Satellite altimeters provide continuous information of the sea level variability and mesoscale processes for the global ocean. For estimating the sea level above the geoid and monitoring the full ocean dynamics from altimeters measurements, a key reference surface is needed: The Mean Dynamic Topography (MDT). However, in coastal areas, where, in situ measurements are sparse and the typical scales of the motion are generally smaller than in the deep ocean, the global MDT solutions are less accurate than in the open ocean, even if significant improvement has been done in the past years. An opportunity to fill in this gap has arisen with the growing availability of long time-series of high-resolution HF radar surface velocity measurements in some areas, such as the south-eastern Bay of Biscay. The prerequisite for the computation of a coastal MDT, using the newly available data of surface velocities, was to obtain a robust methodology to remove the ageostrophic signal from the HF radar measurements, in coherence with the scales resolved by the altimetry. To that end, we first filtered out the tidal and inertial motions, and then, we developed and tested a method that removed the Ekman component and the remaining divergent part of the flow. A regional high-resolution hindcast simulation was used to assess the method. Then, the processed HF radar geostrophic velocities were used in synergy with additional in situ data, altimetry, and gravimetry to compute a new coastal MDT, which shows significant improvement compared with the global MDT. This study showcases the benefit of combining satellite data with continuous, high-frequency, and synoptic in situ velocity data from coastal radar measurements; taking advantage of the different scales resolved by each of the measuring systems. The integrated analysis of in situ observations, satellite data, and numerical simulations has provided a further step in the understanding of the local ocean processes, and the new MDT a basis for more reliable monitoring of the study area. Recommendations for the replicability of the methodology in other coastal areas are also provided. Finally, the methods developed in this study and the more accurate regional MDT could benefit present and future high-resolution altimetric missions.

中文翻译：

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为增强型海岸平均动态地形整合 HF 雷达观测

卫星高度计为全球海洋提供海平面变化和中尺度过程的连续信息。为了估计大地水准面以上的海平面并通过高度计测量监测完整的海洋动态，需要一个关键的参考表面：平均动态地形 (MDT)。然而，在沿海地区，原位测量很少，运动的典型尺度通常比深海小，全球 MDT 解决方案不如公海准确，即使在过去几年。随着某些地区（例如比斯开湾东南部）高分辨率 HF 雷达表面速度测量的长时间序列可用性的增加，出现了填补这一空白的机会。计算沿海 MDT 的先决条件，使用新获得的表面速度数据，是为了获得一种稳健的方法，以从 HF 雷达测量中去除地转信号，与高度计解析的尺度相一致。为此，我们首先过滤掉了潮汐和惯性运动，然后，我们开发并测试了一种去除 Ekman 分量和流动的剩余发散部分的方法。使用区域高分辨率后报模拟来评估该方法。然后，处理后的 HF 雷达地转速度与额外的原位数据、高度测量和重力测量协同使用，以计算新的沿海 MDT，与全球 MDT 相比，该数据显示出显着改进。这项研究展示了将卫星数据与连续、高频、和来自沿海雷达测量的天气现场速度数据；利用每个测量系统解决的不同尺度。原位观测、卫星数据和数值模拟的综合分析为了解当地海洋过程提供了进一步的帮助，新的 MDT 为更可靠地监测研究区奠定了基础。还提供了该方法在其他沿海地区的可复制性的建议。最后，本研究中开发的方法和更准确的区域 MDT 可以有益于当前和未来的高分辨率高度测量任务。和数值模拟为了解当地海洋过程提供了进一步的步骤，新的 MDT 为更可靠地监测研究区域奠定了基础。还提供了该方法在其他沿海地区的可复制性的建议。最后，本研究中开发的方法和更准确的区域 MDT 可以有益于当前和未来的高分辨率高度测量任务。和数值模拟为了解当地海洋过程提供了进一步的步骤，新的 MDT 为更可靠地监测研究区域奠定了基础。还提供了该方法在其他沿海地区的可复制性的建议。最后，本研究中开发的方法和更准确的区域 MDT 可以有益于当前和未来的高分辨率高度测量任务。