Surface Water and Ocean Topography Mission (SWOT)
Lee-Lueng Fu (Jet Propulsion Laboratory)
A snapshot of sea surface height anomalies from T/P and ERS altimeters
Spatial scales of the AVISO T/P-ERS merged data
Overlapping
at 150 km
210 km
km
Correlation of SSH time series as
function of spatial separation
A spatial scale computed as follows:
T/P-ERS
mapped
T/P
alongtrack
T/P
mapped
∞
L=
2
2
[
]
[
]
C
(
s
)
ds
C
(
0
)
∫
−∞
L= 210 km
Scales shorter than 150-200 km are
not resolved.
SSH wavenumber spectra (Ducet et al. 2000)
Small-scale Variability of the Ocean Unresolved
by Nadir-looking Altimeter
ground tracks of Jason (thick) and
T/P (thin) Tandem Mission
100 km scale eddies
resolvable by WSOA
100 km
10 km scale eddies
Resolvable by HM
Coastal currents have scales less
than 10 km
42.5º N
< 10 km
< 10 km
δ h ~ 5 cm
δ v ~ 50 cm/sec
41.9º N
< 10 km
Observations made by
ADCP offshore from the US
West Coast
T. Strub
SWOT Measurement Concept
Near-global coverage with
16-day repeat orbit
8
7
6
5
4
3
2
1
• 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
A SAR interferometry radar altimeter
Observations/cycle
SWOT Orbit Sampling
Altimetry SSH wavenumber spectrum
Power density (cm2/cycles/km)
T/P
Jason pass 132 (147
cycle average)
?
Noise level of HM for 2
cm measurement noise at
2 km resolution
T/P
?
Wavenumber (cycles/km)
Much reduced noise floor will enable the study
of the spectrum at sub-mesoscales which have
not been well resolved from existing data.
Stammer (1997)
Geostrophic velocity error spectrum
Velocity error (cm/s)2/cyc/km
100
50
25 km
σ= 2cm/7km
σ= 2cm/2km
(or 1 cm/7km)
σ= 1cm/km
(or 0.4 cm/7km)
Wavenumber (cyc/km)
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
Sub-mesoscale variability
Radius of
deformation
Radius of
deformation
Sub-mesoscale processes are poorly observed but
important to the understanding of the dissipation
mechanism of ocean circulation.
McWilliams (2006)
Hydrology applications
• Our knowledge of the spatial and
temporal distribution of surface
waters is poor
– Unconfined hydraulics of wetlands
– Lakes and reservoirs are globally
distributed but not measured
– Under-developed economic and political
infrastructures
• Hydrologic Science and Applications
Issues:
– Need to constrain water and energy cycle
models with surface water discharge and
storage changes, globally & consistently
– Improve understanding of flow hydraulics,
especially for flood hazards
– Trans-boundary water flows are poorly
known but critical for water resource
management
High-resolution ocean
surface topography
workshop
SWOT is based on the
technology of SRTM
and WSOA.
WATER Mission was
developed for hydrology
SWOT is recommended by
the Decadal Survey as a
combined mission for
oceanography and
hydrology. A Science
Working Group has been
formed with additional
~200 participants from ~30
countries, and growing!
Errors in coastal tide models up to 20 cm are revealed
from the Jason-T/P Tandem Mission.
What is the state of the art in
coastal tide modeling?
Andersen and Egbert (2005)
Besides the intrinsic science of internal
tides, they introduce 2-5 cm/sec error in
ocean current velocity. Is predicting
internal tides from models feasible ?
R. Ray/GSFC
Wet Tropospheric Corrections
• Land
contamination in microwave radiometer
observations
• OSTM has the potential for pushing the limit of
coastal distance to 10 km in favorable conditions
• What are the energy containing scales of water vapor
in coastal zones?
• How good is model analysis of water vapor in coastal
zones?
• What new technologies are feasible for making highresolution water vapor measurements?
Conclusions
• Using SAR interferometry, SWOT has the capability of
mapping ocean topography at 1 km resolution next to the
coast.
• Coastal tides: need models with much improved accuracies.
• 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. Need developing predictive models.
• Wet tropospheric corrections: Need to assess (1) the
performance of mesoscale meteorological analysis in coastal
zones, (2) new technologies for making high-resolution water
vapor measurements.