Intercomparisons Among Global Daily SST Analyses

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Intercomparisons Among Global Daily SST Analyses
Richard W. Reynolds (NOAA, NCDC) Dudley B.
Chelton (Oregon State University)
NOAAs National Climatic Data Center Asheville,
NC, USA
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Introduction

• GHRSST has resulted in many high resolution SST
  products
• Differences in input data, grid resolution,
  analysis procedures
• Important differences in analyzed SSTs and
  analysis resolution
• The purpose here is try to identify analysis
  problems and determine which analyses are
  superior
• If an analysis resolution is set too fine, all
  you will get is noise

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Goldilocks the 3 Bears

• Need to help users find the SST analysis that is
  just right

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Satellite Data for 25 March 2006

• AVHRR day night
• Note data scarcity due to clouds
• AMSR day night
• Note swath width
• Missing data due to land, precipitation
• Day night differences not always due to diurnal
  warming

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4 Days of Buoy SST Data 0Z 22 March 22 - 0Z 26
March 26, 2006

• Resolution
• 1 minute
• Averaged to
• 1 hour
• 1 day
• Consider random sampling
• AMSR could approach 2 obs/day
• AVHRR would be lower especially in winter

Thanks to Bob Weller
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SST Analyses,1 January 2007

• RSS OI
• (1/11) grid
• NCEP RTG-HR
• (1/12) grid
• UK OSTIA
• (1/20) grid
• NCDC Daily OI (AMSR AVHRR)
• (1/4) grid
• This is a daily average
• What spatial scales are justified?

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SST Analyses,1 January 2007

• RSS OI
• (1/11) grid
• NCEP RTG-HR
• (1/12) grid
• UK OSTIA
• (1/20) grid
• NCDC Daily OI (AMSR AVHRR)
• (1/4) grid
• This is a daily average
• What spatial scales are justified?

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Moored Buoys with Good Daily DataFocus on Red
Regions
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Gulf Steam East Coast Auto SpectraAveraged over
5 buoy locations

• Spectra for Buoys and 4 Analyses
• Vertical axis is logarithmic
• Note
• All daily analyses lower than buoys at middle and
  high frequencies
• RSS slightly higher than other analyses

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Tropical West Pacific Auto SpectraAveraged over
10 buoy locations

• RSS higher than buoys and other analyses at
  middle and high frequencies
• RSS highest resolution details persist between IR
  satellite observations

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Average Cross Correlation Analysis with respect
to buoys
Cross Correlation
GS East Coast
Trop. W Pacific
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Gulf Steam East Coast Average Squared
CoherenceBetween Buoys and 4 Analyses 5 buoy
locations

• OSTIA drops below 0.28 at 0.2
  cycles/day (5 day period)
• Daily OI drops below 0.28 at 0.15
  cycles/day (7 day period)
• RTG-HR drops below 0.28 at 0.1 cycles/day (10
  day period)
• RSS drops below 0.28 at 0.05 cycles/day (20 day
  period)

95
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Tropical West Pacific Average Squared
CoherenceBetween Buoys and 4 Analyses 10 buoy
locations

• OSTIA drops below 0.28 at 0.2 cycles/day
  (5 day period)
• Daily OI drop below 0.28 at 0.1
  cycles/day (10 day period)
• Other analyses drop below 0.28 at
  0.05 cycles/day (20 day period)

95
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Buoy vs. Satellite Data in OI

• Buoy Point measurement, good daily average
• Satellite Spatial average, good snapshot

S S S S S S S S S S S
S B/S S S S S S S S S S
S S S
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N. American West Coast Average Squared
CoherenceBetween Buoys and 4 Analyses 22 buoy
locations

• OSTIA drops below 0.28 slightly at 0.45
  cycles/day
• Daily OI drop below 0.28 at 0.2
  cycles/day (5 day period)
• Other analyses drop below 0.28 at
  0.1 cycles/day (10 day period)

95
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Spectral Results - 1

• Squared Coherence between buoys and analyses
  drops below 0.28 at frequencies greater than
  0.2 cycles/day (5 day period)
• May be due to difference between spatial point
  value (buoy data) and spatial average (satellite
  data/analysis) satellite spatial coverage

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Spectral Results - 2

• In the tropics RSS has too much variability
  compared to buoys at middle and high frequencies
• To fix RSS more smoothing needed, for example
• Wider time window
• Temporal damping when no new observations
  available
• Longer spatial e-folding scales

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The End