Luke Colosi, Nicholas Pizzo, Laurent Grare, Nick Statom, and Luc Lenain. Observations of Surface Gravity Wave Spectra from Moving Platforms. Journal of Atmospheric and Oceanic Technology, accepted.
Surface waves play an important role in the ocean-atmosphere coupled climate system by mediating the exchange of momentum, heat, and gas between the atmosphere and the ocean. Pseudo-Lagrangian autonomous platforms (e.g., Boeing Liquid Robotics Wave Gliders) have been used to investigate the underlying physical dynamics involved in these processes to better parameterize the air-sea exchange occurring at the scale of the surface waves. This requires accurate measurements of directional surface waves down to short scales (O(1) meter), as these shorter waves support most of the stress between the atmosphere and the ocean. A challenge to overcome for pseudo-Lagrangian autonomous vehicles is that the platform's velocity causes the observed frequency of the waves to be Doppler shifted. This leads to a modulation of the wave spectrum, particularly at high frequencies, that depends on the platform’s speed, the wave frequency, and the relative angle between the direction of wave and platform propagation. In this work, we propose a method to account for Doppler effects that considers the full directionality of the wave field. The method is validated using a unique dataset collected from a fleet of two Wave Gliders off the coast of Southern California in September 2019 operating on the perimeter of a tight square (500-m edge length) track over a three-day deployment. This technique can be used to estimate wave spectra derived from other slow-moving surface vehicles such as Saildrones that use platform motion to characterize the surface wave field. MATLAB routines to implement this method are publicly available.
Ocean surface waves play an essential role in the Earth's climate system by influencing how the ocean and atmosphere interact. Autonomous surface vehicles (ASVs) that glide across the ocean surface collect measurements to better understand air-sea physics. Improving climate models requires accurate measurements of short wavelength waves which support most of the fluxes between the ocean and atmosphere. Uncrewed vehicle platform motion influences the wave period observed from the perspective of the vehicle. Here, we introduce a general approach to correct this effect that can be applied to any ASV. This method is validated using a data set from a fleet of uncrewed ocean surface vehicles. MATLAB routines to apply this method are publicly available.
The purpose of this study is to introduce a general approach that corrects observations of ocean surface waves collected onboard autonomous surface vehicles (ASVs) for the effects on the wave period due to the vehicle's forward motion. This is important because improving climate models require accurate measurements of short wavelength waves which can be readily obtained from ASVs. Our method provides the tools for ASVs to better understand air-sea physics and the larger role ocean surface waves play in the Earth's climate system.
- Luke Colosi<lcolosi@ucsd.edu>
- Nicholas Pizzo <npizzo@ucsd.edu>
- Laurent Grare <lgrare@ucsd.edu>
- Nicholas Statom <nstatom@ucsd.edu>
- Luc Lenain <llenain@ucsd.edu>
All data needed to reproduce the analysis in this paper is available for download here.
In this study, observations collected from two experiments are investigated. The first was conducted as part of the ONR Task Force Ocean (TFO) initiative. Observations from two Wave Gliders, referenced as "Planck" and "Stokes", were collected off the coast of Del Mar, California (see Figure 1 from the manuscript) during a 3-day deployment from 9 to 11 September 2020 (referred hereon as DELMAR2020). The two Wave Gliders moved along two tight square tracks: a large square with a 1000-meter edge length and a small square with a 500-meter edge length. Here, we focus on data from the small square where Doppler effects on the frequency spectrum are more apparent due to rapid changes in platform heading.
The second experiment was conducted as part of the NASA Submesoscale Ocean Dynamics Experiment (S-MODE) program, a project that aims to determine whether submesoscale ocean dynamics make important contributions to the vertical exchange of physical and biological variables in the upper ocean using a combination of aircraft-based remote sensing, research vessels, and autonomous oceanographic platforms measurements along with numerical modeling (Farrar et al. 2020). The pilot experiment considered here was conducted off the coast of San Francisco, California (see Figure 2 from the manuscript) from 29 October to 4 November 2021 (referred hereon as SMODE2021). In this work, we focus on the observations from one of the Wave Gliders deployed in the experiment, referenced throughout as WHOI43.
Instrumented Boeing Liquid Robotics SV3 Wave Gliders are used to collect ocean and atmospheric observations at the air-sea interface (Grare et al. 2021, see Figure 1). Included in the present study are atmospheric measurements from a Vaisala WXT (model 530) mounted ~1 meter above the ocean surface that records wind speed and direction among other atmospheric parameters. The wind speed 10 meters above the ocean surface is estimated using the modified version of the Charnock relation (Charnock 1955) used in the TOGA COARE model (Fairall 2003). Surface wave observations are derived from the motion of the platform (Lenain and Melville 2014, Thomson et al. 2018, Grare et al, 2021) measured by a coupled GPS - Inertial Motion Unit (IMU) system (Novatel SPAN OEM7720 - Epson EG320N for the DELMAR2020 experiment and a Vectornav VN-300 GPS/IMU for the SMODE2021 experiment) (Hodges et al. 2023). Uncertainties in these observations measured by Wave Gliders are discussed in detail by Grare et al. (2021), Thomson et al. (2018), and Lenain and Melville (2014). In these studies, wind and wave observations have been shown to agree well with independent ground truth data over a range of environmental conditions thus demonstrating the high accuracy and precision of these measurements.
This work was supported by the Office of Naval Research (Grant N00014-19-1-2635) and NASA (Grant 80NSSC19K1688).
All figures in Colosi et al. (2023) can be reproduced using the MatLab scripts from this repository (or from Zenodo) and processed data published to the UCSD library digital collections. To do so, follow these steps:
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Make a local copy of this repository by either cloning or downloading it.
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Download the processed data, unzip the files, and move all directories to
datain the project root. After doing so, your directory tree should look like this:
WaveSpectrum/
├── data
│ ├── DElMAR2020
│ ├── SMODE2021
│ └── BATHY
├── figs
├── src
└── tools
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Make sure that you have downloaded the R2022a version of MATLAB. Other versions may work, but small differences in keyword argument and outputs of functions may arise.
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If you follow the steps above, you should be able to reproduce all figures, by running
figXX.mfrom thesrcdirectory without having to adjust any paths.
Note on MatLab software version: Code was developed using MatLab R2022a. Versions released before and after this may run into some errors.
Note on the WAFO toolbox: For more information about the WAFO toolbox, please see their website.
If you wish to use the code from this repository, you may cite it as:
Colosi, Luke V. (2023, June 30). Source code for: 'Observations of Surface Gravity Wave Spectra from Moving Platforms'. Zenodo. (https://doi.org/10.5281/zenodo.8122034)