Analysis of waves observed by synthetic aperture radar across ocean fronts

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Analysis of waves observed by synthetic aperture radar across ocean fronts Weizeng Shao 1,2,3 & Xingwei Jiang 1,3 & Ferdinando Nunziata 4 & Armando Marino 5 & Zhehao Yang 2 & Youguang Zhang 1,3 & Valeria Corcione 4 Received: 20 February 2020 / Accepted: 23 August 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract In this study, synthetic aperture radar (SAR) imaging of waves across ocean fronts was investigated using C-band Sentinel-1 VVpolarized SAR imagery collected over the Yangtze and the Zhujiang estuaries. The presence of ocean fronts in the study area was confirmed by collocated sea surface temperature (SST) data provided by the Advanced Very High Resolution Radiometer (AVHRR) and sea surface current information from the National Ocean Partnership Program (NOPP) based on the HYbrid Coordinate Ocean Model (HYCOM). The experimental results revealed that as the current speed increased, the cut-off wavelength (λc) increased as well. The effect of the increasing azimuth cut-off wavelength, however, was relatively weak in terms of variations of the normalized radar cross-section (NRCS), i.e., it was within 2 dB for λc ≤ 60 m. Hence, it was weaker than the NRCS variation related to SST. Larger NRCS variations (i.e., within 5 dB) occurred for λc values up to 120 m. In addition, the experimental results also demonstrated that the parameterized first-guess spectrum method (PFSM) wave retrieval performance was affected by ocean fronts. In particular, overestimations occurred when ocean fronts were present and λc was < 100 m. Keywords Wave . Ocean front . Synthetic aperture radar

1 Introduction Ocean fronts are common marine phenomena characterized by distinctive features of sea surface currents and temperatures and always accompanied by eddies and upwelling. These phenomena include a frontal boundary region featuring strong horizontal and vertical velocity gradients. Ocean fronts are of great importance in marine ecosystems and global climate change (Sydeman et al. 2014) due to their associated heat and Responsible Editor: Richard Signell * Weizeng Shao [email protected] 1

National Satellite Ocean Application Service, Beijing 100081, China

2

Marine Science and Technology College, Zhejiang Ocean University, Zhoushan 316000, China

3

Key Laboratory of Space Ocean Remote Sensing and Application, Ministry of Natural Resources of the People’s Republic of China, Beijing 100081, China

4

Dipartimento di Ingegneria, Università degli Studi di Napoli Parthenope, 80133 Naples, Italy

5

Natural Sciences, University of Stirling, Stirling FK9 4LA, UK

nutrient exchanges. Operational products derived from remotely sensed measurements, and in particular by optical sensors, have generally been used to monitor ocean front events (Fragiacomo and Parmiggiani 2002; McClain et al. 1984). There are some disadvantages in relying exclusively on optical images, however. Images can be noisy, they are not available when there is cloud cover, and increased biological activity in upwelling regions can