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26 February 2002

A calibration/validation tool for the Envisat ASAR Wave Mode

asar spectraWaves are being measured over the whole surface of the ocean by ESA satellite ENVISAT. Before being assimilated operationaly in forecasting models, the ENVISAT/ASAR wave data have to be fully calibrated and validated. In order to make this task easier, CERSAT is providing users with colocated datasets of ASAR (onboard ENVISAT) cross-spectra and wave spectra with in-situ measurements (wind, waves, sst) from various buoy networks. Envisat ASAR Wave mode products are also colocated with WAM data from the ECMWF model.

 

 envisatThe inversion algorithm designed by IFREMER and the Norvegian laboratory NORUT is designed to retrieve waves 2D spectra from SAR images. A first version of the inversion inversion algorithm applied to ERS1/2 SAR imagettes was already porivding significant wave parameters such as dominant wave length and direction. Now, with the complex or "dynamic" imagettes delivered by ESA from the ASAR onboard Envisat, the new IFREMER/NORUT inversion algoithm enables also the determination of the propagation direction. The 2D wave spectra from the ASAR level 2 product can be compared with wave spectra from ECMWF/WAM model outputs, or with buoy spectra (see graphic below).

 

Source/reference

1D wave spectra (wave frequency)

2D wave spectra (wave length)

Envisat ASAR wave spectra

date : 26/06/2002 06:51
latitude : 40.96 N
longitude : -136.70 E

(as read in the colocated product ASA_WVW_2P/buoy)

wvw1d large wvw2d

WAM wave spectra

date : 26/062002 06:00

latitude : 41.00 N

longitude : -136.50 E

(as read in the colocated product Envisat ASAR Wave Mode/WAM)

wam1d large wam2d large

buoy (NODC) wave spectra

date : 26/06/2002 06:50

latitude : 40.84 N

longitude : -137.49

(as read in the colocation product ASA_WVW_2P/buoy)

buo1d0 large

IFREMER/CERSAT is involved in the calibration/validation for the Envisat ASAR Wave Mode products of the European Space Agency (ESA). CERSAT is especially a web portal for the distribution of calibration/validation data. To help this step, CERSAT process and distributes two colocated datasets : buoys wave spectra colocated respectively with Envisat/ASAR Wave mode cross spectra (ASA_WVS_1P) and wave spectra (ASA_WVW_2P). Each product is described its related section :

  • Data > Catalog > Colocated products > satellite/buoys > Envisat ASAR/ASA_WVS_1P - Buoys
  • Data > Catalog > Colocated products > satellite/buoys > Envisat ASAR/ASA_WVW_2P - Buoys

A third product, developped and processed at ECMWF by Dr. Abdallah Saleh, contains WAM spectra colocated with Envisat ASAR Wave mode measurements. This product is available through the CERSAT web site in the following section :

Thanks to these datasets, the cal/val team for the Envisat ASAR Wave Mode can compare the measurements of the satellite with the WAM output model or with buoy measurements. The first triple colocation Envisat ASAR/buoy/WAM wave spectra can be seen above.