METOCEAN Data Systems Drifter data from the Southern Oceans from 1997-11-08 to 1998-02-18 (NCEI Accession 9900163)

Drifter details

Two types of METOCEAN Data Systems' drifters were deployed in the Southern Ocean: physical drifters and optical drifters.

Physical drifters

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.

Optical drifters

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 measurements.

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:

Despiking Method

The despiking method used was based on a combination of statistical and subjective criteria.

The input parameters and typical values used (in brackets) were as follows:

* numav : number of data points to average in a running average (30)

* 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 zero).

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 removed.

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.

Optical despiking

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.

Time

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.