cfosat wind and wave observations from space: a french … · 2009. 6. 5. · α=cotθ−4tgθ− 1...
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CFOSAT
Wind and wave observations from space:
A French-Chinese mission
Scientific Pis:
Danièle Hauser (France): CNRS/Univ. Versailles
Liu Jianqiang (China): NSOA
and the Mission and Technical Team
Globwave Workshop, Brest, September 2007
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The Chinese French Oceanographic Satelllite CFOSAT
Two payloads
- Directional wave spectrum form SWIM (France)
- Wind scatterometer SCAT (China)
- Dedicated to ocean wind and waves obervation
- SWIM: ≈similar to the former SWIMSAT proposal to ESA in 2002
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Outline• Mission objectives
• Overall mission description
• Wave measurements
- Scientific requirements- Scientific requirements
- Proposed instrument (SWIM)
- Principle of measurements
- Preparation studies
• CFOSAT in the context of GLOBWAVE
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CFOSAT Objectives
1) Monitor at the global scale the wind and waves at the ocean
surface
⇒Improve wind and wave prediction and sea-state monitoring
=> improve knowledge and modeling of sea-surface processes
• sea-state evolution
• role of waves in the atmosphere and ocean,
• sea-ice properties and evolution in marginal ice zones,• sea-ice properties and evolution in marginal ice zones,
• coastal processes
• determination of ocean surface parameters by remote
sensing.
2) Opportunity for additional estimates of land surface
parameters (in particular soil moisture and soil roughness)
3) Opportunity for studying polar ice sheet characteristics
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Overall mission characteristics
-Recently agreed between China and France
-Launch planned in 2012-2013
-Platform and launcher under Chinese responsibility
-Ground segment shared between China and France
-SWIM designed and built by France
-SCAT designed and built by China
-Polar orbit (inclination ≈ 97°)
-Altitude 500 km approximately
-Near Real time transmission of data (within 3 hours)
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-Ku-Band
-Almost global coverage in one day
Two alternatives under study
Scatterometer on CFOSAT (Chinese part):
- Quickscat type (2 rotating pencil beams, with
incidence around 50°)
- intermediate between Quickscat and ERS (or
Ascat): rotating fan-beam with incidence from
about 20 to about 55°.
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•Measurement of directional wave spectrum at global scale with a
70x70 km spatial resolution
• Revisit better than 10 to 13 days (TBC) for latitudes > 35°
• Wavelengths to be measured : � ~ [70 - 500] m
• Variation of backscattering coefficient from 0 to 10° incidence
SWIM Scientific requirements
with absolute accuracy < 1dB
• Sea surface parameters to be derived :
- Significant Wave Height
- Wind speed
- Radial wave spectrum every 15° in azimuth
- Directional wave spectrum over scales of about 70 x 70 km
- Mean square slope of short waves and tentatively other
parameters of the slope pdf
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Instrument SWIM
Ku-Band radar(13.6 GHz)
–1 nadir beam (altimeter-type)
–5 off-nadir beams (2 to 10°), rotating over 360°, real-aperture
Altitude : 500 km
Incidences : 0-2-4-6-8-10°°°°3dB beamwidth: 2°°°°x2°°°°
Sequential in time
3dB beamwidth: 2°°°°x2°°°°
88km
17.7 x17.7km²
6 tr.min-1
Nadir :
-Hs
-Normalized Radar Cross-Section σ0
Off-nadir :
- Each 15° in azimuth, wave slope
spectrum from the modulation spectrum of
σ0
- σ0 for each incidence θ. σ0(θ) => pdf of
surface slopes
Expected products
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Principle for estimating wave spectra => similar to
STORM/RESSAC
Φ
θ
X(elevation)x
y
zZ
Y (azimuth)
m(X,Φ)
βθ
• In each azimuth direction, the normalized
radar cross-section σ0 is modulated by the tilt of
the long waves
=> Measurement of these modulations m(x,φ),
calculation of their spectrum Pm(k,φ)σ
Waves with
wave-number k
X(elevation)
Look direction
σ0
R
(θ)
Swath covering 2° incidence range
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Pm(k,φ) =2π
Lyα2k2F(k,φ )
• Linear relationship between modulation spectrum
Pm(k,φ) and wave slope spectrum k2F(k,φ)
Ly: azimuth footprint dimension
• Complete directional information using the 360° scans
α = cotθ − 4tgθ −1
cos2θ
∂ ln p
∂(tanθ),
where p is the slope pdf
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H= 500 km, Rotation= 6 rpm, incidences 6, 8, 10°
100
Surface patterns for incidence beam 6, 8 10°
track distance
Scale at which directional spectra will be retrieved
70 x 70 km by combining 3 incidences
90 x 90 km for 10° incidence
-100
-80
-60
-40
-20
0
20
40
60
80
-200 -100 0 100 200 300 400 500 600
Along-track distance (km)
Tour 2, 6°
Tour 3, 6°
Tour 4, 6°
Tour 5, 6°"
Tour 1, 6°
Tour 1, 8°
Tour 2, 8°
Tour 3, 8°
tour 4, 8°
Tour 5, 8°
tour 1, 10°
Tour 2, 10°
tour 3, 10°
Tour 4, 10°
Tour 5, 10°
Along-track distance
Across-track distance
Zone
1
Zone
2
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0,02
0,025
0,03
0,035
backscatter modulation radial spectrum in the direction of wave propagation (averaged over 15°)
Swell case, Hs=2mU=13 m/s
Spectral density of backscatter modulation
Direct simulation, without_noise
, λ, λ, λ, λpeak≈200m
0,02
0,025
0,03
0,035
backscatter modulation radial spectrum in the direction of wave propagation (averaged over 15°)
Swell case, Hs=2mU=13 m/s
Spectral density of backscatter modulation
Direct simulation, without_noise
Inversion, after correction from speckle and thermal noise
0,02
0,025
0,03
0,035
Spectral density of backscatter modulation
Direct simulation, without_noise
Inversion, after correction from speckle and thermal noise
Preparation studies (1): simulations of observations and inversions
Objectives : - extraction of directional wave
spectrum
- instrument performance &
optimization
Example of simulated wave field (4x4 km ; res 1m)
wind waves + swell
Simulator developed through a CNES/CETP
cooperation
-0,005
0
0,005
0,01
0,015
0,02
0 0,05 0,1 0,15 0,2
Spectral density of backscatter modulation
Wave number (rad/m)
-0,005
0
0,005
0,01
0,015
0,02
0 0,05 0,1 0,15 0,2
Spectral density of backscatter modulation
Wave number (rad/m)
-0,005
0
0,005
0,01
0,015
0,02
0 0,05 0,1 0,15 0,2
Spectral density of backscatter modulation
Wave number (rad/m)
Speckle level
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Other illustration: 2D plot mixed sea case with swell Hs=4m, )peak=200 m,wind sea with U = 3 m/s, 90°°°° angle between swell and wind seapropagation
Direct simulation
of the 2D modulation spectrum
2D modulation spectrum
after inversion
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Preparation studies (2): development of
assimilation schemes, study of assimilation impact
Two kind of studies (Meteo-France, CETP, BOOST, SHOM,
IFREMER):
- simulation of SWIM-like data (observations simulated
with perturbed analyzed fields)
- assimilation of data from the ENVISAT SAR wave
modemode
Spectral data: assimilation of main parameters (significant
wave height, mean direction, mean frequency) of partitions of
the 2D spectrum
Model: WAM, cycle 4, global (1 x 1 °), 25 frequencies (0.04-0.41 Hz), 24/36 directions, 6 hourly analyzed ECMWF wind-
fields
Assimilation scheme: Optimal Interpolation
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Description of the assimilation scheme
• Decomposition in partitions of the wave spectrum
(first guess and observation)
frequency [Hz]
0.19779
0.29668
0.39557
30
60300
330
0
15
20
Partition 2
frequency [Hz]
direction [degrees]
0.098894
210
240
90270
120
150
180
5
10
15
Partition 3Partition 1
Each partition is describedby Its mean parameters
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SWIMSAT synthetic caseSee also Aouf et al, JAOTech, 2006
-Simulation of a reference case (forecast without assimilation for a
period of 8 days )
-Simulation of synthetic “noisy” SWIM observations along the
orbit, including noise
-First guess perturbed wave field
-Assimilation for a period of 4 days, every 3 hours, then forecast
without assimilation
-Comparison between runs with and without assimilation
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Main results of the simulations
1 day after
beginning of
assimilation
1 day of forecast after
end of assimilation
Difference in Hs between runs with and without assimilation of
SWIMSAT synthetic data
2 days after end of
assimilation
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1 day after
beginning of
assimilation
1 day of forecast after
end of assimilation
Difference in mean wave period between runs with and without
assimilation of SWIMSAT synthetic data
2 days after end of
assimilation
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Comparison to assimilation of Hs only (case of an altimeter)
Difference of Hs , 2 days after end of assimilation, between runs
with and without assimilation of:
Spectral data
(SWIMSAT case)Non spectral data
(altimeter case)
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Summary on efficiency of assimilation
- On H10 (waves with periods longer than 10 s): 6% 1 day after assimilation) ,
larger than on Hs and larger for spectral data (SWIM) than for non spectral
data
- On mean wave frequency and mean direction: larger for spectral data (about
- Use of the assimilation index: quantifies the reduction due to assimilation,
of rms error between model and observations
Spectral data (SWIMSAT case) brings more information in
the assimilation than standard altimeter data (Hs), efficiency
of assimilation larger than for SAR data (less cutoff effect)
- On mean wave frequency and mean direction: larger for spectral data (about
9% and 4%, respectively) than for non-spectral (5% and 2% respectively)
- Efficiency decreases significantly when the cutoff is larger than 150 m (case
of a SAR)
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Impact of the assimilation of upgraded ASAR wave
spectra (cut-off de 244 m) in forecast mode
Mean wave periodWave height H10 (for
waves with period > 10 s)
28 November 2005 at 18:00 (UTC)
1-day forecast
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Impact of the assimilation of upgraded ASAR wave
spectra (cut-off de 244 m) in the forecast mode
Wave height H10 (for
waves with period > 10 s)Mean wave period
November 29 2005, 18:00 (UTC)
2-day forecast
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Using Jason-1 and GFO altimeter data as independent
verification data set at crossovers points
JASON-1 GFO
Sep-Oct-Nov-Dec 2005
���� Significant reduction of RMS error (AI) by 10.2 % and 7.6%
1
44
10
49
0
20
40
60
BIAS RMS
ASAR-SAR-NEW NOASSI
0
45
11
49
0
2040
60
BIAS RMS
ASAR-SAR-NEW NOASSI
Nb crossovers: 4421 Nb crossovers: 7806
in centimeters
in centimeters
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Effect of combined assimilation (ASAR+RA2)
in the forecast period
Comparison of Significant wave heights at Jason-1 orbit tracks
After two months of
assimilation
(Jan et Féb 2004)
forecast period of8
10
12
14
Assim
ilation index for SWH(%
)
comparison with Jason-1 in forecast period
ASAR-200-RA2RA2onlyASAR-240-RA2
forecast period of
three days
� The spectral information induces a longer impact in the
period of forecast
-18 -12 -6 0 6 12 24 36 48 600
2
4
6
8
Time in forecast (hours)
Assim
ilation index for SWH(%
)
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Concluding remarks on assimilation studies
� SWIM simulation : positive impact for H10, mean
period, direction; larger impact than in case of non-
spectral assimilation (Hs from altimeter)
�WAM with ASAR : positive impact for significant wave
height as shown by comparison with independent wave
data (Jason-1, GFO and NDBC buoys)
�Need for long term studies combining assimilation of
ASAR wave spectra and altimeter Ra-2 in WAM model
data (Jason-1, GFO and NDBC buoys)
�Need for better knowledge of correlation functions of
spectral parameters
�Need for independent data for validation, scores,…
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-CFOSAT will provide ocean winds and spectral characteristics
of surface waves (starting 2012-2013)
- coverage for wind should be global almost daily
- coverage for waves will be global over 10 to 13 days (to be
confirmed)
⇒CFOSAT must be used as one element of a « virtual »
CFOSAT in the context of a “Globwave system”
⇒CFOSAT must be used as one element of a « virtual »
constellation, with complementary characteristics :
•other altimeter missions (Sentinelle 3, Jason3,
SARAL/AltiKa,…)
• SAR missions (Sentinelle 1) with wave mode or image
mode over the ocean, Radarsat2, …
• Scat missions (METOP/ASCAT, Chinese HY2B??,
other missions of Quickscat type??)
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CFOSAT in the context of a “Globwave system”, (2)
-Real-time aspects will be under space agencies responsibility
-In addition: need for easy data access for science and
applications (not real-time)
-Need for in situ observations (wind, spectral characteristics of
waves) for validation of retrieval algorithms, assimilation
scores,..
-Need for models as observation integrating systems
(assimilation), as tool for open ocean forecast, as boundary
conditions and tool for coastal processes forecast and
analysis.
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Personal recommandation
Wind and waves from various sources (satellites, buoy,
ships,..) in the same data base or via same interface
For remote sensing: radar cross-sections also archived
Ancillary useful data: rain, current, SST,…
Need for more in situ observations of spectral properties
of waves.
Even wit the present 1D buoy network : access to
the 1D spectrum rather than to the parameters of a
the spectra would be useful
Need for more 2D spectra
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First option (pencil-beam, Quickscat type)
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Second option: rotating fan-beam
Overlap between adjacent scans
for large number of azimuth view
for each pixel
Nadir Point
footprint
Sub-satellite track
Resolution pixel
Resolution pixel
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Radio-Frequency part
Frequency 13.575 GHz
Peak RF power (TWTA) 120-150 Watt
Waveform Chirp (frequency modulated)
Bandwidth 200 to 320 MHz depending on incidence
Pulse duration 50 µs
PRF 2 to 6.5 kHz, depending on incidence
Antenna part
Incidence 0, 2, 4, 6, 8, and 101
Polarisation HH (TBC)
3 dB beamwidth 21 x 21
Rotation 6 rotations/minute (TBC)
Design 80 cm diameter passive offset parabole with six
Tentative design of the antenna part
Offset antenna (80 cm)
for the 5 offset beams +
small antenna (40 cm)
for nadir beam
Tentative specifications (to be confirmed)
Design
80 cm diameter passive offset parabole with six rotating feeding horns
On-board Processing Unit
Range Compression Deramp technique (TBC)
Range resolution after compression 0.47 to 0.75 m depending on incidence
Time integration 17 to 48 ms (depending on incidence
with correction of the advection using tracking-loop)
Real time processing
- nadir beam: distance, radar cross-section and significant wave height - off-nadir beams: backscatter power versus distance, time integration accounting for migration due to satellite displacement
Data transmission
Data rate/data Link 8 Mbits, X-Band
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α = cotθ −4tgθ −1
cos2θ
∂lnp
∂(tanθ)
∂lnp
∂(tanθ)relatedto
1
σ0
∂σ0
∂ tanθ,
α is wind dependent
Pm(k,φ) =2π
Lyα2k2F(k,φ )
σ0(θ) for different wind speeds
(from Chapron- and TRMM data)
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Observation locations for 4 days assimilation of SWIMSAT synthetic
Coverage for 4 days of assimilation (preliminary specification)