NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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Intercomparisons Among Global Daily SST Analyses

NOAA’s National Climatic Data Center

Asheville, NC, USA

Richard W. Reynolds (NOAA, NCDC)Dudley B. Chelton (Oregon State University)

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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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

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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Goldilocks & the 3 Bears

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

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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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

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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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

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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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?

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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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?

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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Moored Buoys with Good Daily DataFocus on Red Regions

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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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

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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• RSS higher than buoys and other analyses at middle and high frequencies

• RSS highest resolution details persist between IR satellite observations

Tropical West Pacific Auto SpectraAveraged over 10 buoy locations

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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Average Cross Correlation Analysis with respect to buoys

GS East Coast Trop. W Pacific

Cro

ss C

orre

latio

n

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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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%

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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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%

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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Buoy vs. Satellite Data in OI

S S S S SS S S S SS S B/S S SS S S S SS S S S S

• Buoy: Point measurement, good daily average

• Satellite: Spatial average, good snapshot

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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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%

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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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

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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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

NOAA Climate Observation Annual ReviewNOAA Climate Observation Annual ReviewSilver, Spring, MD Sept. 3-5, 2008Silver, Spring, MD Sept. 3-5, 2008

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The End