Synergistic utilization of spaceborne SAR observations for monitoring
the Baltic Sea flow through the Danish straits
Abstract
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 (≤0.08) and correlation coefficient (R≈0.75) but lower standard
deviations and
rms errors (0.15 m/s) than those exhibited by the ocean model (0.31
m/s).