A Mediterranean drifters' dataset

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}.

意大利国家研究委员会（CNR）于1998年7月至2022年4月（最后一次浮标回收的月份）的12年间，在地中海海域开展了138项非连续的近岸与远海表面拉格朗日漂流浮标（Lagrangian drifter）实验。实验期间共使用4家不同企业生产的漂流浮标，各浮标在数据传输方式、结构、实验重复性、尺寸、电池续航以及实验管理流程上均存在差异，且分阶段投入使用：
1. 1998-1999年：使用意大利InnoTech S.c.r.l.公司研发的近岸拉格朗日漂流浮标（Coastal Lagrangian Drifter, CLD），该浮标专为近岸环境设计，搭载Trimble Lassen™ SK8模块实现GPS定位，通过GSM手机网络以5分钟的频次传输位置数据。CLD的外壳采用PVC材质，内置电子单元、可充电电池组与顶部天线，尺寸为高140cm×直径27cm，重量12.5kg，浮标下方搭载锚定浮筒（drogue）。
2. 2009-2010年：使用美国Technocean公司（FL, USA）生产的Argo漂流浮标（ArgoDrifter，又称CODE漂流浮标），其主体为高100cm×直径10cm的圆柱体，搭配4个呈90°分布的帆翼，总帆面积约2㎡。该浮标采用不可充电电池供电，可通过ARGOS卫星发射器完成最长1年的数据传输，搭载GPS定位模块与温度传感器，海上位置可通过卫星三角定位与GPS双重获取。
3. 2014年：使用西班牙Albatros Marine Technology SA公司生产的铱星海洋漂流浮标（Iridium Ocean Drifters, ODi），该浮标为小型低成本紧凑型表面浮标，采用基于铱星短突发数据（Short Burst Data, SBD）传输系统的GPS模块实现定位。其外壳由两个完全相同的球形浮标半壳组成，通过直径20cm的O型圈密封，总重量3kg，浮标下方搭载锚定浮筒（drogue）。
4. 2015-2022年：使用西班牙SouthTEK Sensing Technologies S.L.公司生产的近岸与远海Nomad漂流浮标，分为近岸GPRS版（Coastal Nomad）与远海卫星版（Offshore Nomad）两类。两类浮标均采用黄色塑料材质制成，尺寸为高72cm×直径22cm，重量2.895kg。其搭载的可充电锂电池组可支持GPRS版浮标连续工作7天，卫星版浮标则可连续工作数月。该浮标的圆柱形头部仅16cm露出海面。Nomad漂流浮标包含多种亚型：LCA（GPRS版）、LCE（卫星版）、LCH（混合版，支持GPRS与卫星传输）、LCF（搭载温度传感器的卫星版）。

漂流浮标的原始数据需经过预处理：首先手动删除重复位置或时间/日期错误的数据——此类错误多由GPS接收机故障引发，可通过绘制的轨迹图直接识别。随后按照OGS制定的编辑流程（Gerin与Bussani, 2011; Menna等人, 2017）进行二次处理：首先调取部署信息并录入OGS的PostgreSQL数据库，补充仪器类型与参数、所属单位、首席研究员等重要元数据；随后基于对各类误差来源的评估，将超出地中海海域范围、位于陆地区域、重复数据、超出实验时段获取的数据以及错误流速数据对应的位置误差替换为非数值（NaN）。

仍存在的错误数据将通过人工目视检查进一步移除。部分实验中，因存在显著的时间间隔或采集频次变更，原漂流浮标轨迹被拆分为两段独立的部署段，此时需补充新的回收/部署信息至数据库，并重新运行自动编辑流程。

后续采用克里金最优插值法（kriging optimal interpolation method, Hansen与Poulain, 1996）对编辑后的数据进行均匀间隔插值：对于采集频次为数分钟至2小时的数据，以1小时间隔进行插值；采集频次不超过6小时的数据，以3小时间隔插值；采集频次高于6小时的数据，则以6小时间隔插值。流速通过插值后位置的有限差分法计算得到。

最终生成包含158条插值后漂流浮标轨迹的数据集。

本数据集提供两种格式：哥白尼（Copernicus）格式与类NASA/NOAA格式（可通过NASA开发的数据查看器Panoply读取）。两种格式均采用NetCDF文件存储插值后的数据，包含时间、纬度、经度、纬向与经向流速以及元数据。辅助数据如温度、电池电量或锚定浮筒状态等因并非所有浮标平台均支持，故未纳入数据集。

本数据集包含搭载表层锚定浮筒（drogue，即水下数米深度的锚定浮筒）的漂流浮标数据，仅少数实验的锚定浮筒布设深度为14m或20m——此类实验对应数据集中的arib_LCE234、brib_LCE234（20m深度）以及arib_LCE236、arib_LCE354（14m深度）文件。

本数据集可通过SeaDataNet基础设施公开获取，访问地址为：https://cdi.seadatanet.org/search/welcome.php?query=2610&query_code={9F00DF80-1881-42DD-9DF1-B9BD0282F2B0}。