Synergistic Utilization of Spaceborne SAR Observations for Monitoring the Baltic Sea Flow Through the Danish Straits

Artikel i vetenskaplig tidskrift, 2024

Synthetic aperture radar (SAR) has emerged as a key instrument in oceanography due to its high spatial resolution and sensitivity to ocean surface dynamics. The main limitation of a single spaceborne SAR is the long repeat cycle (e.g., 12 days for Sentinel-1), which hinders its capability to monitor the temporal evolution of oceanic processes. The principal objective of this study is to demonstrate the potential of spaceborne SAR to monitor the temporal variation of ocean surface circulation. This is assessed using the Baltic Sea flow through the Danish strait Fehmarn Belt as a case study. In order to overcome the temporal sampling limitation, data from three satellites are combined, namely Sentinel-1A, Sentinel-1B and TanDEM-X. The average revisit time achieved by combining the three satellites is 1.2 days. Two months of opportunistic SAR data (June and July 2020) covering the Fehmarn Belt are used. The radial surface current derived from SAR is compared to ocean model and in situ data. It is shown that the dominant processes that govern the circulation in the Fehmarn Belt exhibit time scales larger than 2 days. Subsequently, it is demonstrated that SAR effectively captures the synoptic-scale features (time scales larger than 2 days) of the Baltic Sea circulation, thereby enabling monitoring the temporal variations of flow dynamics. Comparison of the SAR-derived radial surface current against in situ measurements yields comparable bias (<= ${\le} $0.08 m/s) and correlation coefficient (R approximate to ${\approx} $ 0.75) but lower standard deviations and rms errors (0.15 m/s) than those exhibited by the ocean model (0.31 m/s). Synthetic aperture radar (SAR) provides high resolution images of the ocean surface from space. These images contain information on the roughness of the ocean surface, which is related to wind speed and wave height, and information on the motion of the surface which is related to the wave propagation and surface currents. The ocean surface is dynamic. However, it changes rapidly in some regions and slowly in other regions. This depends on the time scale of the forces driving the circulation in that region. The main limitation of satellites for ocean observation is that they do not pass often over the same area. In this study, we investigate the time scale of the dominant driving processes like sea level and wind in the Danish straits area. This area contains important channels for water exchange between the Baltic Sea and the North Sea. Our investigation reveals that the dominant time scale governing forcing processes in this region exceeds 2 days. Furthermore, through the synergistic utilization of data from three satellite platforms, we demonstrate the capability to effectively resolve the surface flow dynamics of the Baltic Sea with a high degree of precision. The dominant processes that govern the sea surface circulation in the Danish strait Fehmarn Belt exhibit time scales larger than 2 days To properly sample Baltic Sea time scales, data from three satellites are combined to achieve an average revisit time of 1.2 days SAR is capable of monitoring the synoptic scale circulation of the Baltic Sea with adequate temporal resolution and high accuracy