Mapping Ocean Surface Topography With a Synthetic-Aperture Interferometry Radar:

A Global Hydrosphere Mapper

Lee-Lueng Fu

Jet Propulsion Laboratory Pasadena, CA, USA

x

seamount

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Geoid (1-100 m)Ocean surface topography (1-100 cm)

Ocean surface

Gravity anomaly ~ x Ocean current velocity ~ x

Ocean Surface Topography and Geoid

x

Progress in Satellite Altimetry for Measuring Ocean Variability

Circa 1984

Circa 2000

A snapshot of sea surface height anomalies from T/P and ERS altimeters

22 )0()( CdssCL

Correlation of SSH time series as function of spatial separation

A spatial scale computed as follows:

km

L= 210 km

SSH wavenumber spectra (Ducet et al. 2000)

T/P along-track

T/P-ERS mapped

T/P mapped

Overlapping at 150 km

Spatial scales of the AVISO T/P-ERS merged data

Scales shorter than 150-200 km are not resolved.

210 km

Asymmetry in ocean current velocity and sea surface slope (gravity anomaly)

Latitude

(v/u

)2

-60 -20

2

0

4

6Theoretical Noise

Morrow et al (1994)

Sandwell et al (2001)

Requirement

Requirement

-30-50

Satellite track

equator

A Global Hydrosphere MapperA SAR interferometry radar altimeter Near-global coverage with

16-day repeat orbit

• Same technique as WSOA – radar interferometry• Use of SAR to enhance the along-track resolution• 2 cm measurement precision at 2 km resolution• 1 micro-radian precision in mean sea surface slope at 2 km resolution • No data gap near the coast

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7

6

5

4

3

2

1 Nu

mb

er o

f O

bse

rvat

ion

Small-scale Variability of the Ocean Unresolved by Nadir-looking Altimeter

100 km

ground tracks of Jason (thick) and T/P (thin) Tandem Mission

100 km scale eddies resolvable by WSOA

10 km scale eddiesResolvable by HM

41.9º N

42.5º N

< 10 km

< 10 km

Coastal currents have scales less than 10 km

< 10 km

Observations made by ADCP offshore from the US

West CoastT. Strub

h ~ 5 cm v ~ 50 cm/sec

Errors in coastal tide models up to 20 cm are revealed from the Jason-T/P Tandem Mission.

Andersen and Egbert (2005)

R. Ray/GSFC

Besides the intrinsic science of internal tides, they introduce 2-5 cm/sec error in ocean current velocity.

McWilliams (2006)

Sub-mesoscale variability

Sub-mesoscale processes are poorly observed but important to the understanding of the dissipation mechanism of ocean circulation.

Radius of deformation

Radius of deformation

Altimetry SSH wavenumber spectrum

Wavenumber (cycles/km)

Pow

er d

ensi

ty (

cm2 /

cycl

es/k

m)

Noise level of HM for 2 cm measurement noise at 2 km resolution

Jason pass 132 (147 cycle average)

Stammer (1997)

T/P

T/P

?

?

Much reduced noise floor will enable the study of the spectrum at sub-mesoscales which have not been well resolved from existing data.

= 1cm/km(or 0.4 cm/7km)

= 2cm/2km (or 1 cm/7km)

= 2cm/7km

For the three cases, velocity error is reduced from 7.8 to 3.6, 1.3 cm/sec at 25 km resolution;

or 27, 15, 5 cm/sec at 10 km resolution

Wavenumber (cyc/km)

Vel

ocit

y er

ror

(cm

/s)2

/ cyc

/km

Geostrophic velocity error spectrum100 50 25 km

k -2 spectrum

TOPEX/PoseidonJason, or OSTM

HydrosphereMapper

Oceanic Processes Resolved by Various Missions

• SAR interferometry provides the capability of mapping ocean topography approaching 1 km resolution.

• Coastal processes: upwelling, jets, fronts, and biological-physical interactions. Coastal tides must be removed.

• Sub-mesoscale variability: important to the understanding and modeling of the dissipation mechanism for ocean circulation.

• Internal tides: sources of mixing in the ocean which is linked to the overall meridional overturning circulation. Also sources of errors for estimating ocean current velocity if not corrected.

• Determination of ocean current velocity and marine gravity anomalies with much improved accuracy.

• Sun-synchronous orbits should be avoided to ensure the observation of coastal and internal tides.

Conclusions

Internal tides from altimetry

Wavenumber spectrum

100 km

Scales are less than the T/P-Jason Tandem track spacings.

Ray & Mitchum (1997)

Besides the intrinsic science of internal tides, they introduce 2-5 cm/sec error in ocean current velocity.

10 km/day

Eddy drift velocity (vectors) and SSH standard deviation (color) determined from T/P-ERS

Fu (2006)

Sinking by gravity

Rising by mixing

Ocean Mixing and the Overturning Circulation

Ocean mixing is important in determining the strength of the meridional overturning circulation