MREA07 - LASIE07 SURFACE DRIFTER DATABASE
Buoy systems, positioning and data telemetry
The surface drifter data contained in this database come from CODE drifters equipped with GPS receivers and SST sensors, and manufactured by Technocean.
The drifters were released in the central Ligurian Sea between 14 May 2007 and 22 June 2007 in small clusters of 5 units from ITN Galatea and R/V Urania.
All drifters were tracked by, and transmitted data to, the Argos Data Collection and Location System (DCLS) onboard the NOAA polar-orbiting satellites. Transmissions were programmed every 90 seconds onboard the NOAA polar-orbiting satellites.
Transmissions were programmed every 90 seconds. Additionally they are equipped with the GPS.
Data Reduction and editing
2.1 Drifter Data
The data for each
were read, reduced and written into individual files. The
data were processed and reduced in the following way. The sensor data
time, voltage and temperature) records telemetered during a single
overpass were decompressed, that is, each record was repeated by a
of times equal to a given compression index and the repeated records
shifted back in time by successive 90 s increments. The data were then
sorted in increasing sequential order and the median values were
These median statistics were assigned to the drifter location and were
written into the output raw file. For the passes with good sensor data
but for which no
location was provided by Service Argos, the output raw latitude and
were assigned with the NaN default value.
and reduction process, the times were converted into MATLAB time (serial days
where 1 corresponds
to 01-Jan-0000). The deployment
(time, latitude and longitude) were added to the drifter
series as the initial record. A location class 4 was assigned to
of time of last good fix and type of death
The type of
or the circumstances of the termination receipt of good quality
data have been carefully investigated by examining the suspect records
in the context of their proximity to the coast line, the values and the
probability that they were picked up by seafarers. Thus, the time for
last good fix was determined and the type of death was classified into
one of five categories: Recovered (intentionally), Grounded; Picked-up;
of time of drogue loss
The tether strain
sensor located in the carrot below the surface float provides
submergence data which are used to determine whether or not the
holey-sock drogue is attached. These data are interpreted as follows:
low values (<20) correspond to no drogue, and high counts (> 30)
indicate the presence of the drogue. The time of drogue loss was
manually estimated by examining the time
series of submergence data versus time.
The position and
were edited through automatic and manual procedures. The
criteria utilized in the automatic procedure for the position data are
a maximum distance of 1000 m, a maximum speed of 50 cm/s and a maximum
45 degrees, between successive points. This means that the longitude
latitude of a point are flagged with NaN if (i) the distances
and successive points are greater than the limit; (ii) the
previous or the
successive velocities are greater than the limit and (iii) the angles
with the previous and successive points are both within 180+/-45
. This procedure
is iterated twice. Thereafter, a manual procedure is performed to
eliminate the remaining residual outliers visualized on the drifter
were edited using the following conditions: the temperatures are
with NaN if (i) the temperature gradient between successive points
and (ii) the temperature difference
with respect to a blackman
running mean exceeds 5, 4, 3, 2 and 1 degree (in iterative successive
editing based on the maximum
gradient is then repeated. Finally the manual procedure is performed
to remove remaining outliers appearing in time series plots.
interpolation and filtering
The despiked data
interpolated onto regular 2-hour intervals using an optimum analysis
known as Kriging.
Kriging used here employed an analytic function fit to the empirical
computed from the entire despiked data set. Both the interpolated
value and an estimate of its accuracy were
positions and temperature were then low-pass filtered with a Hamming
with cut-off period at 36 hours
order to remove high frequency current components, especially the
tidal and inertial currents. The low-pass time series were finally
every 6 hours and the velocity was computed by finite centered
the 6-hourly interpolated/filtered position data. The processed data
contain 6-hourly values of position, velocity and
The velocity for the first and last records of each drifter, the
after failure of the SST sensor, and all the variables during temporary
grounding, were assigned the NaN default value. If the time
between the interpolated point and the closest edited observation is
than 3 days, the corresponding velocity was assigned NaN in order
avoid meaningless interpolated velocity estimates in large data gaps.
I. Taupier-Letage, R. Gerin, R. Barbanti, D. Deponte and G.
Notarstefano (2006) Oceanographic
Cruise EGITTO-1 R/V OGS - EXPLORA 11-18 November 2005 Trieste, Italy to
Port Said, Egypt. Cruise Report.
OGS Tech. Report 29/2006 - OGA/14, OGS, Trieste, Italy.