Coastal Pencil Beam Wind Processor

In the context of the OSI SAF Visiting Scientist Program, Giuseppe Grieco from the Instituto di Scienze Marine (ISMAR-CNR) in Napoli, worked on the implementation of a Land Contribution Ratio (LCR) based Normalized Radar Cross Section (σ0) correction scheme applied to QuikSCAT measurements. This work was carried out in 2021/2022 and was supervised by Ad Stoffelen (KNMI) in collaboration with Marcos Portabella (ICM-CSIC).

Objectives and framework of the study

This study shows the implementation of a σ0 correction scheme based on the calculation of the LCR index. In addition, it shows a sensitivity analysis of the four integrated views to the integration methods. Finally, it shows a refinement of the estimates of the noise that affects σ0s and an analysis of the intra-footprint biases caused by the variation of the acquisition incidence angle.

Report conclusions

The σ0 correction scheme applied in this study is based on the hypothesis that the dependence of σ0 on LCR is linear, the variations of σ0 on land and sea in the surrounding of the desired wind vector cell (WVC) are negligible and that the noise level does not depend on σ0.

The results show that the methodology is effective in correcting the contaminated acquisitions (see Figure 1), even if the occurrence of negative σ0s after correction (blue dots in Figure 1b) appears to be excessive in both number and intensity. This outcome suggests that the hypothesis that the noise level is constant appears to be too strong for QuikSCAT measurements.

In the future, proper noise ”regularization” will be attempted to mitigate such effects.

The integration procedure gives different results according to the methodology used, but the preliminary results are not sufficient to select the most performant method. The wind retrieval step is necessary for this purpose. This was not done in the current work.

The refined estimates of the noise confirm the results shown in [1]. The high-resolution backscatter slices within a radar footprint (‘egg’) are clearly biased due to the variation of the acquisition incidence angles, but their impact on noise estimates is always less than 7%. The opportunity to mitigate these biases before the retrieval step by applying a multi-collocation inter-calibration procedure will be evaluated in the future.

Benefits for the SAF

• σ0 noise characteristics and an accurate procedure aimed at mitigating the land contamination of coastal measurements are essential to improve the quality and sampling of coastal winds derived from scatterometers.
• The large amount of pencil-beam scatterometers from NASA’s QuikScat to the Indian OSCAT and the Chinese scatterometers on HY make coastal wind 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

Coastal PenWP

Authors

• Giuseppe Grieco, Institute of Marine Sciences (ISMAR-CNR)
• Ad Stoffelen, Jur Vogelzang Anton Verhoef, Koninklijk Nederlands Meteorologisch Instituut (KNMI)
• Marcos Portabella, Barcelona Expert Center (BEC ICM-CSIC)

References

[1] G. Grieco, M. Portabella, J. Vogelzang, Verhoef, and A. A., Stoffelen, “Quikscat normalized radar cross section noise characterization for coastal wind field retrieval,” tech. rep., Barcelona Expert Center (BEC ICM-CSIC), 2020. OSI-SAF Technical Report # VSA 20 01.