Remote Sensing:Observing a BIG COUNTRYDavid Griffin & Edward KingCSIRO Marine and Atmospheric Research

David Griffin

It’s a big country, with a lot happening

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David Griffin

Forecasting currents is like weather forecasting

• People talk about using models to interpolate observations (time and space) and/or extrapolate to other variables• That’s true but remember: • 1) models only represent a subset (length- and time-scales) of

reality• -> some details cannot be forecast• 2) eddy-resolving models generate eddies spontaneously, i.e.,

creates spectrum• - > a critical number of obs are required before skillful forecasts

(and accurate hindcasts) can be made• What can satellites provide?

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Satellites are the principal means of making models track reality• Satellites are a cost-effective way of obtaining broad-scale

observations of a few key variables, continuously. • Especially cost effective for Australia. Remember that.• Sea level (-> geostrophic velocity), wave height, sea surface

temperature, salinity, gravity (->mass), colour (-> chl-a, bathymetry, etc)• But there’s a catch. The ‘oceanography’ needs to be carefully

teased from the raw data. Eg: ‘coastal altimetry’.• Hence the formation of space agency mission-specific science

teams.• We are invited to respond to the ESA Sentinel-3 AO (Nov)• We are already members of various STs. Typically in ‘spare time’.

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Altimeters measure lines of sea level anomaly

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Sea level anomaly: OI using 3 altimeters

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Why measure sea level?

• Because you can.• Because SLA(x,y) = f(T(x,y,z), S(x,y,z))• But there’s a catch: the Earth is lumpy (>100m). • GRACE and GOCE measure the geoid down to 100km scales.• Sea level = Range – orbit - geoid – path_delays – aliases• Sea level = Range – orbit – MSS + MDT – path_delays – aliases • 0.20m = 1350km – 1300km – 50m + 1m – 0.2m – 0.2m• Spectrum of errors: 1cm to 2m• Mostly well controlled. Under-sampling is the main problem.

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Altimetric sea level = steric height (+ barotropic)

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We read track data from NOAA/TUDELFT RADS

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Sea Surface Temperature

• Crucial for correcting drift in models• Meso-scale information mostly lost during assimilation• issues: cloud clearing, atmospheric corrections, geo-location,

diurnal variability• - > good validation information (when skies are clear)• Feature-track currents: a mix of good and bad• IMOS processes NOAA AVHRR and MODIS SST to GHRSST standard• Recently discovered: geolocation errors in CAPS• Microwave radiometers: low res but all-weather• Need to prepare for AVHRR replacements

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Ocean Colour

• SeaWiFS, MODIS, MERIS• Under-exploited for a decade or more• IMOS now processing MODIS (aqua) routinely, from L0 data• NASA SeaDAS 6.1 -> 6.4 algorithms. • Back-process pre-2011 soon

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Summary

• For modelling, altimetry, SST and Ocean colour are the mainstays• Gravity and salinity missions are the new kids on the block• Next talk: Synthetic Aperture Radar• Immediate future: Australian involvement in the ESA Sentinels. AO

announced in a month.• Australia – ESA collaboration agreement• Australia – India, Aust-China, …..

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David Griffin & Edward King

+61 3 6232 5244David.Griffin@csiro.au

oceancurrent.imos.org.au

CSIRO MARINE AND ATMOSPHERIC RESEARCH

Thank you

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