A new family of multi-purpose, multi-sensor, small sized, easy-to-handle, low-cost, spherical surface drifters is about to appear in the market, with its first three models. The WAVY Ocean is a current, wave and near-surface temperature gradient tracker for the open Ocean; the WAVY Littoral is a current and wave tracker designed for nearshore operations; and the WAVY Basic is a current and nearsurface temperature tracker intended for riverine and estuarine environments, and suitable for citizen science. The WAVY optimized buoyancy - only 30% of the axes above the surface - ensures that their displacement is essentially due to the current, while their small size makes them sensitive to the average vectors in the very near surface. Examples of WAVY Ocean full resolution data from recent observations illustrate the performance and the potential of these drifters in operational missions requiring near-real time data of the very near surface of the ocean.This is a contribution to Project MELOA (Multipurpose/Multi-sensor Extra Light Oceanography Apparatus - H2020-SC5-2017-OneStageB). This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 776825.Peer ReviewedPostprint (published version

Chumbinho, Rogério

da Silva, António Jorge

de Saint Léger, Philippe

Gonçalves, Pedro

Oliveira, M.

Pedrera, Félix

Pinto, José P.

Río Fernandez, Joaquín del

Sousa, Alexandre

English

UPCommons. Portal del coneixement obert de la UPC

Lisboa, 21, 22 e 23 de junho de 20223597as Jornadas de Engenharia Hidrográfica / 2as Jornadas Luso-Espanholas de Hidrografia Generation WAVY: a New Breed of Surface Drifters for all Ocean Domains A.Silva (1), A. Sousa(2), F. Pedrera (3), J. del Rio (4), J.P. Pinto (5), M. Oliveira (6), P.Gonçalves (7), P. de Saint Léger(8), R. Chumbinho (9) (1) Deimos Engenharia, silva.antonio@deimos.com.pt (2) OceanScan Marine Systems & Technology (3) Deimos Space (4) Universitat Politècnica de Catalunya (5) Instituto Hidrográfico (6) Instituto de Engenharia de Sistemas e Computadores, Tecnologia e Ciência (7) Laboratório de Sistemas e Tecnologia Subaquática – University of Porto (8) Collecte Localisation Satellites (9) BlueWise Marine  Abstract: A new family of multi-purpose, multi-sensor, small sized, easy-to-handle, low-cost, spherical surface drifters is about to appear in the market, with its first three models. The WAVY Ocean is a current, wave and near-surface temperature gradient tracker for the open Ocean; the WAVY Littoral is a current and wave tracker designed for nearshore operations; and the WAVY Basic is a current and near-surface temperature tracker intended for riverine and estuarine environments, and suitable for citizen science. The WAVY optimized buoyancy - only 30% of the axes above the surface - ensures that their displacement is essentially due to the current, while their small size makes them sensitive to the average vectors in the very near surface. Examples of WAVY Ocean full resolution data from recent observations illustrate the performance and the potential of these drifters in operational missions requiring near-real time data of the very near surface of the ocean. Keywords: current tracking, ocean surface, surface drifters, temperature gradient, wave tracking  1. INTRODUCTION Surface drifters have been extensively used to map ocean currents, follow oil spills and marine debris or aid search and rescue operations. Depending on the application, they have taken different configurations and sizes, but for most purposes the major concern of drifter’s developers was how to reduce the effect of direct wind and wave forcing. For the mapping of average currents in the surface layer (upper 10-100 m), the solution has been to tether a drogue to a surface buoy in such a way that the drogue was placed at the core of the layer being tracked. The first successful drifter of this kind used a holey sock drogue with a cross-sectional area 40 times larger than the cross-sectional area of all other components (Lumpkin, Özgökmen and Centurioni, 2017). Such conception has been adopted by NOAA’s  (GDP). The drifter itself remained an equatorially buoyant sphere that became quite useless when the drogue was lost. Very popular in near surface studies was a drifter developed by Russ E. Davis: a 1 m long central vertical tube containing a satellite transmitter, antenna and batteries, with four vanes attached to 50 cm long arms forming a cruciform drogue (Davis et al. 1982, Davis 1985). Stokes drift and direct wind forcing were minimized while measuring current in roughly the upper metre of the Ocean. Due to the bulkiness and fragility of the Davies drifter, an evolution of its conception is the Carthe drifter (http://carthe.org/) developed for the Lagrangian Submesoscale Experiment. Whether drogued or undrogued, most surface drifters on the market tend to be spherical or otherwise shaped, floating equatorially. Although this poses no problem when attempting to track waves, this type of buoyancy allows direct drifter exposure to the wind. This is a major inconvenient when attempting current tracking, even when the current is wind induced. On the other hand, the sensitivity to Stokes drift may be an asset when attempting to follow an oil spill or the spreading of plastic debris. The H2020 Project MELOA (https://ec-meloa.eu) used the WAVY drifter concept introduced by the RAIA Coastal Observatory (www.marnaraia.com) to further develop a family of small-size, sturdy, easy-to-handle, cost-effective, multi-purpose surface drifters. 2. THE WAVY DRIFTERS The original WAVY was a 12 cm-diameter, 750 g sphere containing a payload of a GPS receiver and antenna, a microprocessor, Bluetooth module and GPRS module and antenna. Two perfectly adjusted semi-sphere caps of polyurethane resin protected the electronics from impacts, and a Clear Flex® layer completely sealed off the system (Jorge da Silva et 7.as Jornadas de Engenharia Hidrográfica / 2.as Jornadas Luso-Espanholas de Hidrografia360  Lisboa, 21 a 23 de junho de 2022  al., 2016). Magnetic switches were used to start/stop data acquisition and data download upon recovery of the drifter. GPS sampling frequencies were possible up to 1 Hz. Positions were stored in a logger and could be displayed in near-real time through a viewer. This drifter was intended to estimate the current in the upper 10 cm of the littoral ocean, i.e., the shoaling, breaking and swash zones. If going aground with a wave, its spherical shape would favour the return to water with the next wave. The buoyancy was optimized in such a way that the drifter was submerged just enough to reduce wind drag effect while allowing the GPS antenna to stay above water. The mass was also distributed in a way that would reduce the amplitude of high frequency oscillations and, most of all, prevent capsizing. This was understood to have been achieved, as revealed by the very high acquisition rate of GPS positions at 1H, even in the wave breaking zone. Project MELOA added two members to the WAVY family - one for open ocean operations (WAVY Ocean) and another for inshore and estuarine waters (WAVY Basic) – and gave the original one (WAVY Littoral) the capacity to acquire wave data in the littoral area (Table I). The main characteristics of the original WAVY were kept, namely the spherical shape, no more than 30% height exposure above the water surface and no components out of the sphere. Table. I. WAVY family new members and possible uses. WAVY version Main Features Endurance/Acquisition Rate Possible uses WAVY Ocean 20cm, 3450g, solar panels, ARGOS sat-coms, GNSS, IMU, 2 thermistors (SST + grad-SST), directional waves Temperatures: 20 min (nominal) Waves (4096 4Hz-bursts) at 1,2,3,5 h Optional: no waves Endurance: ca. 90 days at 1 h wave obs ca. 140 days at 3 h wave obs Coastal, shelf and deep-sea, long-lived missions  Lagrangian current measurements Ground-truth for satellite-derived SST Near surface temperature gradients Numerical models feeding WAVY Littoral 12 cm, 770g, GPRS-coms, GNSS, IMU, directional waves 36 h at 4Hz continuous Exposed littoral areas, short-lived missions Wave & current measurements in nearshore areas Safety in public beaches Littoral and nearshore dynamics WAVY Basic 10cm, 400g, GPRS-coms, GNSS,  1 thermistor (SST) 24 h at1Hz continuous Estuarine & sheltered areas, short-lived missions, Citizen science The performance of the WAVY Ocean and the WAVY Littoral as wave tracking devices faced a serious challenge. Possible non-linear interactions in the very high-frequency domain, close to the natural frequency of the drifters, gave rise to a spurious energy band in the low-frequency side of the wave spectrum. As this band was still situated within the surface waves domain, it was not possible to digitally    filter it. The adopted solution was the addition of an equatorial ring to the drifters, protruding 3 cm out of the WAVY Ocean and 2 cm out of the WAVY Littoral, to damp the high frequency oscillations. No such addition was necessary on the WAVY Basic, as this drifter does not have wave measurement capacity. Although it was no longer possible to fulfil the requirement of having no protruding parts (Figure 1), the rings did not interfere with the current tracking efficiency of the drifters. Work is, nevertheless, underway to robustly find the minimum ring size that, while optimizing the drifters’ performance, does neither significantly reduce their spherical elegance nor introduce major packaging difficulties. 3. THE WAVY OCEAN DRIFTER Clearly the most ambitious member of the family, the WAVY Ocean sits at the core of Project MELOA. It aims to occupy a place of its own in a market where a large number of drifters is being offered although none with its characteristics. It was purposely intended not to be biodegradable, to avoid polluting the Ocean with its payload components, and under the expectation that all units will eventually be recovered or end up at a beach or other coastline. This would be the very long-term target and, for that purpose, MELOA worked on prototypes of energy harvesting devices. In the meantime, one needs to rely on a (small) number of battery cells to power all elements of the drifter. Figure. 1. WAVY drifters (proportions not respected): WAVY Ocean (top left), WAVY Littoral (bottom left), WAVY Basic (top right; detail of thermistor at bottom right). Notice, in the WAVY Ocean, the two thermistors (inside screws), the solar panels (above the ring) and the ARGOS sat-com antenna. When adrift, the base of the antenna approximately coincides with the water surface. Lisboa, 21, 22 e 23 de junho de 20223617as Jornadas de Engenharia Hidrográfica / 2as Jornadas Luso-Espanholas de Hidrografia Another target was to use European systems, both for geolocation and communications. The geolocation module on board the WAVY Ocean is compatible with all positioning constellations, including Galileo. ARGOS system, initially operated by CLS and presently by Kinéis (a CLS group company) was chosen for satellite communications, and a dedicated antenna has been developed from scratch by CLS to fit inside the encapsulation. The communications module is already prepared for ARGOS 4, and is ready to accept (with minor modifications) the new Kinéis system as soon as it becomes available. Unfortunately, as a consequence of the Covid-19 pandemic, that imposed drastic delays in the launch of new satellites, MELOA had to rely on ARGOS 3, with a low number of satellites and long time intervals with no passes. CLS devised a way to minimize the number of message losses, but this was done at the cost of data resolution (in position coordinates, temperature values and derived wave parameters). If it were not for the few drifters that went aground, it would not have been possible to determine the real performance of the WAVY Ocean as a sea-surface observation device. Indeed, the MELOA consortium was fortunate to have been invited to participate in the CALYPSO22 exercise that took place in the Balearic Sea in February-March 2022, under the coordination of the NATO Centre for Maritime Research and Experimentation (CMRE), in La Spezia, Italy. What follows is totally based on the results of the CALYPSO22 exercise. Table II. CALYPSO22 - valid WAVY Ocean drifting time span WO# Validated start Validated end 61 04 MAR 2022, 09:49 16 MAR 2022, 05:29 68 25 FEB 2022, 10:37 21 MAR 2022, 09:59 76 04 MAR 2022, 10:11 22 MAR 2022, 16:51 78 01 MAR 2022, 10:01 24 MAR 2022, 15:21 79 01 MAR 2022, 11:22 25 MAR 2022, 05:22 82 01 MAR 2022, 12:03 18 MAR 2022, 11:23 83 04 MAR 2022, 10:32 16 MAR 2022, 13:32 84(a) 25 FEB 2022, 10:56 20 MAR 2022, 19:35 (a) not recovered Eight WAVY Ocean drifters were deployed between 25 February and 4 March, all of them ending up in the Spanish coast of the Mediterranean by 25 March (Table 2). Seven drifters were collected and returned to Universitat Politècnica de Catalunya, a member of the MELOA consortium. The drifters were set up to acquire temperature at 20 minutes intervals and waves data every hour. The valid drifting start time of each drifter was set based on the reports of time of deployment and the observed change of temperature value upon immersion. The valid end time for drifting was offset based on location, sometimes complemented by the occurrence of anomalous wave heights, as a result of grounding. Table III. Success rate of internal data acquisition and after satellite transmission  Internal Logging Received after satellite transmission WO# Theoretical no data points Theoretical no wave data points No. GPS logging failures No. identified outliers Total no. data points % Total no. wave data points % 61 852 284   761 89.3 164 57.7 68 1727 576 2 1 GNSS, 1 (T1+T2) 1553 89.9 331 57.5 76 1317 439   1233 93.6 289 65.8 78 1673 557  1 GNSS 1536 91.8 356 63.9 79 1711 570   1574 92.0 365 64.0 82 1223 407   1118 91.4 232 57.0 83 874 291 1  840 96.1 194 66.7 84 1683 561   1605 95.4 361 64.3 The number of theoretical data points was calculated for each drifter and compared with the corresponding number of data points in the drifters’ internal logs as well as with the data points arriving via ARGOS. The same procedure was done for the waves data points only. Only two drifters showed less than 100% data position logging success: WAVY Ocean 83 and 68, respectively, with 1 and 2 failures (Table 3). These results are in line with those reported by Jorge da Silva et al. (2016) for the original WAVY, now upgraded to WAVY Littoral. On the other hand, three evident outliers were identified: 2 in WAVY Ocean 68 (GNSS and temperature) and 1 in WAVY Ocean 78 (GNSS). A different picture is revealed by the data returned via ARGOS, with an average success of 92.4% for temperature data, but only 62.2% for waves data. The reason for this difference probably resides in the strategy used to reduce data loss. While a message is built every 20 minutes with temperature data from the actual and the previous 3 temperature data readings, no similar strategy was adopted for waves data. An inspection of the time series of the physical variables derived from the internally logged data values may be looked upon as a preliminary assessment of the drifters’ performance. The current velocities estimated from the (unfiltered) positions observed at 20 minutes intervals reveal values generally within 50 cm/s until 12 March (Figure 2). During this time interval the wave heights were generally below 1.5 m, decreasing to 0.5 m with peak periods decreasing from 8 s to about 4 s. The current velocity and the wave minima occurred in 9-10 March when a longer swell from SW became apparent. After 12 March an easterly storm structured 7.as Jornadas de Engenharia Hidrográfica / 2.as Jornadas Luso-Espanholas de Hidrografia362  Lisboa, 21 a 23 de junho de 2022   Figure. 2. Time series of observed and derived variables from internally logged data (from top to bottom): significant wave height, wave peak period, wave peak direction, temperature at WAVY bottom and current velocity. the wave field around 7-9 s NE-SE, with the wave height increasing to 3-4 m. The undrogued drifters may have been carried in the direction of the waves by the breaking crests, at nearly the speed of the wave crest, hence the apparent increase in the current velocity to 40-60 cm/s. By 20 March, after the storm, a current minimum was apparent in association with a relative minimum in the wave height and a maximum in the temperature. The WAVYs did reveal the wind and wave induced dynamics typical of the near-surface of the ocean. The apparent wave height and current outliers in WO 83 on 16 March result from a validation process where nearshore wave breaking was understood to have occurred. 4. FINAL REMARKS The results obtained during the CALYPSO exercise are quite encouraging, suggesting that the WAVY Ocean is an adequate drifter to track the wind and wave induced current dynamics in near-surface of the Ocean, a capacity that is not exhibited, or has been purposely avoided, by other drifters. This capacity may be invaluable in the tracking of oil spills or plastics in the Ocean. Further analysis of the performance of the thermistor pair in the characterization of the near-surface temperature gradient (not analysed in this contribution) is still necessary. Acknowledgements Thanks are due to Dr. Pierre-Marie Poulain for the kind invitation to participate with WAVY Ocean drifters in the CALYPSO 22 exercise. The authors are also indebted to the anonymous citizens that recovered the drifters that went aground, as well as to the Spanish authorities for all the help in the drifters’ return. The contribution of Reviewer 2 to the improvement of the text was also highly appreciated. This is a contribution to Project MELOA (Multi-purpose/Multi-sensor Extra Light Oceanography Apparatus - H2020-SC5-2017-OneStageB). This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 776825. REFERENCES Davis, R.E. (1985) Drifter observations of coastal surface currents during CODE: the method and descriptive view. Journal of Geophysical Research, 90:4741–55. Davis R.E., J.E. Dufour, G.J. Parks and M.R. Perkins (1982) Two inexpensive current-following drifters. Ref. 82–28, Scripps Institute of Oceanography, La Jolla, California Jorge da Silva, A., D. Mendes, J. Pinto, B. Loureiro, M. Oliveira and A. Rocha (2016) Observation of near-shore circulation with Lagrangian drifters developed by the RAIA coastal observatory. 4as Jornadas de Engenharia Hidrográfica, Instituto Hidrográfico, Lisboa, 455-458. Lumpkin R., T. Özgökmen and L. Centurioni (2017) Advances in the Application of Surface Drifters. Annual Reviews of Marine Science, 2017 9:59–8 

Generation WAVY: a new breed of surface drifters for all ocean domains

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Generation WAVY: a new breed of surface drifters for all ocean domains

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