Envisat ASAR Wave Mode Products - Quality Assessment and Algorithm Upgrade
Harald Johnsen(1) , Vincent Kerbaol(2) , Fabrice Collard(2) , Geir Engen(1) , and Bertrand Chapron(3)
Box 6434, Forskningsparken,
(2) Boost Technologies, 135 rue Claude Chappe, 29280 Plouzané, France
(3) IFREMER, BP 70, 29280 Plouzane, France
Global calibration, validation and long-term quality assessment of the ASAR WM products have been conducted on a monthly basis since January 2003, using collocated WAM model as provided by ECMWF. The monitoring of the geophysical calibration constant indicates a slow decrease in the instrument gain of about 1dB in the period June 2003 to September 2005. The ground segment processing has been updated with the new value of the calibration constant.
The geophysical validation consist of systematic regional and seasonal comparisons with collocated wave parameters of the WAM model. Although the agreement is good on a global basis for the swell part of the wave spectra, significant deviation is observed for the swell peak wave period (RMS=2.1sec, BIAS=0.7sec) and significant waveheight (RMS=0.5m, Bias=0.2m). The deviation is mostly due to an imperfect detrending of the images as well as the use of improper modulation transfer function (MTF) at low wavenumbers.
The outcome of the geophysical validation process has resulted in an upgraded algorithm for the ASAR WVW (Level2) product. The upgrade consists of both refinements and new developments such as: - a new spectral detrending procedure, - modification of MTF at low wavenumbers, - improved ambiguity removal, - improved azimuth cut off estimation. Validation of the upgraded processing shows a significant improvement in the geophysical quality of the Level2 product as well the number of products passing the quality control. The latter is of importance for the assimilation of Level2 products into NWP models.
The new Level2 algorithm is currently used for the reprocessing the whole Envisat ASAR WM archive at IFREMER/Cersat.