Radar and severe weather:

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Radar and severe weather

• Tornadoes
• Supercell morphology
• Radar detection
• Mesoscale convective systems
• case study 10 Feb 98 - synoptic situation
• Symmetric and asymmetric squall lines
• Squall line formation
• Severe vs non-severe squall lines
• Wind shear and longevity
• Leading convective line and trailing stratiform
  region why ?
• Bow echoes

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Supercell (rotating updraft)
tornado
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hook echoes, and bounded weak-echo regions
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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.
• Dynamically defined as a storm updraft coincident
  with a vort max

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anvil
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How does the (bounded) weak echo region (WER)
form ?
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High resolution reflectivity image of an occluded
tornado

• View of a tornado as seen from the Doppler on
  Wheels (DOW) - image is about 2x2 km

forward flanking line
clear slot
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Newcastle tornado, 110 km from radar
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Doppler-radar view of a mesocyclone
Mesocyclones and tornadoes
Radar down this way
Radar down this way
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Doppler-radar based tornado detection

• Ground-based Doppler radars generally do not see
  a tornado.
• Most tornadoes are associated with mesocyclones.
• The higher the radar-estimated mesocyclone
  vorticity, the more likely it is to find a
  tornado on the ground.

Most wall clouds (seen by people) are at the base
of a mesocyclone (as seen by radar)
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Reflectivityradial velocity
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Find the mesocyclones
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Gate-to-gate shear
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Typical mesocyclone evolutions prior to observed
tornado touchdowns spinup rate
mesocyclone
height
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Tornado probability(as a function of mesocyclone
characteristics)
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How likely is it that a tornado is found below a
radar-observed mesocyclone ? Is a tornado more
likely when the mesocyclone is found to be ...

• closer to the ground OR higher up
• deeper OR just on one elevation scan
• smaller in diameter OR larger
• longer-lived OR more transient
• more intense (larger max rotational velocity) OR
  less intense

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Mesoscale convective systems

• Radar reflectivity loop, Squall Line with
  Straight-Line Winds on 15 April 1994 in Missouri
• Radar reflectivity loop, squall line initiation,
  with redevelopment along gust front, and
  maturation to form a trailing stratiform region,
  25 June 1997 in Illinois

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1. Case study on 10 Feb 98 several squall lines
in SE Texas produced flash floods, hail and wind
damage, and a couple tornadoes
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SHV
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2. Symmetric vs non-symmetric squall lines
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A symmetric squall line (2D) Identify
the convective line, the transition region, the
trailing region of stratiform precip
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an asymmetric squall line (3D)
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3. Formation
4 ways in which squall lines seem to form (view
broken line formation here)
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PPI Time Series (WSR-88D)
2031 UTC on 20 June 1991
Broken line of convective cells
Dominant component of shear is directed WNW-ESE
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2051 UTC
Two fine lines can be seen eminating from the
convective band.
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2111 UTC
Gust front propagation
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2131 UTC
New cell growth is evident as the gaps between
cells begin to fill
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2245 UTC
System is beginning to develop a region of
stratiform rain on its upshear side.
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4. Severe versus non-severe squall lines
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5. Wind shear and squall longevity
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For squall lines, it is the low-level shear
normal to the line that matters most
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Shear and squall longevity

• When the low-level, line-normal wind shear is
  moderate-to-large, then the shear generated by
  the storms cold pool may become equal opposite
  to the ambient shear.
• This supports upright lift, which triggers new
  convection and makes the squall long-lived.

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6. About half of all squall lines have a leading
convective line and a trailing stratiform region.
Why ??

• insight arose from Doppler radar data revealing
  the internal flow structure.
• We ll look at a famous symmetric squall line,
  the 10-11 June 1985 case

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Transition Zone
Convective Cells
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Wake Low
divergence
convergence
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5.7 degrees
20 km elevation
Melting Level
Cells are developing ahead of low level
reflectivity
Anvil
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Strong Divergence from convective cells
Mid-Level Rear Inflow
Upper-Level Outflow
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Mature
Developing
Dissipating
Melting Level
Divergence from Convection
Rear to Front Descending Jet
Outflow from Downdrafts
Front to Rear Ascending Motion
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Conceptual Model of Mature Stage Structure
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Why a trailing stratiform region (TSR) ? Summary

• Squall lines with a TSR appear to have, in the
  TSR, a broad ascending front-to-rear flow and a
  more jet-like descending, dry rear-to-front flow.
• Dynamically, there seems to be a symbiotic
  relationship between the convective line and the
  TSR ice crystals pumped into the upper
  troposphere in the convective line are carried to
  the rear by the FtR flow, and they continue to
  grow their because of ascent. They then fall thru
  the RtF flow which they cool by melting and
  evaporation. The cooled RtF flow then enhances
  the convectively generated cold pool below the
  convective line, which strengthens the gust front
  and sustains the convective line.

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7. Bow echoes

• A bow echo is a (part of a) squall line that bows
  out
• Many bow echoes produce straight-line wind damage
  - those are called derechoes
• Bow echo squall lines form with strong speed
  shear
• Rear-downdrafts transport UL momentum to the
  surface
• Damaging winds are the result of the rear inflow
  jet, combined with convective downdrafts

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8. Mesoscale Convective Complexes
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MCC - definition

• MCCs are defined by means of satellite IR
  imagery
• total size the lt-32C area must be at least
  100,000 km2
• core size the lt-52C area must be at least
  50,000 km2
• duration total and core size criteria must be
  met for at least 6 hours
• shape eccentricity (ie total size minor/major
  axis) must be at least 0.7 at the time of maximum
  extent
• (from Maddox et al 1982)

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Cloud top temperatures as low as -75C
Enhanced infrared satellite image, showing an MCC
over the central Plain States
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Enhanced IR image of the same MCC
This MCC probably has a squall line along its
southeastern flank
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Mesoscale Convective Complexes

• MCCs may be large squall lines, with a large
  trailing anvil (ie the stratiform region).
• MCCs usually produce severe weather, esp.
  flashflooding.
• The are long-lived, as long as 3-4 days.
• They develop during the afternoon and reaching
  max development at midnight.
• They form in a uniform (barotropic) environment
  with little wind shear (except in the lowest
  10,000 ft).

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MCC's are the closest thing to hurricanes on land
-they often have a mid-level cyclonic
vortexThe two satellite IR images on the right
are just 5 hours apart(source Fritsch)
7/7/'82, 1130 Z
7/7/'82, 1630 Z