DOI: 10.17882/90537
title: A Mediterranean drifters' dataset

description: The CNR realised 138 experiments in the Mediterranean basin with surface Lagrangian drifters in 12 years, not continuously, between July 1998 and April 2022 (month of the last recovery), at coastal and offshore level. Lagrangian drifters produced and sold by 4 different enterprises have been used in the years, with different characteristics in data transmission, structure, repeatability of the experiments, dimensions, batteries, management of the experiments. The four drifters were used in different periods:
 - in 1998-1999 the Coastal Lagrangian Drifter (CLD), by Italian company InnoTech S.c.r.l., designed just for coastal use with GPS transmission of its position, by a Trimble Lassen™ SK8, at a frequency of 5 minutes by a GSM mobile phone. The CLD had a housing in PVC with electronic unit, rechargeable battery pack and antennas at its top. Its dimensions were 140 cm high x 27 cm in diameter with a weight of 12.5 Kg. A drogue was used below the CLD;
 - in 2009-2010 the ArgoDrifter or CODE drifter by Technocean (FL, USA) consisting in a cylinder of 100 cm height x 10 cm in diameter with four sails placed at 90°, for a total area of about 2 m2. Its not rechargeable batteries permitted transmissions till a year by an ARGOS satellite transmitter, a GPS for its localisation and a temperature sensor. Its position at sea was given by both satellite triangulation and GPS;
 - in 2014 the Iridium Ocean Drifters (ODi) by the Spanish Albatros Marine Technology SA were small, low-cost, and compact surface buoys localised by a GPS module based on Iridium satellite data transmission system (Short Burst Data – SBD). Its housing were two identical halves of a spherical drifter, sealed with an O-ring of 20 cm in diameter and 3 Kg of weight. Drogues were used below drifters;
 - in 2015-2022 coastal and offshore Nomad drifters by the Spanish SouthTEK Sensing Technologies S.L. were coastal GPRS, namely the Coastal Nomad, and offshore satellite, namely the Offshore Nomad. Both types were made in plastic, yellow colour, 72 cm in height x 22 cm in diameter and 2.895 Kg of weight. The lithium rechargeable batteries allowed operations up to 7 days to the GPRS and several months to the satellite drifters. In the water, only 16 cm of the cylindrical head were over the sea surface. The Nomad drifters were of different types: LCA (GPRS), LCE (satellite), LCH (hybrid, GPRS and satellite), LCF (satellite with temperature sensor).
Drifter data were pre-processed and repeated positions or wrong date/time, usually a failure of GPS receiver and visible on plotted tracks, were manually deleted. This was followed by an editing procedure implemented at OGS (Gerin and Bussani, 2011; Menna et al., 2017) starting with the retrieve of the deployment information then filled into the OGS PostgreSQL database, enriched with other important metadata as the type and characteristics of the instruments, the owner, the principal investigator. Here location errors were also replaced with NaNs based on the evaluation of different potential origins of error like positions outside the Mediterranean or on land, duplicated data or data acquired outside the date/time of the experiment or wrong velocities.
Further remained erroneous data were then manually removed through a visual check. In some cases, the drifter trajectory was considered as two different deployments and split into more segments due to important temporal gaps or acquisition frequency modifications during the experiment. A new recovery/deployment information were included in the database and the automatic editing procedure is relaunched.
A following step was the interpolation of edited data at uniform intervals using a kriging optimal interpolation method (Hansen and Poulain, 1996): at 1-hour intervals data with a frequency in acquisition between a few minutes and 2 hours; at 3-h intervals with frequency till 6 hours; at 6-h intervals with frequency higher than 6 hours. The velocities were calculated as finite differences of the interpolated position.
At the end, a final dataset of 158 interpolated drifter tracks was generated.
The presented datasets are available in two formats: Copernicus and NASA/NOAA-like (manageable by Panoply, a NASA-developed data viewer). They are both composed of the interpolated data in NetCDF files which include time, latitude, longitude, zonal and meridional speed, and metadata. Ancillary data like temperature, battery level or drogue presence were not considered as not available for all platforms. The dataset includes drifters’ data with subsurface drogue (in the first metres) apart from a few experiments when the drogue was at 14 or 20 m depth. These experiments correspond to the files arib_LCE234 and brib_LCE234 (20 m), arib_LCE236 and arib_LCE354 (14 m) of the dataset. The dataset is publicly available also at the SeaDataNet infrastructure at https://cdi.seadatanet.org/search/welcome.php?query=2610&query_code={9F00DF80-1881-42DD-9DF1-B9BD0282F2B0}.