OAS accession Detail for 9900163
|<< previous|||revision: 3|
|accessions_id:||9900163 | archive|
|Title:||METOCEAN Data Systems Drifters (NODC Accession 9900163)|
|Abstract:||Sea surface temperature data were collected using drifting buoys in the Southern Oceans from 08 November 1997 to 18 February 1998. Data were submitted by Oregon State University (OSU) with support from the Joint Global Ocean Flux Study / Southern Oceans (JGOFS/Southern Oceans) project.|
|Seanames:||South Atlantic Ocean, Southern Ocean|
|Instrument types:||buoy - drifting buoy|
|Datatypes:||SEA SURFACE TEMPERATURE|
|Submitter:||Abbott, Mark R.|
|Contributing projects:||JGOFS/Southern Ocean|
|Number of observations:||15|
|Supplementary information:||Drifter details |
Two types of METOCEAN Data Systems'
drifters were deployed in the Southern
Ocean: physical drifters and optical
The physical drifters used were WOCE
SVP GPS drifters, which measure
location and sea surface temperature.
The basic drifter design is shown in
the schematic to the right.
GPS location data are acquired once
per hour. Sea surface temperature
measurements are made just before and
after each GPS fix, and averaged.
These hourly data are then transmitted
to ARGOS every 400 s. ARGOS
positioning occurs if 5 or more GPS
fix attempts have failed. The typical
lifetime of these drifters in the
Southern Ocean is 4.5 months.
The optical drifters used
were WOCE/OCM/GPS (Ocean Color
Monitor) Lagrangian drifters. The
basic drifter design is similar to
that of the physical drifters
(schematic to the right), except that
optical sensors are included in the
surface unit (diagram below) and the
drogue dimensions are different (40 cm
diameter, 12.27 m long) to decrease
its influence on the optical
These drifters contain sensors that measure the water temperature
and optical sensors from Satlantic, Inc., that measure
downwelling irradiance above the sea surface and upwelling
radiance just below the sea surface . The irradiance is measured
at a center wavelength of 490 nm and the radiances are measured
at seven wavebands centered at:
412, 443, 490, 510, 555, 670, and 683 nm.
The 683 nm sensor has a bandpass of approximately 10 nm. The
remaining sensors have a bandpass of 20 nm. These optical
sensors were calibrated by Satlantic, Inc., before deployment.
Optical measurements were made every 100 s and averaged over a
one hour period. Sea surface temperature measurements were made
alternately with GPS location fixes each hour, hence SST and GPS
location data are not available at the same time. Each set of
hourly data was transmitted to ARGOS every 400 s. The typical
lifetime of these drifters in the Southern Ocean is 3 months.
Drifter data processing
The data were first converted from binary to ASCII format (as raw
counts) and then converted to physical units. Optical data were
calibrated using the calibration factors provided by Satlantic,
Inc. Missing, saturated, and anomalous data were replaced with
NaNs. Repeated and out-of-order data were removed. The data
were then despiked as follows:
The despiking method used was based on a combination of
statistical and subjective criteria.
The input parameters and typical values used (in brackets) were
* numav : number of data points to average in a running
* n : least number of standard deviations from the mean that
is acceptable (2)
* minstd : minimum standard deviation used (value varies)
* initav : estimated mean for the good data points in the
first set of 30 data points (value varies)
The steps taken were as follows:
1. A running average and standard deviation is calculated in
groups of 30 (numav) data points.
2. If the standard deviation is less than a guessed lower
estimate for the standard deviation (minstd), the guessed
estimate (minstd) is used instead (this prevents stds of
3. For the first 30 points, if any of the points deviate from a
guess for the initial mean (initav) by more than n minimum
standard deviations (n*minstd), they are removed.
4. After the initial set of 30 points, the despiking method is
as follows: Each data point is compared with the mean and
standard deviation of the previous 30 points. If it differs
from the mean by more than n standard deviations, it is
5. After the initial despiking has been made, a second
despiking is performed on data where any remaining spikes
are obvious (such as latitude, longitude, and occasionally
SST data). This second type of despiking is simply based on
visual estimation of the maximum deviation that should occur
between data points.
Before applying the above method to radiance data, each set of
radiance measurements was first divided by Ed490 to remove
diurnal variations. Poor data points were then determined by
recording the positions of spikes present in these ratios. The
corresponding data points were removed from the radiance data.
Poor data points in the Ed490 data set were assumed to correspond
to the poor data points present in the ratio of Lu412 to Ed490.
Note that this method also removes some good data points, since
some of the data points removed from the radiance data may have
been caused by poor Ed490 values, and vice versa.
The decimal day was calculated from the day of year and datatime.
The datatime is the satellite GMT time minus the data age.
Location and Sea Surface Temperature
Because of storage constraints during the data retrieval from the
optical drifters, several of the optical drifter parameters were
recorded alternately. Hence, note that measurements of location
(latitude and longitude) do not occur simultaneously with
measurements of sea-surface temperature (SST) in the optical
drifter data sets.