Initial development of pencil-beam scatterometer coastal processing
In the context of the OSI SAF Visiting Scientist Program, Giuseppe Grieco from the Institut de Ciències del Mar (ICM - CSIC) in Barcelona, worked on the implementation and validation of the QuikSCAT spatial response function (SRF) and the computation of the derived land contribution ratio (LCR). This work took place in 2020 and was supervised by Marcos Portabella (ICM) and Ad Stoffelen (KNMI).
Objectives and framework of the study
Recent developments on ASCAT coastal wind processing show that by thoroughly characterizing the size and orientation of the spatial response function (SRF) for each beam, a better coastal sampling and wind quality product can be derived. The wind processing also benefits from earlier work on the so-called land contribution ratio (LCR), which aims at identifying backscatter measurements contaminated by land signal in the wind retrieval process and correcting or removing them. This work aims at fully characterizing the SeaWinds SRFs before revisiting the LCR method in order to set the grounds for the development of an OSI SAF coastal wind product for rotating pencil-beam scatterometers.
The SeaWinds spatial response function (SRF) has been analytically modeled, and it has been visually validated by comparing it with that derived from the look-up-table (LUT) provided by Prof. Dave Long of the Brigham Young University.
The two SRF models are in good agreement, even if some differences are apparent. In particular, the LUT derived SRFs are generally more jagged with respect to those from the analytical model. Furthermore, analytical SRFs may be asymmetric with respect to the slice centroid, while those derived from the LUT are not because of a re-positioning step. It has been demonstrated that these asymmetries are physical, therefore expected.
The derived land contribution ratios (LCRs) are compared with each other, showing a general agreement and visual consistency with the shoreline. The assessment of the impact of their differences (up to 3%) on the wind field retrievals deserves further investigation.
A coastal test case in the Gulf of Taranto (south of Italy) has been analyzed. Two main aspects are revealed in this analysis: a) the distribution of the slice σ0 values is excessively wide and many values are much higher than expected; b) the σ0 has a linear trend with respect to the LCR, when land contamination is present (LCR>0). It is concluded that a thorough characterization of the slice σ0 noise is fundamental for a successful retrieval of the wind field. This aspect deserves a dedicated study.
Benefits for the SAF
• OSI SAF users repeatedly request winds closer to the coast for a wide variety of applications and pencil-beam scatterometer developments of a LCR are not publicly available; extension of the LCR to PenWP would make this happen;
• The large amount of pencil-beam scatterometers from QuikScat to the OSCAT and the Chinese scatterometers on CFOSAT, HY and FY make coastal winds production from this type of scatterometer particularly interesting for the OSI SAF users for both near real time (NRT) and climate applications.
Report on this study: Initial development of pencil-beam scatterometer coastal processing