How horizontal current shear affects the remote sensing of current depth profiles

Abstract

<p>Detailed knowledge of the subsurface current in the ocean environment allows for more accurate modelling of, e.g., exchange of mass and heat with the atmosphere. Measuring the vertical profile of the current <em>in situ</em> poses a range of costs and difficulties. Therefore, methods were developed to infer this information from measured Doppler shift velocities, i.e., from changes in the waves’ phase velocities owing to the background current, which are obtainable from, e.g., optical or radar imaging of the surface. Notably, the “polynomial effective depth method” (PEDM), due to Smeltzer et al. [1], was shown to be a promising candidate. These methods, however, typically assume the current to be uniform in the horizontal plane.</p><p>In this work we study the effects of slow horizontal variations on the accuracy of the extracted Doppler shifts. Synthetic data is generated by propagating waves from still water into a region of horizontal and vertical shear, where the propagation is governed by the dispersion relation as given by Steward and Joy [2]. The numerically generated wave fields then serve as the raw video data for the extraction of current-induced Doppler shifts whence the vertical shear current is estimated and compared to the prescribed one.</p><p>The simulation of the wave fields is based on the method of characteristics. Given a wave spectrum in the quiescent region, for each wavelength, a phase field is obtained from propagating waves along rays. These fields then form the basis for constructing a “movie”.</p><p>Results for different horizontal velocity fields and wave spectra are compared to investigate their effect on the accuracy of the vertical profile retrieved by the PEDM.</p><p>[1] Smeltzer, B. K., Æsøy, E., Ådnøy, A., & Ellingsen, S. Å. (2019). An improved method for determining near-surface currents from wave dispersion measurements. <em>Journal of Geophysical Research: Oceans</em>, 124, 8832– 8851.</p><p>[2] Stewart, R. H. & Joy, J. W. (1974). HF radio measurements of surface currents. <em>Deep Sea Research and Oceanographic Abstracts</em>, 21, 1039-1049</p>