• 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

• 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).

• 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

• 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)

• 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..

• 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.

High-Resolution Marine Wind Retrieval Using Synthetic Aperture RadarRick Danielson, Michael Dowd

Dalhousie UniversityLuc Fillion, Harold Ritchie

Canadian Meteorological Centre

Introduction Methodology

Data Spatial Correlation ReferencesCMOD Observation Operator

Results Conclusions

Hersbach, H., 2003: An improved geophysical model function for ERS C-band scatterometry, ECMWF technical memorandum 395 (available at www.ecmwf.int/publications)

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), 2587-2600

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:10.1029/2001JC000925

Stoeffelen, A., 1998: Toward the true near-surface wind speed: Error modeling and calibration using triple collocation, J. Geophys. Res., 103(C4), 7755-7766

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, 306-313

Radar ( )Cross-Section (dB)

Sea LevelPressure (hPa)

AcknowledgementsSAR data were obtained through the Integrated Satellite Tracking of

Oil Polluters (ISTOP) Program and was provided by RadarsatInternational (RSI). The Canadian Centre for Remote Sensing

made available code to manipulate this data. Bridget Thomas ofEnvironment Canada assisted with the vertical adjustment of obs.

• 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).