1. CMOD Observation Operator
High-Resolution Marine Wind Retrieval Using Synthetic Aperture Radar
Rick Danielson, Michael Dowd
Dalhousie University
Luc Fillion, Harold Ritchie
Canadian Meteorological Centre
Radar ( )
Cross-Section (dB)
Sea Level
Pressure (hPa)
Introduction
Methodology
Results
Conclusions
The desired surface wind field is a
weighted combination of the SAR obs
and model winds that minimizes a cost
function J (assuming Gaussian errors)
We choose the denominators (errors in
backscatter and winds) to best fit the
ship/buoy winds : given wind errors of
1.7 m/s, the best choice for SAR errors
appears to be 15% of the backscatter
(double that of Portabella et al. 2002).
As a result of the scatterometer wind
calibration effort (e.g. Stoeffelen 1998),
we have a relationship between wind
and backscatter (CMOD).
An a priori justification for the form of the
cost function (J) is that the difference
between the ship/buoy obs and either the
model or retrieved wind speeds is quite
Gaussian. This
also appears to
apply well to
differences in
backscatter, but
not as robustly
Scatterometers permit a retrieval of
marine winds at ~25-km resolution, but
that of synthetic aperture radar (SAR) is
much higher (~500m). However, a wind
calibration specific to SAR has yet to be
performed
If SAR wind errors could be quantified,
this would constitute an initial step
toward the assimilation of such data into
coastal models. Simple estimates of
these errors are derived here using a
variational approach
Use of SAR observations in a variational
context appears to improve the surface
wind analysis of a model. Monaldo et al.
(2001) further claim that 1-2 m/s RMS
wind speed errors can be achieved
In situ observations are also erroneous
(Stoeffelen 1998) because they are not
representative of large areas. In order to
avoid pseudobiases in SAR observations
all error types should be considered (i.e.
a SAR wind calibration is required)
Preliminary SAR obs errors are given
here using ship and buoy obs as a
reference. An equivalent weighting
scheme is used to produce these winds
As the “Retrieved-Observed” bias is smaller, the winds have improved (but only insofar as ship obs are correct).
Data
CMOD Observation Operator
Spatial Correlation
References
Collocations of ship/buoy, model, and
SAR data are assembled using 400
acquisitions in May-December, 2004.
Grids are processed to 1-km resolution.
The ship and buoy observations are our
reference dataset. Model winds are from
the GEM 15-km operational model
Ship/buoy obs are visually inspected and
SAR data is masked over land, sea ice,
and where winds are beyond 3 to 33 m/s.
Buoy observations within 90 minutes of
an acquisition are employed. These are
adjusted to the 10-m level and are
considered valid within 50 km. A total of
7497 ship/buoy-model-SAR collocations
are obtained.
Bragg scattering from capillary waves
varies with wind speed, wind direction,
SAR beam angle, and other processes
The empirical relation used here is
CMOD5 (Hersbach 2003)
CMOD has been derived using ERS
scatterometer data (ERS is vertically
polarized). Radarsat-1 is horizontally
polarized, so we employ a polarization
correction (Vachon and Dobson 2000)
Hersbach, H., 2003: An improved geophysical model function for
ERS C-band scatterometry, ECMWF technical memorandum 395
(available at
Monaldo, F. M., D. R. Thompson, R. C. Beal, W. G. Pichel and P.
Clemente-Colon, 2001: Comparison of SAR-derived wind speed with
model predictions and ocean buoy measurements, IEEE Trans.
Geosci. Remote Sens., 39(12),
Portabella, M., A. Stoffelen, and J. A. Johannessen, 2002: Toward an
optimal inversion method for synthetic aperture radar wind retrieval,
J. Geophys. Res., 107(C8), 3086, doi: /2001JC000925
Stoeffelen, A., 1998: Toward the true near-surface wind speed: Error
modeling and calibration using triple collocation, J. Geophys. Res.,
103(C4),
Vachon, P. W. and F. W. Dobson. 2000: Wind retrieval from
RADARSAT SAR images: Selection of a suitable C-band HH
polarization wind retrieval model. Can. J. Remote Sens., 26,
Length scales of wind speed and
backscatter error covariance are about
150 km and 100 km, respectively. Here,
the reference backscatter is defined by
ship and buoy winds and CMOD5..
Acknowledgements
SAR data were obtained through the Integrated Satellite Tracking of
Oil Polluters (ISTOP) Program and was provided by Radarsat
International (RSI). The Canadian Centre for Remote Sensing
made available code to manipulate this data. Bridget Thomas of
Environment Canada assisted with the vertical adjustment of obs.