Fundamentals of Radar in Severe Weather Detection

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Fundamentals of Radar in Severe Weather Detection

• Scott Lincoln
• ISU AMS Severe Weather and Radar Workshop
• November 10th, 2006

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Outline

• Observing Severe Weather Parameters
• The Basic Products of Radar
• Using Radar to Indicate Severe Weather
• Radar and Ground Truth
• GR2Analyst Case Studies
• Warning Workshop Simulation

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Observing Severe Weather Parameters

• Weather Balloons
• Record and send back information on
• Temperature
• Pressure
• Dewpoint
• Location
• Allows inference of wind direction, wind speed,
  and height above ground level
• This information is put together to make a Skew-T
  Log-P Diagram

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Observing Severe Weather Parameters
TEMPERATURE
PARAMETERS
HODOGRAPH
TEMPERATURE / DEWPOINT UNITS
DEWPOINT
WIND BARBS
PRESSURE
http//www.rap.ucar.edu/weather/
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Observing Severe Weather Parameters

• Skew-T Definitions
• Lapse Rates
• ?Temp/?Height
• Dry air cools more quickly than saturated air
• As saturated air cools, water vapor condenses
• Condensation is a warming process, therefore
  causes the saturated air to cool more slowly
• Parcel Path
• Path taken by a parcel of air if it starts at
  the surface of the earth and is raised upward
  into the atmosphere. Follows a different lapse
  rate depending on if it is dry or saturated.

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Observing Severe Weather Parameters

• Skew-T Definitions
• Lifted Condensation Level (LCL)
• Dry air from the surface lifted and cools at the
  dry lapse rate
• Temperature of the parcel reaches the dewpoint
  temperature of the parcel
• Parcel of air now cools at the moist lapse rate
• This changeover is called the Lifted Condensation
  Level
• Is a good indicator of cloud bases of developing
  storms
• Lower LFCs are generally more favorable for
  storms to form

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Observing Severe Weather Parameters

• Skew-T Definitions
• Level of Free Convection (LFC)
• Hot air is more buoyant than surrounding cooler
  air
• In most cases lifted air parcels from the ground
  cool off to temperatures below that of the
  environment at mid and upper levels (Stable
  Airmass)
• If surface temperatures and dewpoints are high
  enough to allow the lifted parcel to become
  warmer than the environment at mid and upper
  levels, the parcel can become buoyant and
  continue upward without forcing
• The point at which no forcing becomes necessary
  for the parcel to accelerate upward is called the
  Level of Free Convection
• Lower LFCs are generally more favorable for
  storms to form

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Observing Severe Weather Parameters
SATURATED LAPSERATE CURVED GREEN LINES
DRY LAPSERATE STRAIGHT RED LINES
PARCELPATH
THIS IS A STABLE AIRMASS -No LFC
LCL
http//www.rap.ucar.edu/weather/
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Observing Severe Weather Parameters

• Skew-T Definitions
• Convective Available Potential Energy (CAPE)
• Sum of the amount of energy available to storms
  based on how much warmer the parcel path is
  compared to the environmental temperature
• Higher CAPE generally means higher chance of
  severe weather
• Inversion
• When the environmental temperature increases with
  height
• On a Skew-T this would be the environmental
  temperature generally turned to the right at more
  than a 45 angle
• Inversions can block rising air, but can be
  overcome by incoming weather disturbances
• An inversion becomes a cap when it inhibits
  convection

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Observing Severe Weather Parameters
PARCELPATH
VERY UNSTABLE AIRMASS
CAPE
0 LINE (Black)
Frzg Level
INVERSION
LFC
LCL
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Observing Severe Weather Parameters

• Skew-T Definitions
• Speed Shear
• Speed shear is change in wind speed with height
• Good speed shear generally means increasing
  wind speed as a parcel rises from the surface to
  the mid-levels
• Directional Shear
• Directional shear is change in wind direction
  with height
• Good directional shear change of about 90 of
  direction from the surface to the mid-levels
• Surface winds out of the south or south-southeast
  under mid-level winds out of the wind are
  generally considered ideal directional shear
  conditions

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Observing Severe Weather Parameters
WSW _at_ 45 knots
Wind direction change of about 50 - 70
Wind speed change of about 25-30 knots
S to SSW _at_ 15-20 knots
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Observing Severe Weather Parameters

• Severe Weather Parameters Summary
• More Favorable for Severe Weather
• More CAPE
• More directional shear
• More speed shear
• Lower LCLs
• Lower LFCs
• Higher Dewpoint

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The Basic Products of Radar
Volumetric Radar Display - Today
Simple Radar Display 1960
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The Basic Products of Radar

• How Radar Works
• When hit by electromagnetic radiation,
• objects re-emit it in all directions

Distance from the radar site is determined by
time taken for the re-emitted wave to reach the
dish More energy sent back from a particular
area generally means heavier rain or hail
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The Basic Products of Radar

• How Radar Works
• Moving objects change the frequency of re-emitted
  waves depending on movement
• towards or away from the radar

Higher frequencies correspond to towards, lower
frequencies correspond to away
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The Basic Products of Radar
Base Reflectivity Assigns a value in DBZ to a
particular area based on how much energy is
re-emitted to the radar unit from that point.
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The Basic Products of Radar
Base Velocity Assigns a wind speed and the
direction of toward/awa to a particular
area. Uses the doppler effect to determine
direction towards or away from the radar unit,
as well as the speed in knots.
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The Basic Products of Radar
Vertically Integrated Liquid (VIL) Estimates the
mass of water above an area the size of a square
meter on the ground
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The Basic Products of Radar
Vertically Integrated Liquid Density
(VILD) Turns VIL into a density by dividing VIL
by the height of a storm. Estimates chance of
heavy rain and hail better than VIL. Short
storms with the same VIL as a taller storm
generally contain larger hail.
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The Basic Products of Radar
Hail Algorithms Probability of Severe Hail
(POSH) An algorithm that uses the mass of a
storm above the freezing level and determines
the likelihood that such a storm would produce
hail larger than 0.75 in diameter. Maximum
Estimated Hail Size (MEHS) An algorithm that
uses the mass of a storm above the freezing
level and determines the size of the largest
hail stones expected under those conditions.
What is shown on the radar display represents the
size of the hail core aloft, generally takes 2-3
radar scans for that hail to reach the ground
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The Basic Products of Radar
Storm Relative Motion Velocity Removes the
direction and speed of storms from the velocity
image. Makes it easier to to see areas of
rotation.
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The Basic Products of Radar
Spectrum Width Displays a variance in the values
of velocity for a given area. Can emphasize areas
of sharp changes in wind direction and speed, as
well as small areas of rotation under certain
conditions.
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The Basic Products of Radar

• Volume Coverage Pattern (VCP)
• Each NEXRAD unit can be switched to different
  scanning patterns that scan
• the sky at different speeds and with varying
  elevation angles.
• Two Basic Types
• Clear Air Mode
• Precipitation Mode

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The Basic Products of Radar

• Volume Coverage Pattern (VCP)
• Clear Air Mode
• VCP 31 and VCP 32
• Used for periods when no major precipitation is
  occuring, such as light snow or very light rain
• 12 minutes to complete a volume scan of the
  atmosphere
• The most sensitive to precipitation and other
  things

VCP 3132
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The Basic Products of Radar

• Volume Coverage Pattern (VCP)
• Precipitation Mode
• VCP 11, VCP 12, VCP 21, and VCP 121
• Used for periods when heavier precipitation is
  occuring
• Fastest volume scan takes 4 minutes 6 seconds,
  longest takes 6 minutes

VCP 1112
VCP 21121
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Using Radar to Indicate Severe Weather
MEHS Display in GR2Analyst
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Using Radar to Indicate Severe Weather

• Weak Echo Region
• Areas of reflectivity suspended above areas of
  lower or absent reflectivity
• Representative of the location of the storm
  updraft
• Good indication of a strong/severe storm

WER
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Using Radar to Indicate Severe Weather

• Bounded Weak Echo Region (BWER)
• Weak echo region that is surrounded on all sides
  by higher reflectivity
• Like a cave in the storm due to a very strong
  updraft
• Good indication of a severe storm and is a
  supercell characteristic
• BWERs can be found by taking two cross-sectional
  storm slices or by using a volumetric radar
  display.

WER
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Using Radar to Indicate Severe Weather

• Hook Echo
• Rotation in the weak echo region pulls a lower
  reflectivity area into the storm and a higher
  reflectivity area out of the storm
• A fairly typical indication of a supercell storm
• Velocity Couplet
• Two radar pixels of strong winds in opposing
  directions side by side
• An indication of strong rotation and possible
  tornado formation if conditions are favorable

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Using Radar to Indicate Severe Weather

• Hidden Rotation Signatures
• Not always an obvious hook echo, but velocity
  image shows rotation
• May develop quickly, or may not appear to be a
  hook that is not associated with tornadic
  rotation

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Using Radar to Indicate Severe Weather

• Bow Echo
• Indication on radar of strong/severe straight
  line winds
• Strong downdraft occurs that begins pushing part
  of a storm or line of storms ahead of other
  storms, resembles a bow

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Using Radar to Indicate Severe Weather

• Hail Spike (TBSS)
• Indication on radar of severe hail
• Large hail in a storm reflects the radar beam
  into the ground, some of that energy is reflected
  right back to the hail and then back to the radar
  unit
• The energy reflected back to the radar unit
  usually shows up where no precipitation is
  actually occurring
• Shows up often in tilts above the lowest tilt of
  the volume scan

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Using Radar to Indicate Severe Weather

• Outflow Boundary
• Indication of a change air density and
  temperature from a storms rain-cooled downdraft
• Outflow boundaries can cut off the inflow to
  another storm, causing it to weaken
• Outflow boundaries can trigger more convection
• Outflow boundaries can sometimes affect tornado
  formation

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Using Radar to Indicate Severe Weather

• Outflow Boundary
• Boundary can indicate location of strong winds in
  a storm outflow or along a gust front
• Act similarly to a cold front, lifting air in
  front and causing precipitation to fall along and
  behind the boundary

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Radar and Ground Truth
Woodward, IA Tornado The chaser and radar
perspectives
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Radar and Ground Truth

• Classic Supercell
• Core of hail and heaviest rain in front of the
  rotation
• The most common type of supercell thunderstorm

Simon Bishop, 2001
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Radar and Ground Truth

• High Precipitation Supercell (HP)
• Core of hail and heaviest rain behind and
  wrapping around the rotation
• Tornadoes from HP Supercells are very difficult
  to see

Greg Stumpf, 1997
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Radar and Ground Truth

• Low Precipitation Supercell (LP)
• Usually a lack of hail and heavy rain in the
  thunderstorm cell, but is a supercell because of
  rotating updraft
• Usually form in areas of dry low levels and low
  CAPE values

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Radar and Ground Truth

• Rear Flank Downdraft (RFD)
• Area of downward motion behind the main updraft
  that makes up the hook echo
• RFDs rotate around the updraft and can contain
  very strong winds

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Radar and Ground Truth

• Tornado Formation
• Wall cloud forms because of the reduction in
  pressure and cooling air beneath the rising the
  air of the updraft
• Wall clouds begin to rotate when rotation
  increases at the low levels

Mike Hollingshead
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Radar and Ground Truth

• Tornado Formation
• Within as little as one radar scan, quickly
  increasing rotation near the ground can produce a
  funnel cloud or a tornado

Mike Hollingshead
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Radar and Ground Truth

• Tornado Formation
• The chaser observing this tornado is not in an
  ideal location. He is situated in the hail core
  looking to the southwest.
• The tail cloud points from the updraft/tornado to
  the rain shaft

Mike Hollingshead
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Radar and Ground Truth

• Non-Supercell Tornadoes
• Often called landspouts, these are officially
  considered tornadoes but cause far less damage
• They must be connected to the cloud base but are
  not generally associated with large scale
  rotation in a thunderstorm

www.ontariostorms.com
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GR2Analyst Edition

• Goal
• Learn how to use GR2Analyst by
• Setting up a live polling source
• Personalizing the GIS Settings
• Examining cases of severe weather