Assimilation of ENVISAT ASAR Wave Mode Level 2 Product
Abdalla, Saleh; Janssen, Peter; Bidlot, Jean-Raymond

The wave mode of the Synthetic Aperture Radar (SAR) provides a wealth of information regarding the detailed description of the surface sea state with global coverage. Unfortunately, SAR is not able to sense the whole spectrum of ocean waves especially in the azimuthal direction and misses quite a large range of short waves. Although this range usually carries the most energetic part of the ocean surface spectrum, the resolvable part of the spectrum can be very useful in a wide range of the oceanic applications including data assimilation in ocean wave models.

The European Centre for Medium-Range Weather Forecasts (ECMWF) has been assimilating SAR Wave Mode (WM) Level 1b (L1b) product operationally since January 2003. This work started with the ERS-2 SAR WM L1b product. The corresponding product from ENVISAT replaced the ERS-2 product on the first of February 2006. WM L1b SAR spectrum product is inverted in-house using the iterative MPI-M (Max Planck Institute for Meteorology) nonlinear mapping scheme, which was developed by Hasselmann and Hasselmann (1991), to obtain the ocean wave spectra before assimilation. The assimilation of L1b product, both from ERS-2 and ENVISAT, proved to be beneficial.

On the other hand, operational assimilation of ENVISAT ASAR WM Level 2 (L2) product has never been realised at ECMWF. Several experiments were carried out to assess the impact of assimilation of L2 product on wave model predictions. The obtained results were not encouraging. It seems that the information included in the BUFR (Binary Universal Form for the Representation of Meteorological data) template of the ASAR WM L2 product is not sufficient to carry out a proper quality control (QC) of product to achieve a successful data assimilation process. With the already available information in the BUFR template, relaxed QC criteria result in an adverse impact when WM L2 product is assimilated. Restricting the QC criteria improves the situation leading only to very limited impact which is mainly positive. The restricted QC criteria led to the rejection of most of L2 products, thus the limited impact. However, the negative impact, despite being very limited, in some areas could not be avoided. This prevented an operational assimilation of the product at ECMWF.

The current plans of ESA for the future mission of Sentinel-1 do not accommodate the distribution of any Wave Mode Level 1b product. Therefore, it was decided to carry out a new series of ASAR assimilation experiments. Those experiments are planned to implement extra QC criteria using some information that is not available in the BUFR product. This work is based on the recommendations of Johnsen (2005) who suggested the use of the normalized image variance and the ambiguity factor. Both are not included in the BUFR product received at ECMWF. The results obtained from the new series of experiments will be presented and discussed. An overview of the whole experience of ASAR WM L2 assimilation will also be presented.