Validation of surface wave spectral measurements from velocity profiling floats

EM-APEX floats have primarily been used to measure subsurface ocean velocities for the purpose of studying ocean dynamics and the vertical structure and shear of currents. However, the motionally-induced voltage sensed by the EM-APEX also contains signals from surface wave orbital velocities, and the time taken to pass through the top ∼100 m of the water column is sufficient to estimate a full spectrum of surface wave amplitudes with periods from 3–25 seconds (i.e., including both wind seas and long swell) with each profile. The addition of surface wave measurements expands the abilities of the EM-APEX to include measuring the dynamics of waves and their interactions with surface and sub-surface currents. Following procedures developed by Hsu (2021) and D’Asaro (2015), we tested and analyzed the performance of EM-APEX floats as surface wave measuring instruments against other in-situ wave-measuring platforms. Data from a 2017 field study off the coast of California (ONR’s Langmuir Circulation Department Research Initiative, or LC-DRI) were used to test the ability of the EM-APEX floats to accurately measure surface-wave spectra, as well as bulk wave parameters such as significant wave height and energy weighted period. This field campaign included the deployment of 6 EM-APEX floats, 8 SWIFT buoys and a Datawell Waverider buoy, which were used for the analysis presented here. In this paper we discuss the limitations and uncertainties inherent in the EM-APEX wave measurements, and determine the uncertainty on each individual 1 Hz velocity measurement to be on the order of 1.6 cm/s, resulting in a minimal detectable significant wave height of ?? = 0.6 m. Results indicate that the EM-APEX surface wave measurements are in good agreement with the other in-situ bulk wave measurements, with correlation coefficients of ? = 0.87 for significant wave height, and ? = 0.81 for energy-weighted period. As EM-APEX floats are already a well-used platform for directly observing subsurface ocean currents, vertical shear, and turbulent mixing in the ocean, this work demonstrates the potential for these floats to be used as a powerful tool for direct measurements of surface-wave driven mixing processes, Langmuir turbulence, and the interactions between surface waves and ocean currents.