Validation of wind retrieval algorithms from spaceborne SAR

The exploitation of Synthetic Aperture Radar (SAR) imagery as a tool to obtain geophysical information of the marine environment has been object of intense studies by the scientific community in the recent years. Several methods have been proposed to retrieve quantitative estimates of ocean wave spectra and of wind speed and direction at a given reference height from SAR imagery. These tools appear very interesting in view of the future dual-polarised ASAR instrument onboard the European ENVISAT satellite, whose launch is planned for June 2001.

In preparation of ENVISAT, we planned an experiment to validate wind retrieval algorithms in the Mediterranean Sea. Two recent ERS-2/SAR scenes over the North Tyrrhenian Sea were selected as case study. The empirical backscatter model CMOD4 [2] was used to retrieve the wind stress while wind direction was resolved without ambiguity by the output of the ECMWF atmospheric model. CMOD4 analysis was performed on a strip 512 pixels wide in azimuth, spanning the whole SAR swath. The strip had been previously calibrated, with corrections for antenna gain pattern, range losses and ADC saturation. The backscatter value as a function of the incidence angle value was computed by averaging over range lines.

Our source of reference data is the buoy ODAS Italia 1, located in the North Tyrrhenian Sea at 43.8 N and 9.1 E. The buoy is indeed an open sea laboratory for multidisciplinary research, equipped with a full set of geophysical and biochemical instruments [1]. All the relevant meteorological parameters (wind intensity, wind direction, air/sea temperature) were collected by the buoy during a total of 1 hour of acquisition, centered around the time of satellite pass.

A comparison between SAR retrieved wind data and buoy measured ones will be presented and discussed. For the near future, we plan to make an extensive use of the buoy ODAS Italia 1 for the validation of ASAR/ENVISAT products.

REFERENCES

[1] A. Siccardi, R. Bono, F. Grosso, R. Amore, R. Mantovani and A. Mistrangelo, ``Open sea monitoring: ODAS Italia 1, an operational system and a challenge for the future'', Oceanology Internat. 96, Brighton, U.K., March 1996.

[2] A.C.M. Stoffelen and D.L.T. Anderson, ``Scatterometer data interpretation: estimation and validation of the transfer function CMOD4'', J. Geophys. Res., vol. 102, C3, 1997, 5767-5780.