Radar / Satellite Meteorology

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Radar / Satellite Meteorology

• Lesson 5
• Section 305
• 02/29/2008

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Satellites
Liam Gumley, Space Science and Engineering
Center, University of Wisconsin-Madison
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History of Satellites

• 1957 Russia launched the first satellite,
  Sputnik
• 1959 Scientists at the Space Science and
  Engineering Center (SSEC) at UW-Madison conducted
  pioneering meteorological satellite research

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History of Satellites

• April 1, 1960 First satellite completely
  dedicated to satellite meteorology, named TIROS
  was launched
• TIROS Television and InfraRed Observational
  Satellite
• Life span of TIROS was 79 days

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Types of Weather Satellites

• There are two main types of weather satellites
• GOES Geostationary Operational Environmental
  Satellites
• POES Polar Operational Environmental Satellites
  (also referred to as LEO Low Earth Orbit)
• They are defined by their orbital characteristics

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Geostationary Vs. Polar Orbiting
http//cimss.ssec.wisc.edu/satmet/modules/sat_basi
cs/images/orbits.jpg
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Geostationary Satellites

• Geostationary satellites orbit as fast as the
  earth spins
• They maintain a constant altitude and momentum
  over a single point
• Approximate altitude 36,000 km (22,300 mi)
• In order to maintain their location, they must be
  located over the equator

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Geostationary Satellites

• Good Temporal Resolution Imagery is obtained
  and displayed approximately every 15 minutes
• In the case of severe weather, or hurricanes,
  passes over smaller areas are able to be obtained
  every 2-5 minutes
• Poor Spatial Resolution At a high altitude and
  fixed point, geostationary satellites can view a
  large, fixed area
• Equatorial regions are covered well, polar
  regions are covered poorly

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Polar Orbiting Satellites

• Polar orbiting satellites travel in a circular
  orbit moving from pole to pole
• Significantly closer to the earth (879 km 500
  miles) than geostationary
• Collect data in a swath as the earth rotates on
  its axis
• Sees the entire planet twice in a 24 hour
  period
• Takes 1 hour and 42 minutes to complete a full
  orbit

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Polar Orbiting Satellites

• Good Spatial Resolution Lower altitude results
  in higher resolution images and atmospheric
  profiles
• Poor Temporal Resolution Over any point on
  Earth, the satellite only captures two images per
  day

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Types of Satellite Images

• There are three widely used atmospheric windows
  (channels)
• Visible (0.6 microns)
• Infrared aka IR (10 12 microns)
• Water vapor (6.5 6.7 microns)
• Remember, 1 micron 1x10-6 m

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Visible

• Visible images record visible light from the sun
  reflected back to the satellite by cloud tops,
  land, and sea surfaces
• Equivalently a black and white photograph from
  space
• Visible images can only be made during daylight
• Dark areas Regions where small amounts of
  visible light are reflected back to space. i.e.
  forests, oceans
• Bright areas Regions where large amounts of
  visible light are reflected back to space. i.e.
  snow, thick clouds

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Infrared

• Infrared images record infrared radiation emitted
  directly by cloud tops, land, or ocean surfaces
• Cooler temperatures shown as light gray tones
• Warmer temperatures shown as dark gray tones

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Water Vapor

• Water vapor images record infrared radiation
  emitted by water vapor in the atmosphere
• Bright, white shades represent radiation from a
  moist layer or cloud in the upper troposphere
  (cold brightness temperature)
• Dark, gray/black shades represent radiation from
  the Earth or a dry layer in the middle
  troposphere (warm brightness temperature)

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Geostationary Satellite Coverage
http//www.ssec.wisc.edu/mcidas
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Geostationary Satellite Coverages
http//www.ssec.wisc.edu/mcidas
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Geostationary Satellite Coverages
http//www.ssec.wisc.edu/mcidas
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Geostationary Satellite Coverages
http//www.ssec.wisc.edu/mcidas
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Geostationary Satellite Coverages
http//www.ssec.wisc.edu/mcidas
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Geostationary Satellite Coverages
http//www.ssec.wisc.edu/mcidas
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LEO Satellite Coverage
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LEO Satellite Coverage
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Liam Gumley, Space Science and Engineering
Center, University of Wisconsin-Madison
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Liam Gumley, Space Science and Engineering
Center, University of Wisconsin-Madison
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Liam Gumley, Space Science and Engineering
Center, University of Wisconsin-Madison
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Visible Pros/Cons

• Pros
• Seeing basic cloud patterns and storm systems
• Monitoring snow cover
• Shows nice shadows of taller clouds (has a 3-D
  look to it)
• Cons
• Only useful during the daylight hours
• Difficult to distinguish low clouds from high
  clouds since all clouds have a similar albedo
  (reflect a similar amount of light)
• Hard to distinguish snow from clouds in winter

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IR Pros/Cons

• Pros
• Distinguishing higher clouds from lower ones
• Observing storms at night
• Distinguishing clouds from snow cover
• Cons
• Sometimes hard to distinguish between a thick
  cirrus and thunderstorms
• Makes clouds appear blurred with less defined
  edges than visible images

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RADAR
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History

• Radio Detection and Ranging
• Initially designed to track enemy ships and
  aircraft during WWII
• Rain often obstructed the detection of advancing
  enemy attackers
• Radar technology has greatly advanced since then,
  mainly due to high-speed computers

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How it works

• Similar to an x-ray machine that examines the
  body
• Radar uses the microwave region of the EM
  spectrum
• Wavelengths typically between 3 and 10 cm
• Sends out millions of microwaves which interact
  with frozen and non-frozen water particles
  throughout the atmosphere
• Rain, snow, hail, clouds, etc.

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How it works

• When microwaves encounter particles, their energy
  is scattered in all directions
• Some of this energy is returned to the radar
• Beam is typically 1 inclined and 1.5 wide, and
  rotates to see a full circle or sweep
• Typically sweeps out 200 nautical miles

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How it works

• Time difference between transmission and return
  of signal distance to the storm
• Strength of signal precipitation intensity
• Large/numerous particles reflect waves with
  greater intensity than smaller/fewer ones
• Intensity amounts referred to as echoes
  refers to the reflection of the waves off
  particles
• Like the echoes you hear when yelling in a canyon
• An image showing precipitation intensity is
  called a reflectivity image
• Intensity measured in decibels (dBZ)

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Reflectivity Image

• Intensity usually depicted on computer image by
  scaling various colors
• Reds/purples heavier precipitation
• Blues/greens lighter precipitation
• Black clear

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Types of Radar

• There are two main radar types
• Conventional Radar
• Doppler Radar

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Conventional Radar

• Conventional Radar
• Echoes displayed on radar screen
• Only produces reflectivity images
• Not only sweeps in circles, but also up and down
  to look at different levels and individual storms

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Doppler Radar

• Doppler Radar
• One of the most advanced versions of radar
• Does everything a conventional radar can do, PLUS
  more
• Operates on principle of the Doppler Effect
• Doppler Effect
• Usually described using sound waves
• Definition the change in the observed frequency
  of waves produced by the motion of the wave
  source and/or wave receiver

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Doppler Radar

• Example of Doppler Effect passing ambulance
• The movement of the ambulance alters the
  orientation of the waves
• Approaching siren pitch increases to higher
  frequency
• Passing siren pitch decreases to lower
  frequency
• Waves compressed
• together in direction
• of moving object

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Doppler Radar

• Meteorological use of Doppler Effect is very
  similar
• Movement of precip towards the radar increases
  the frequency of reflected pulses
• Movement away decreases frequency

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Doppler Radar

• Thus, Doppler analysis of winds can determine
  their speed and direction
• Allows us to determine internal structure of a
  thunderstorm
• Shown in Velocity (or Storm Relative Velocity)
  Images
• Note Doppler radar give us BOTH reflectivity
  and wind velocity images, as opposed to only
  reflectivity images with a conventional radar

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Velocity Image

• Wind velocities are also scaled by color
• Greens/blues winds moving toward the radar
  (i.e. inbound)
• Reds/oranges winds moving away from the radar
  (i.e. outbound)
• Measured in knots

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Doppler Radar

• Note Most Doppler Radars can operate in either
  a conventional or Doppler mode
• The weather community shows reflectivities
  (a.k.a. conventional data) to the general public
• Therefore, as a civilian, you will almost never
  see a velocity (a.k.a. Doppler wind field image)

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Doppler and Severe Weather

• Before Doppler Radar, tornado warnings could not
  be issued until the tornado was on the ground
• Now, we know that potentially tornadic
  thunderstorms often have characteristics
  recognizable using Doppler Radar
• Thus, we can give the general public an advanced
  warning before the tornado hits

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Doppler and Severe Weather

• Conceptual Model of a Tornadic Thunderstorm
  (a.k.a. Supercell)
• Seen in reflectivity images
• Curling of the reflectivity in back corner of
  storm is called a Hook Echo
• Most likely place for tornadoes to form

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Doppler and Severe Weather

• Supercell thunderstorms have a unique feature
  that separates them from all other thunderstorms
• The mesocyclone
• A mesocyclone is a 5-10 km wide region within the
  low- to mid-levels of a storm that is rotating
  counterclockwise
• This rotation forces air to rise from the
  surface, which can then be potentially twisted
  into a tornado under the right conditions
• Since mesocylones are associated with wind flow,
  they are recognizable using the Doppler velocity
  images
• Denoted by a small region of reds and greens
  directly adjacent to each other

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Doppler and Severe Weather

• Tornado Vortex Signature
• An image of a tornado on a Doppler velocity
  image
• Shows up as a small region of rapidly changing
  wind speeds inside a mesocyclone
• Velocity criteria
• Difference between max inbound and outbound
  velocities (shear) greater than or equal to 90
  knots at less than 30 nmi, or greater than or
  equal to 70 knots between 30 and 55 nmi

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Oakfield Tornado Case

• The Oakfield Tornado occurred on July 18th, 1996
  in Oakfield, WI
• F5 tornado developed
• Winds greater than 261 mph (most severe tornado
  possible on Fujita Scale)
• It was spawned from a classic supercell at 715
  pm
• Makes a very good case to study
• The following images are from the Doppler Radar
  in Green Bay, WI

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Oakfield Tornado Case

• Using reflectivity images, forecasters noticed
  the classic kidney-bean shape of a supercell
  beginning to form, as well as a well-defined Hook
  Echo

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Oakfield Tornado Case

• Using the velocity images, forecasters verified
  the presence of a mesocyclone (i.e. the storm was
  rotating)
• A TVS was then identified
• Notice that the TVS is in the same location as
  the Hook Echo

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Doppler and Severe Weather

• Sometimes, tornadoes are spawned from
  thunderstorms that are part of a squall line, and
  thus not supercells
• Squall line organized line of thunderstorms
• Usually develop along/ahead of cold front
• Severe winds are main threat, but tornadoes form
  occasionally
• Tornadoes associated with them tend to be weaker
  and shorter-lived than supercell tornadoes

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False Data

• Ground Clutter
• Portion of radar beam hits buildings, trees,
  hills
• Also can be due to dust, aerosols in the air near
  the radar
• Gives false indication that precip is present
• Radar location is in the black area surrounded by
  blue/green reflectivities

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False Data

• Anomalous propagation (AP)
• Occurs when temperature inversions are present in
  low-levels
• Radar beam bent into ground, returning strong
  signal
• Common during early morning hours after a clear
  night
• Again, no precip really present

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False Data

• Virga
• Radar detects precip occurring at upper levels,
  but not making it to the ground
• Precip quickly evaporates in dry air below cloud
• Precipitation is thus overestimated

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False Data

• Overshooting Beam
• Some precip can form from clouds with minimal
  height
• Beam may overshoot a large portion of the cloud,
  underestimating the intensity of the
  precipitation

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False Data

• Storm Interference
• Storms closest to radar may absorb or reflect
  much of the radar energy
• Leaves reduced amount of energy available to
  detect distant storms
• Underestimates precipitation

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False Data

• Beam Blockage
• Buildings, trees, mountains, etc. prevent
  portions of the radar beam from reaching
  precipitation that may be on the other side of
  them
• Underestimates precipitation

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False Data

• Wind Shear
• Falling precip may be displaced by the wind as it
  falls
• Some regions may be experiencing precip where the
  radar indicates nothing, and vice versa