Advisory and Research Group on
Geo Observation Systems and Services
Waveclimate.com
Satellite Data Sources
September 2004
waveclimate.com
Contents
1
Background and availability of wave and wind data ................................................................................... 1
1.1
1.2
1.3
September 2004
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Sensors, sampling, data ............................................................................................................................. 1
Coverage .................................................................................................................................................... 5
Current sources of wave and wind climate products ................................................................................ 7
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waveclimate.com
1
Background and availability of wave and wind
data
1.1
Sensors, sampling, data
Radar Altimeter
Radar altimeter sends pulse downward and measures return.
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waveclimate.com
Radar altimeter measures orbit height, significant wave height and wind speed.
Scatterometer
Wind scatterometer obtains wind vector from directional dependence of radar backscatter.
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2
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SAR, Synthetic Aperture Radar
SAR is an imaging radar; positions are obtained from delay and doppler shift of return.
Example of swell waves visible in a SAR image (nearshore wave propagation)
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Wave spectral data from ERS SAR wave mode backscatter image
1.
2.
3.
4.
5.
6.
7.
8.
Compute spectrum of radar image (in wave number plane)
Calibrate spectrum using known properties of noise
Find coincident wind vector from scatterometer
Obtain wind-sea spectrum from wind; simulate image spectrum
Compare with measured spectrum; adjust wind-sea spectrum, …
When converged: compute swell spectrum by inverting a simplified model
“Gap” in SAR spectrum is filled by wave model data
Climate: significant wave height matched to radar altimeter data (consistency)
Examples of spectra retrieved from SAR imagettes, compared to buoy and wave model data
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waveclimate.com
1.2
Coverage
Temporal coverage of missions/instruments
variables(s)
significant wave height
sensor(s)
radar altimeter
satellite(s)
Geosat
significant wave height and
wind speed
radar altimeter
wind speed and direction
scatterometer
ERS1/2, Topex,
Poseidon,
Jason,
GFO
ERS1/2
Wave spectral density and mean
direction in 25 frequency bands,
together with coincident wind
speed and direction
synthetic aperture radar
(SAR)
and
scatterometer
ERS1/2
period covered
from March 1985 to
Dec 1989
from Aug 1991 to Dec
2003
from Jan 1993 to Dec
2000
from April 1993 to Jun
2003
Data density altimeter: very uniform also near coast
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Data density wind ERS scatterometer: high except certain coastal zones (related to SAR image
mode acquisition)
Data density SAR wave mode: lower, in particular in semi-enclosed basins and some other
strips along coast ((related to SAR image mode acquisition)
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waveclimate.com
1.3
Current sources of wave and wind climate products
statistic for total seastate
histogram
prob. distribution of wind speed
prob. distribution of wave height
prob. distribution of mean period
prob. distribution of zero-crossing period
monthly distribution
monthly prob. distribution of wave height
monthly prob. distribution of wind speed
scatter table
joint prob. distribution of wind speed and wind direction
joint prob. distribution of sign. wave height and wind speed
joint prob. distribution of sign. wave height and mean period
joint prob. distribution of sign. wave height and zero-crossing period
joint prob. distribution of sign. wave height and wave direction
extreme value analysis
extreme value distribution of sign. wave height
extreme value distribution of wind speed
source of data
altimeter
altimeter
SAR
SAR
altimeter
altimeter
scatterometer
altimeter
SAR
SAR
SAR
altimeter
altimeter
Properties satellite wind and wave data
+
Accurate/reliable
Fairly wide range of parameters measured, also spectral
Spatial variability (e.g. geographic effects)
_
Irregular sampling: no persistence
For some areas/parameters sampling is not frequent enough
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