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New satellite missions for surface current,
waves, and air-sea interactions research
Town-Hall meeting :
What can be achieved ?
What are the needs and requirements of the ocean community ?
Speakers :
Fabrice Ardhuin (LOPS, France), PI for SKIM – EE9 pre-selected
Co-I for CFOSAT – launch in 2018
Christine Gommenginger (NOC, UK), PI for SEASTAR – EE10 candidate
Mark Bourassa (FSU, USA), co-I for WaCM – NASA proposal
Today's menu :
- common context, objectives & challenges
- some news about CFOSAT
- the SKIM mission
- the WaCM proposal
- the SEASTAR proposal
Q & A
SKIM SKIM in 10 slides. February 2018
1. Context
ESA « Earth Explorer » program
ESA's Living Planet Programme
- Sentinel missions
- Earth Explorer missions
Together with other satellites, EEs & Sentinels data archives will
be used to produce Essential Climate Variables
EEs are research missions
→ address key scientific challenges identified by
the science community
→ breakthrough technology in observing techniques.
science community involved right from the beginning :
- definition of new missions
- peer-reviewed selection process
8 EEs selected so far (opportunity missions ~ 250 M€, core missions ~ 450 M€):
EE1 : GOCE 2009-2013. EE2 : SMOS 2009-... EE3 : Cryosat 2010-...
EE4 : Swarm 2013-… EE5 : ADM-Aeolus 2018 EE6 : EarthCARE 2018
EE7 : Biomass EE8 : FLEX EE9 : FORUM or SKIM… TBD in 2019
EE10 : ??? → 21 proposals submitted in March 2018 (including SEASTAR)
SKIM SKIM in 10 slides. February 2018
1. Context
the U.S. decadal survey & program of record
SKIM SKIM in 10 slides. February 2018
1. Context
the U.S. decadal survey & program of record
5 missions
3 out of 6 to 8
SKIM SKIM in 10 slides. February 2018
2. Why we need (more) current measurements
- where is it all going ?
plankton, plastics, heat, salt, sea ice,
man overboard…
- How much and where wind works on ocean ?
- Wave heights, periods,and directions?
What impact on extreme sea level at coast ?
- How waves and currents interact ?
- How waves and sea ice interact ?
- from meters (waves, Stokes drift …)
to global scales (garbage patches ...)
SKIM SKIM in 10 slides. February 2018
2. Why we need (more) current measurements
Models give currents… but poorly constrained by
- sea level variability from altimeters
- gravimetry (GOCE / GRACE)
- in situ drifters (used in Mean Dynamic Topography)
Especially near & under sea ice … and in tropics,
SKIM SKIM in 10 slides. February 2018
2. Why we need (more) current measurements
Fu and Ubelmann (JTECH 2014).
SKIM SKIM in 10 slides. February 2018
2. Why we need (more) current measurements
What we have today
SKIM SKIM in 10 slides. February 2018
2. Why we need (more) current measurements
What we have today
HF radars can cover coastal regions up to ~ 200 km
Kim et al. (JGR 2011)
SKIM SKIM in 10 slides. February 2018
3. « New » technology
Doppler oceanography from space
Chapron, Collard & Ardhuin (JGR 2005)
SKIM SKIM in 10 slides. February 2018
3. « New » technology
Doppler oceanography from space
Zonal current component, 3-month average, Envisat data
Section at 120°W
From Collard et al. (SEASAR 2008)
See also Chapron et al. (JGR 2005),
Rouault et al. (JGR 2010)
SKIM SKIM in 10 slides. February 2018
3. « New » technology :
Doppler oceanography from space
SKIM SKIM in 10 slides. February 2018
3. New technology :
What we can sample
2021 : SWOT will make a big step
towards high resolution,
especially using back-scatter
(Rascle et al. 2017)
2025 : SKIM can give the right
combination & sampling to fully
map the mesoscale
(dx = 30 km, dt = 4 days)
a pathfinder for Doppler vector
Oceanography
~2025 : WaCM can provide
extended space & time + wind
→ synergy of 2 missions
Ideal combination for SKIM?
SWOT
+ SKIM
+ CIMR (Microwave radiometer
considered for future Sentinels)
These 2 could be in a “train” on the
same orbit with MetopSGB&
& SWOT « glitter »
WaCM ?
CIMR (microwave SST)?
SEASTAR ?
SKIM SKIM in 10 slides. February 2018
3. New technology
What we are measuring
- depends on radar band (C, Ku, Ka … )
- depends even more on incidence angle and (at incidence > 15° ) on polarization
- velocity contains some « wave bias ».
For C-band at 23° incidence this amounts to 30 % of wind speed
(this plot is from Chapron et al. JGR 2005).
Planned campaigns to address this :
- platforms (Yurovsky et al. 2018 … )
- ship-based (e.g. Laxague et al. …)
- airborne :
U.S. experiments
French October experiment (ESA-funded)
Next meetings :
7-10 May 2018 : SEASAR, ESRIN, Italy
10-12 October 2018 : Doppler Oceanography from Space, Brest, France
Sea surface KInematics Multiscale monitoring (SKIM) mission
Think Current
Model « truth » SKIM L2b SWOT current from SSH
SKIM SKIM in 10 slides. February 2018
1. What is SKIM ?
It is a combination of Ka-band
radar altimeter, disco ball, and speed gun …
+ + =
- « super-duper altimeter » : best ever flown
(Ka-band, 32 Khz PRF, 200 MHz bandwidth, SAR unfocused)
→ very low noise for sea level, wave height, ice freeboard …
- « disco ball » : fixed reflector (1.2 m parabola – in pink)
but turning « spotlights » (horn feeds on near focal point).
Result : radar beams dancing around ground track
Preliminary design :
1 nadir beam (classic altimeter)
7 other beams
at 6 and 12° incidence
- « speed gun » : Doppler analysis → surface currents, ice drift & wave orbital velocities.
SKIM SKIM in 10 slides. February 2018
2. How SKIM works
As the plate rotates on the
spacecraft, beams illuminate
different footprints.
Switching from one beam to
another we measure the
« radial current » (in color),
that is a projection of the
current vector (in black).
The result is a map of currents
with a full vector at least every
20 km in most of the swath.
SKIM also measures :
- wave spectrum
- sea level
See https://youtu.be/xtgAp_7EmAc
SKIM SKIM in 10 slides. February 2018
2. How SKIM works
… M for « Multiscale »… but sparse
… submesoscales…
Fooprint has same
size as Xband radar
SKIM SKIM in 10 slides. February 2018
2 : expected coverage
(for S1 orbit, better sampling for MetopSGB )
SKIM SKIM in 10 slides. February 2018
3. Science goals
Polar applications :
The Arctic is becoming a giant marginal ice zone (Aksenov et al., Marine Policy 2017):
wave & drift data needed by 2025 to observe this regime shift
.
→ Excellent revisit : example of 1-day coverage
- drift in marginal ice zone - Excellent nadir beam quality
→ freeboard → ice thickness
- wave spectra near the ice edge
SKIM SKIM in 10 slides. February 2018
4. SKIM and waves
But this is only after correcting for wave bias UWB
The Doppler current (projected on horizontal),
contains UWB due to wave orbital velocity :
Non-zero average of
sigma0 x Doppler
(Chapron et al. JGR 2005)
SKIM SKIM in 10 slides. February 2018
4. SKIM and waves
The Doppler current (projected on horizontal), contains UWB .
- 2 scale model in Chapron et al. (2005)
- Kirchoff approximation in Nouguier et al. (in review) :
UWB is proportional to « mean slope velocity » = 2G msv
(NB : msv happens to be Surface Stokes drift /2 ) → UWB = G US
For SKIM : G ~ 25 : validated with AirSWOT :
→
SKIM SKIM in 10 slides. February 2018
4. SKIM & waves
SKIM will resolve much shorter waves (20 m) than Sentinels (~ 150 m) CFOSAT (70 m)
fraction of resolved wave energy or Stokes drift
Measuring shorter waves
→ surface Stokes drift Uss
→ UWB bias for currents :
UWB = G Uss
SKIM SKIM in 10 slides. February 2018
5. SKIM effective resolution
So what are we resolving in terms of ocean scales ?
Here is for the Gulf Stream case : effective resolved wavelength Le = 65 km
L2b
SKIM SKIM in 10 slides. February 2018
5. SKIM effective resolution
Now for 3 regions from equator to Arctic:
in blue, effective resolved wavelength Le
Regions Equator Gulf-Stream Fram
Input Level-2a error (12& 6°
beams)
0.08 & 0.15 m/s 0.16 &0.20m/s 0.10 & 0.16 m/s
Level-2b error
(along&across track) and
e*ective resol.
0.03 & 0.05 m/s
89 km
0.09& 0.14 m/s
65km
0.11 & 0.14 m/s
59km
Level-3a error
(zonal&meridional)
and e*ective resol.
0.14 & 0.18 m/s
290km
0.23 & 0.24 m/s
71km
0.12 & 0.13 m/s
62km
Level-2b W/O instr. err. 53km 0.08& 0.12 m/s
51 km
0.10 & 0.13 m/s
53 km
Level-3a W/O instr. err. 0.14 & 0.17 m/s
277km
0.22 & 0.24 m/s
71km
0.08 & 0.10 m/s
44km
SKIM SKIM in 10 slides. February 2018
5. SKIM effective resolution
What these
scales looks
like ...
SKIM SKIM in 10 slides. February 2018
- SKIM adds new variable for Earth monitoring : total surface velocity
focus : strong currents (in particular tropics) & marginal ice zones
- leap forward for waves :
new applications from nearshore to solid Earth monitoring.
- complementarity with other missions : SWOT, nadir altimeters, SST …
Mission status : Phase A & B1 approved, started Nov. 15, 2017.
Choice between SKIM and FORUM (infrared Earth Radiation mission) in mid – 2019
will be ESA « Earth Explorer 9 »
Launch for SKIM or FORUM : mid-2025 on VEGA from Kourou.
Papers : Ardhuin et al. (in press) : Measuring currents, ice drift, and waves from space: the Sea
Surface KInematics Multiscale monitoring (SKIM) concept. doi : 10.5194/os-2017-65
Nouguier et al. :Sea surface kinematics from near-nadir radar measurements
Ubelmann et al. : Mapping surface currents from Doppler measurements...
http://tinyurl.com/SKIMonRG a
1.5 km
Conclusions
#SKIM4EE9 @FabriceArdhuin
SKIM SKIM in 10 slides. February 2018
Extra slide : How do we know it works ?
Thanks to AirSWOT...
AirSWOT measures SSH and range-resolved Doppler Doppler + ATI & XTI
Doppler in Ka band (12°
incidence) responds to
currents in range direction:
Nouguier et al. (revised)
2016 LASER experiment :
mapped drifter velocity
Winds and Currents Mission
30
EGU 2018: New satellite missions for surface current,
waves, and air-sea interaction research
WINDS AND CURRENTS MISSION
Mark A. Bourassa and Ernesto Rodriguez
With input from
Dudley Chelton, Dmitry Dukhovskoy, Tom Farrar, M. Mar Flexas,
Sarah Gille, Brian Haus, David Long, Rick Lumpkin,
Thomas Kilpatrick, Nikolai Maxeminko, Dimitris Menemenlis,
Steven L. Morey, Alexis Mouche, Dragana Perkovich,
Roger Samelson, Bryan Styles, Andrew Thompson,
Frank Wentz, and Shang-Ping Xie
& the rest of the WaCM team
Winds and Currents Mission
31
EGU 2018: New satellite missions for surface current,
waves, and air-sea interaction research
Winds and Currents Mission
Measurement Concept
Ka-band rotating pencil beam
Doppler scatterometer
Ku-band rotating beam
scatterometer
Winds measured from Ka/Ku
0 measurements at multiple
azimuth angles
Heritage: QuikSCAT,
RapidScat, OSCAT
Surface currents from Doppler
measurements at multiple
azimuth angles
Heritage: SAR Doppler,
Along-track interferometry
Temporal coverage achieved
by a roughly 1600 km swath
800 km
700 km
Winds and Currents Mission
32
EGU 2018: New satellite missions for surface current,
waves, and air-sea interaction research
The Wide Swath Improves Sampling
More accurate surface currents than SWOT
Relatively noisy, but much better sampling
True surface currents (ageostrophic)
Antenna
Spin
Mechanism
Electronics
Antenna
Feeds
1600 km
swath
700 km
Spacecr
aft
Bus
Winds and Currents Mission
33
EGU 2018: New satellite missions for surface current,
waves, and air-sea interaction research
Aircraft-borne DopplerScatt Works!
surface current U- vector
component (both images)
Left: DopplerScatt aircraft
observations
Down: NCOM model
Wind measurements work like
other scatterometers,
producing ‘equivalent netural’
winds
which are a stress scaled like a
wind.
Winds and Currents Mission
34
EGU 2018: New satellite missions for surface current,
waves, and air-sea interaction research
Designated
Measurements
GRACE
Atmospheric
Boundary Layer?
Winds and Currents Mission
35
EGU 2018: New satellite missions for surface current,
waves, and air-sea interaction research
Competed
Concepts
Winds and Currents
•Opportunities for 3 of 6 to be
selected in coming decade
•(Possibility that other concepts
needing tech development
could also be ready for
competition by end of decade,
so maybe 3 of 7 or 3 of 8.)
Winds and Currents Mission
36
EGU 2018: New satellite missions for surface current,
waves, and air-sea interaction research
WACM opportunities
Questions to
esas2017@nas.edu
http://sites.nationalacademies.org/DEPS/ESAS2017/index.htm
Winds and Currents Mission
37
EGU 2018: New satellite missions for surface current,
waves, and air-sea interaction research
Goals & Synergies
Recent results from multiple groups show that there is relatively
vigorous vertical transport in the ocean, air-sea interface and
atmospheric boundary-layer
It appears that key processes that can be investigated with wind
stresses and surface currents
Complimentary with SKIM
Two missions together improve temporal current sampling, SKIM
has wave spectra).
Science goals are related to spatial derivatives of wind (stress) and
currents
Stress is modified by currents and waves
Complimentary with SWOT (balanced vs unbalanced motion)
Compliments wind scatterometer constellation (ASCAT, OSCAT, etc.)
Useful for biology, sea ice, ocean transport, marine weather, ….
Winds and Currents Mission
38
EGU 2018: New satellite missions for surface current,
waves, and air-sea interaction research
Requirements & Conclusion
Resolve mesoscale (atmosphere and ocean)
5km grid space
For coupling with ocean features, which as slowly evolving compared
to the atmosphere, currents can be sufficiently accurately resolved with
0.5 m/s accuracy in speed
assuming Gaussian random vector component errors
which is a good assumption
Assumes wide swath sampling (1600 km wide, 200km nadir gap)
Wind vector component accuracy of 0.8 m/s is sufficient for most
applications, and is achieved with scatterometers
Conclusion: A pencil beam Ka-band conically rotating system with
Doppler capability can achieve these requirements
Winds and Currents Mission
39
EGU 2018: New satellite missions for surface current,
waves, and air-sea interaction research
WINDS AND CURRENTS MISSION
Mark A. Bourassa and Ernesto Rodriguez
With input from
Dudley Chelton, Dmitry Dukhovskoy, Tom Farrar, M. Mar Flexas,
Sarah Gille, Brian Haus, David Long, Rick Lumpkin,
Thomas Kilpatrick, Nikolai Maxeminko, Dimitris Menemenlis,
Steven L. Morey, Alexis Mouche, Dragana Perkovich,
Roger Samelson, Bryan Styles, Andrew Thompson,
Frank Wentz, and Shang-Ping Xie
& the rest of the WaCM team
Winds and Currents Mission
40
EGU 2018: New satellite missions for surface current,
waves, and air-sea interaction research
Doppler Current Measurement Concept
Doppler Phase Difference: = 2kr = fDt
Radial velocity component: vr = r/t = /(2kt)
Vector currents are estimated by combining
multiple ( 2) azimuth observations and
projecting vector to the ocean surface.
•Radars provide coherent measurements: both the phase and the amplitude of a scattered
signal are measured.
•The phase is proportional to the 2-way travel time (or range)
•The amplitude is proportional to the scattering strength of the target
•Doppler measurements, fD, are obtained by measuring the phase difference between
pulses, DF. Noise is reduced by combining multiple pulses.
Winds and Currents Mission
41
EGU 2018: New satellite missions for surface current,
waves, and air-sea interaction research
What is Scattering?
The dominant source of the radar
return occurs when there is coherent
addition of scattered energy (Bragg
scattering)
Bragg scattering predicts that surface
spectral components with
wavenumbers kB=2ksin(
-
) will
dominate the returns. ( is the surface
slope)
The Bragg resonant wavelengths are
given by
B =
/2sin(
)
It has been observed that scattering is
due to independent patches of waves
which satisfy the Bragg condition
Winds and Currents Mission
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EGU 2018: New satellite missions for surface current,
waves, and air-sea interaction research
What Velocity are we Measuring?
Additional platform motion and scattering effects can also appear.
Bragg group velocity can be estimated using the dispersion relation (for water waves)
and knowledge of the wind direction.
Orbital wave velocity component averages mostly out after averaging over kilometer
scales, with the non-cancelling part contributing to the surface current through Stokes drift.
Residual motion (Doppler current minus group velocity) is due to the surface current.
Surface scatterers (resonant
gravity/gravity-capillary waves
satisfying the Bragg condition)
motion is due to several effects:
group velocity of resonant patch;
orbital wave velocity;
advection due to surface
currents.
Winds and Currents Mission
43
EGU 2018: New satellite missions for surface current,
waves, and air-sea interaction research
Bragg Wavelength for Ka-band (~0.8 cm)
The dominant contributors to the
radar scattering mechanism for
WaCM will be capillary waves
(technically ultra-gravity waves)
at wavelengths of approximately
0.5cm.
This wavelength is weakly
dependent on incidence angles
for incidence angles above 45
o
.
Winds and Currents Mission
44
EGU 2018: New satellite missions for surface current,
waves, and air-sea interaction research
Doppler Currents- Experimental Basis
Experimental observations between
Doppler radial current and the sum of the
true current and the Bragg phase speed.
(Yurovsky et al.)
The Bragg speed can be removed when
it lies along the line, which is true for
incidence angles above ~50o at Ka-band.
The measured Doppler
velocity is the sum of the
surface current velocity and
the phase speed of Bragg
resonant waves.
In order to remove Bragg
contribution, it is necessary
to:
Measure the Doppler
from different azimuth
directions
Know the wind direction
(from scatterometer) to
infer net direction of
wave propagation for
Bragg scattering waves
Incidence Angle (degrees)
Observed Current Component
(m/s)
Doppler Current Component
(m/s)