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 Flow into the page Flow out of the page 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

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 km are not resolved. 210 km

Asymmetry in ocean current velocity and sea surface slope (gravity anomaly) Latitude (v/u) Theoretical Noise Morrow et al (1994) Sandwell et al (2001) Requirement Satellite track equator

A Global Hydrosphere Mapper A 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 Number of Observation

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 eddies Resolvable by HM

41.9º N 42.5º N < 10 km Coastal currents have scales less than 10 km < 10 km Observations made by ADCP offshore from the US West Coast T. 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

Altimetry SSH wavenumber spectrum Wavenumber (cycles/km) Power density (cm 2 /cycles/km) Noise level of HM for 2 cm measurement noise at 2 km resolution Jason pass 132 (147 cycle average) Stammer (1997) 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) Velocity error (cm/s )2/ cyc/km Geostrophic velocity error spectrum km k -2 spectrum

TOPEX/Poseidon Jason, or OSTM Hydrosphere Mapper 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