DOLCEVITA
ADRIATIC SURFACE DRIFTER DATABASE
Scientific
document
1.
Buoy systems, positioning and data telemetry
The surface drifter
data contained is this database come from various buoy
designs,including the modified CODE, CODE/Tz, WOCE-SVP, SVP/OCM,
MINIMET (WOTAN) and CMOD (XAN-3) drifters. Most systems (e.g., the
modified CODE
type) measure the surface currents within the first top meter of water
column. Others were drogued at depths ranging between 15 and 50
m. All drifters measure Sea Surface Temperature
(SST). Some drifters measure temperature at various levels in the water
column with a thermistor chain (CODE/Tz and CMOD (XAN-3)). Others
measure water optical properties (SVP/OCM). The MINIMET (WOTAN)
measure wind speed and direction.
The drifter were released in the Middle and Northern
Adriatic between
21 September 2002 and 19 November 2003. Most drifters were released
during oceanographic surveys onboard NRV Alliance (ADRIA02 and
ADRIA03), R/V Knorr (DV-1 and DV-2) and R/V Hidra (EACE). The other
drifters were released from ship-of-opportunity (R/V OGS-Explora, ferry boats, small boats).
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. A repetitive duty cycle with transmission enabled for
8 hours and disabled for the consecutive 16 hours was adopted. In
addition, some drifters were fitted with GPS receivers. Details about
the drifter systems, their positioning and the data telemetry can be
found in a data report (Ursella et al., 2004).
2.
Data Reduction and editing
2.1 Drifter Data
reduction
The data for each
drifter
were read, reduced and written into individual files. The
sensor
data were processed and reduced in the following way. The sensor data
(i.e.,
time, voltage and temperature) records telemetered during a single
satellite
overpass were decompressed, that is, each record was repeated by a
number
of times equal to a given compression index and the repeated records
were
shifted back in time by successive 90 s increments. The data were then
sorted in increasing sequential order and the median values were
estimated.
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
drifter
location was provided by Service Argos, the output raw latitude and
longitude
were assigned with the NaN default value.
During the
reading
and reduction process, the times were converted into MATLAB time (serial days
where 1 corresponds
to 01-Jan-0000). The deployment
coordinates
(time, latitude, longitude and bucket SST) were added to the drifter
time
series as the initial record. A location class 4 was assigned to
this
record.
2.2
Determination
of time of last good fix and type of death
The type of
dysfunction
or the circumstances of the termination receipt of good quality
oceanographic
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
the
last good fix was determined and the type of death was classified into
one of five categories: Recovered (intentionally), Grounded; Picked-up;
Battery
failure; Unknown. For drifter equipped with GPS receivers, the
start and end times of the Argos and GPS position time series were also
identified.
For some drifters, it was necessary to divide the files in two or more
segments to avoid prolonged data gaps due to temporary grounding and/or
dysfunction.
2.3 Data
editing
The position and
temperature data
were edited through automatic and manual procedures. The
criteria utilized in the automatic procedure for the position data are
based on
a maximum distance of 1000 m, a maximum speed of 50 cm/s and a maximum
angle of
45 degrees, between successive points. This means that the longitude
and
latitude of a point are flagged with NaN if (i) the distances
with
the previous
and successive points are greater than the limit; (ii) the
previous or the
successive velocities are greater than the limit and (iii) the angles
formed
with the previous and successive points are both within 180+/-45
degrees
. This procedure
is iterated twice. Thereafter, a manual procedure is performed to
eliminate the remaining residual outliers visualized on the drifter
tracks.
The temperature
data
were edited using the following conditions: the temperatures are
flagged
with NaN if (i) the temperature gradient between successive points
exceeds
1/8 degrees/hour
and (ii) the temperature difference
with respect to a blackman
running mean exceeds 5, 4, 3, 2 and 1 degree (in iterative successive
runs). The
editing based on the maximum
gradient is then repeated. Finally the manual procedure is performed
to remove remaining outliers appearing in time series plots.
3. Data
interpolation and filtering
The despiked data
were
interpolated onto regular 0.5-hour intervals using an optimum analysis
technique
known as Kriging.
The
Kriging used here employed an analytic function fit to the empirical
structure
function
computed from the entire despiked data set. Both the interpolated
value and an estimate of its accuracy were
computed.
The interpolated
positions and temperature were then low-pass filtered with a Hamming
filter
with cut-off period at 36 hours
in
order to remove high frequency current components, especially the
strong
tidal and inertial currents. The low-pass time series were finally
subsampled
every 6 hours and the velocity was computed by finite centered
differencing
the 6-hourly interpolated/filtered position data. The processed data
files
contain 6-hourly values of position, velocity and
temperature.
The velocity for the first and last records of each drifter, the
temperature
after failure of the SST sensor, and all the variables during temporary
grounding, were assigned the NaN default value. If the time
difference
between the interpolated point and the closest edited observation is
larger
than 3 days, the corresponding velocity was assigned NaN in order
to
avoid meaningless interpolated velocity estimates in large data gaps.
4.
Reports
Poulain,
P.M.,
L. Ursella, E. Mauri and D. Deponte (2003) DOLCEVITA-1 Cruise
31 January - 24 February 2003 Report of drifter-related
activities, Rel. 08/2003/OGA/03, OGS, Trieste, Italy, 32 pp (pdf file)
Poulain, P.M.
and R. Barbanti (2003) DOLCEVITA-2 Cruise
26 May - 15 June 2003 Report of drifter-related activities, Rel.
29/2003/OGA/11, OGS, Trieste, Italy, 17 pp. (pdf file)
Ursella,
L., R. Barbanti and P.-M. Poulain (2004) DOLCEVITA Drifter
Program: Rapporto tecnico finale, Rel.
77/2004/OGA/30, OGS, Trieste, Italy, 31 pp. (pdf file)