DOI: 10.17882/100828
title: A Drifter Dataset for the Western Mediterranean Sea collected during the SWOT mission calibration and validation phase

description: This dataset gathered the trajectories of 161 Lagrangian surface drifters that were deployed in the Western Mediterranean sea in 2023 by three campaigns of the SWOT Adopt-A-Crossover Consortium: C-SWOT-2023, BioSWOT-Med and FaSt-SWOT. Drifter trajectories are available between March 27th 2023 and January 22th 2024. The deployment strategy involved releasing drifters to target specific mesoscale and submesoscale structures in the vicinity of selected SWOT passes. These structures were identified using SPASSO software, which combined near-real-time remote data from COPERNICUS (DUACS) and early SWOT data provided by CLS/CNES. Several drifter designs are used in these experiments : SVP drifters drogued at 15m, 50m, and 100m; SVP-B drifters at 15m depth; a customized BGC-SVP drifter drogued at 15m and equipped with additional sensors such as a CTD (for temperature and salinity) and an optical triplet measuring biochemical properties of sea; surface drifters such as CODE, CARTHE, HEREON type with drogue within the first meter depth; and SPOTTER, MELODI-Eodyn devices as wave drifters. The original nominal sampling rates range from 5 minutes to 1 hour. Drifters were deployed in the passes 3 and 16 of SWOT orbit during its fast-sampling (cal-val) phase (1-day revisit until July 10th) and some of the drifters further crossed the satellite ground-tracks afterwords, when the satellite science orbit was set to 21 days. This dataset is a collaborative effort between the SWOT-AdAC consortium and FaSt-SWOT, BioSWOT-Med and C-SWOT cruises. To provide a single interoperable dataset, all drifter trajectories from the different campaigns were processed with the same scripts in a similar manner, resulting in three distinct levels of processing.
 L0 – Harmonised and preprocessed trajectories
All initial trajectories are merged into a single dataset with variables renamed to match database standards. The following steps are applied: removing rows with missing date/time, ordering by ascending time, trimming to valid deployment/recovery periods, dropping rows with missing values, eliminating duplicates, removing rows with repeated times but different positions, and excluding rows with erroneous latitude/longitude (e.g., outliers outside the Mediterranean Sea).
 L1 – Processed trajectories
L1 trajectories are filtered based on acceleration. Velocity and acceleration are calculated at each timestep, and positions with accelerations exceeding 4 times the interquartile range (IQR) are removed. This results in irregularly spaced trajectories that retain the original gps positions and therefore the overall current dynamics signal with its multiscale components but exclude gps fix outliers as defined above.
L2 – Smoothed and regularly interpolated trajectories
L2-trajectories are obtained from the L1-trajectories, that are regularly interpolated and smoothed in order to reduce noise, especially on acceleration. Two methods are used: the LOWESS method (inspired by Elipot et al. 2016) and a variational method developed by M. Demol and A. Ponte (inspired by Yaremchuk and Coelho, 2014). L2 trajectories are available with time steps of 10 minutes, 30 minutes, or 1 hour. For more details on the smoothing and interpolating processing, please refer to the attached PDF.
Data export in NetCDF format
Each drifter trajectory is stored in eight separate NetCDF files, organised into eight distinct folders based on the processing stage and temporal resolution. For a given drifter, the following files are available :
l0_data/BIOSWOT_CARTHE_4388553.nc
l1_data/BIOSWOT_CARTHE_4388553.nc
l2_data_variational_10min/BIOSWOT_CARTHE_4388553.nc
l2_data_variational_30min/BIOSWOT_CARTHE_4388553.nc
l2_data_variational_1hour/BIOSWOT_CARTHE_4388553.nc
l2_data_lowess_10min/BIOSWOT_CARTHE_4388553.nc
l2_data_lowess_30min/BIOSWOT_CARTHE_4388553.nc
l2_data_lowess_1hour/BIOSWOT_CARTHE_4388553.nc
Contact list : 
Maristella Berta (maristella.berta@sp.ismar.cnr.it), Margot Demol (margot.demol@ifremer.fr), Laura Gómez Navarro (laura.gomez@uib.es) and Lloyd Izard (lloyd.izard@locean.ipsl.fr)
PIs contact for the different involved projects: Bio-SWOT-Med  Andrea Doglioli (andrea.doglioli@univ-amu.fr); C-SWOT Pierre Garreau (pierre.garreau@ifremer.fr), Franck Dumas (franck.dumas@shom.fr) and Aurélien Ponte (aurelien.ponte@ifremer.fr); FaSt-SWOT: Ananda Pascual (ananda.pascual@imedea.uib-csic.es) and Baptiste Mourre (bmourre@imedea.uib-csic.es).
References
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Elipot, Shane, Rick Lumpkin, Renellys C Perez, Jonathan M Lilly, Jeffrey J Early, and Adam M Sykulski. “A Global Surface Drifter Data Set at Hourly Resolution.” Journal of Geophysical Research: Oceans 121, no. 5 (2016): 2937–66.
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Lumpkin, Rick, and Mayra Pazos. “Measuring Surface Currents with Surface Velocity Program Drifters: The Instrument, Its Data, and Some Recent Results.” In Lagrangian Analysis and Prediction of Coastal and Ocean Dynamics, edited by Annalisa Griffa, A. D. Kirwan, Jr., Arthur J. Mariano, Tamay Özgökmen, and H. Thomas Rossby, 1st ed., 39–67. Cambridge University Press, 2007. https://doi.org/10.1017/CBO9780511535901.003.
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Yaremchuk, Max, and Emanuel F. Coelho. “Filtering Drifter Trajectories Sampled at Submesoscale Resolution.” IEEE Journal of Oceanic Engineering 40, no. 3 (July 2015): 497–505. https://doi.org/10.1109/JOE.2014.2353472.