads to
  organization and growth of derecho structure

36
Quasi-linear Convective Systems

• A type of forward propagating MCS
• Similar to a squall line or derecho in structure
  and evolution
• Convection takes on the structure of a line with
  embedded discrete elements

(image courtesy NWS WFO St. Louis, MO)
37
Quasi-linear Convective Systems
(image courtesy NWS WFO St. Louis, MO)

• Semi-discrete elements within the QLCS often are
  accompanied by mesovortices and weak tornadoes
• Associated with MARC (Mid-altitude radial
  convergence) signatures in velocity data
• Key attribute rear inflow jet of 25 m s-1
• Form under conditions that have ambiguous signs
  for ultimate storm mode hybrid conditions

38
Line Echo Wave Pattern

• Characterized by an S shape on radar
• Takes on the appearance of a mesolow with
  meso-warm and cold fronts
• Southern bow echo enhanced wind damage potential
• Mesolow region enhanced low-level shear and
  tornado potential

(image courtesy Texas AM Univ.)
39
Splitting Storms and Storm Propagation

• What are the atmospheric conditions that promote
  storm splitting?
• What conditions favor the right splits?
• What conditions favor the left splits?
• What influences both discrete and linear storm
  motion?

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Storm Splitting

• Storms split due to non-linear supercellular
  dynamics!
• Updrafts tilt vorticity into the vertical,
  resulting in horizontal circulations on either
  side of the updraft
• Pressure perturbations are proportional to these
  circulations (p -?2)
• Rising motion is favored with pressure
  perturbations
• In the vicinity of the updraft weaker motion,
  corresponding to our split region!

41
Storm Splitting

• Generally, the right split is favored.
• This is due to advection of the updraft by the
  shear vector and the resultant pressure
  perturbations within the supercell.
• This arises out of cyclonic shear profiles.
• Tends to be those most associated with
  mesocyclone and tornado development!
• Left splitting storms often rapidly weaken under
  most conditions.

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Right Split Hodograph
Note well-defined cyclonic curvature in storm
layer.
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Storm Splitting

• Left moving splits are favored with
  counterclockwise shear profiles.
• Hodographs either curve entirely or partially
  counterclockwise
• Concordant with backing of the vertical wind
  profile at a location
• Oftentimes such storms are elevated in nature and
  rarely tornadic
• Such conditions are seen approximately 10 of the
  time

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Left Split Hodograph
Note anticyclonic curvature between 550-700 hPa
(example courtesy James McCormick)
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Storm Motion

• Two main factors
• Mean deep-layer (roughly 0-6 km) wind
• Deep-layer shear vector
• Linear storms generally move along the mean wind
  and deep layer shear vectors
• Discrete storms generally move in the same
  direction with a disposition to move to the right
  for right moving storms

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Storm Motion Example
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Where Weve Been

• With severe storms, weve already covered a lot
• One case study (5 February 2008)
• Sounding analysis for severe storms
• Synoptic-scale severe forecasting and analysis
• Storm modes, including supercell structures and
  linear convective systems

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Whats Still to Cover?

• Theres still a bit more to take into account,
  however
• Radar analysis rotation, convergence,
  divergence, elevated vs. low-level signatures,
  etc.
• More real-life examples in-class exercise,
  putting everything together
• Well get into these in the next few weeks.