Remote measurement of near-surface currents via wave spectra: currents varying vertically and horizontally

Remote sensing of ocean near-surface currents based on measurements of the wave spectrum is an attractive means of mapping currents over a large area simultaneously. The most common wave measurement method involves marine X-band radar (Lund et al. 2015), with optical video measurements using drones more recently being used as an alternative (Streßer, Carrasco & Horstmann, 2017). In both cases, analysis of the wave dispersion within a subset window of the spatial domain is performed to determine the spatially varying near-surface current. An improved method for determining the depth-dependence of sub-surface currents from measured wave spectra was recently developed by our group (Smeltzer et al 2019).

Our long-term goal is to develop methods whereby the best possible representation of the three-dimensional sub-surface current can be obtained from remote measurement of waves. Methods based on current retrieval from wave spectra must assume that horizontal current variations are slow compared to a typical wavelength, but this is not always so. To resolve horizontal space, retrieved images must be subdivided into windows and the velocity vector at the midpoint is determined from the 3D spectrum of the waves within the window only.

In this work we examine the dependence of the spatial window size on the results of the current reconstruction. When the window size is decreased, greater spatial resolution is achieved being able to capture currents that vary on a faster horizontal length scale, at the expense of lower resolution in wavevector spectral space which may decrease the accuracy of the reconstructed currents, especially when information as the depth-dependence of the flow is desired. When the window size is larger, the reconstructed current may not be representative of the average current within the window. We present experiments conducted in a laboratory where spatially varying currents and waves of can be well-controlled and measured in situ, a valuable test-bed setup compared to field measurements. We investigate the factors involved which determine the optimal choice of window size.

References

Lund, B., et al. A new technique for the retrieval of near-surface vertical current shear from marine X-band radar images. J. Geophys. Res.: Oceans (2015) 120 8466-8496.

Smeltzer, B.K., Æsøy, E., Ådnøy, A. and Ellingsen S.Å., An improved method for determining near-surface currents from wave dispersion measurements. J. Geophys. Res.: Oceans. (2019) 124, https://doi.org/10.1029/2019JC015202.

Streßer, M., Carrasco, R. and Horstmann, J., Video-based estimation of surface currents using a low-cost quadcopter, IEEE Geosci. Remote Sens. Lett. (2017) 14 2027-2031.