Observing Hurricane Winds From Space

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Observing Hurricane Winds From Space

• Christopher Hennon
• Department of Atmospheric Sciences
• UNC Asheville
• NEMAC Chocolate Fridays, October 26, 2007

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Hurricane Rita 9/21/05 1920 UTC
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How far out do the hurricane force winds extend?
How far out do the tropical storm force winds
extend?
How wide is the eyewall?
How strong are the winds within the outer
rainbands?
What are the maximum sustained wind speeds?
What are the maximum sustained wind gusts?
Where are the maximum winds occurring at the
surface in the storm?
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No Satellite/Aircraft Observations Max Wind 36
kt. No Hurricane Force Winds No 50 kt
.winds Weak Tropical Storm
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Importance of Dense Surface Wind Observations in
Tropical Cyclones

• Establish wind radii for watch/warning purposes
• Establish actual intensity of storm
• Better initialization of vortex in numerical
  models
• Better track forecasts
• Better intensity forecasts?
• Determine trends in intensity
• Contracting eyewalls, double wind maxima

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How Do We Fill in the Gaps?

• Dvorak Technique
• Estimate maximum intensity based on Infrared
  satellite image
• Fairly accurate
• Aircraft Reconnaissance Data
• USAF Hurricane Hunters
• Flight level winds, GPS Dropsondes, Stepped
  Frequency Microwave Radiometer (SFMR)
• NOAA P-3 Research Flights
• SFMR, flight level winds, tail doppler winds
• Satellite derived winds

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Active Techniques

• Scatterometer Sends a pulse of radiation
  through atmosphere to ocean surface correlate
  backscatter to surface wind speed and direction
• QuikSCAT (NASA)
• ASCAT (Europe)

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QuikSCAT

• Ku-Band (13.7 GHz) scatterometer
• Able to retrieve high resolution (25 or 12.5 km)
  surface wind speed and direction
• Cross swath width of 1800 km
• Accurate within 2 m/s for wind speeds of 2-20 m/s
  (4 - 40 kt)
• Assuming no rain

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Scatterometer measures surface capillary waves
(wavelengths 1cm), including their orientation
803 km
46 deg
53 deg
900km
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QuikSCAT can identify TC circulation centers that
may be hidden
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Rainfall Contaminates QuikSCAT Wind Retrievals

• Two-way attenuation of beam
• Enhanced backscatter
• Splash effects on ocean surface
• Dependant on true surface wind speed

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Ultra-High Resolution QuikSCAT
Hurricane Katrina, August 28, 2005
http//www.scp.byu.edu/data/Quikscat/HRStorms.html
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Passive Techniques

• SSM/I
• WindSat
• GOES Cloud Track Winds

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How Passive Technology Works

• Measures microwave emission from ocean surface in
  field of view
• Ocean surface, atmosphere, reflected energy off
  of surface
• Ocean surface Emissivity due to temperature,
  salinity, and roughness
• Roughness depends on surface wind speed

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SSM/I

• Special Sensor Microwave/Imager
• Flown aboard Defense Meteorological Satellite
  Program (DMSP) satellites since 1987
• Operates at 4 frequencies, dual polarization
  (vertical/horizontal)
• Wind speed only

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WindSat

• Wind direction and speed
• Fully polarimetric radiometer
• Senses 5 channels of radiation
• 6.8 GHz (SST), 10.7 GHz, 18.7 GHz, 23.8 GHz
  (water vapor), 37.0 GHz
• Wind retrievals in hurricanes not as robust as
  QuikSCAT

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Hurricane Felix (2007) Sept. 3, 2256 UTC Max.
Wind 115 kt.
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GOES Cloud Track Winds
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All available observations Maximum wind 129 kt.
(36 kt.) Hurricane force wind radii 65, 54,40, 65
nm (no hurricane force winds) Category-4
Hurricane (tropical storm)
HWind Display With All Obs
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Ongoing and Future Work

• SFMR instruments on all USAF hurricane hunter
  aircraft
• QuikSCAT nearing end of life
• New scatterometer that better corrects for rain
  is in the works (combines active and passive
  techniques)
• HiRAD
• SFMR-type instrument that will retrieve surface
  wind vectors in a swath rather than just nadir
• Aircraft mounted first, then on satellites later