PROFILING CURRENT METER MOORING DATA C. Eriksen (University of Washington, USA)
N. Bogue (University of Washington, USA)
Announced availability: 1 October 1994
Data Access Profiling Current Meter (PCM) data are available from the COARE Moored Data Archive at NOAA/PMEL's Tropical Atmosphere-Ocean Array (TAO) Project Office (http://www.pmel.noaa.gov/coare/coare-data.html).
Background Two PCM moorings were located at 1 44.315S, 156 31.21 E and 1 15.399S, 156 1.001E, collecting temperature, salinity and velocity data from 24 October 1992 to 8 March 1993.
COARE MOORING DATA
Mooring name : PCM E PCM N
PCM serial number : 04 06
LOCATION
Latitude : 1/44.315 S 1/15.399 S
Longitude : 156/31.210 E 156/01.001 E
TIMING
First profile : 0900Z 24 October 92 0900Z 25 October 92
Last profile : 1800Z 7 March 93 1800Z 8 March 93
Profile interval : 3 hours 3 hours
Number of profiles : 1076 1076
TEMPERATURE AND SALINITY DEPTH BINS
Top bin center : 35.0 m 60.0 m
Bottom bin center : 180.0 m 175.0 m
bin width : 5.0 m 5.0 m
Number of bins : 30 24
VELOCITY DEPTH BINS
Top bin center : 60.0 m
Bottom bin center : 195.0 m
Bin width : 5.0 m
Number of bins : 28
UNITS
Temperature : degrees C degrees C
Salinity : PSU PSU
Currents/velocities: cm/sec
Velocities: v is positive northward, and u is positive eastward.
Data File Information Files are ASCII, named by station, variable, and the tag 'ascii'. For example, N.t.ascii is the temperature data at the N mooring. They are normally stored in UNIX compressed ('.Z') format, but can be made available in uncompressed format.
ASCII data files were created by starting at the top bin and writing its time series of 1076 profiles, then the next deeper bin, and so on to the bottom bin. Ten values were written on each line of text using C (Fortran) format specifier %8.3f (f8.3) for temperature and salinity and C (Fortran) format specifier %8.2f (f8.2) for velocity. Missing data are flagged with -999.
The electromagnetic current meter on PCM S/N 4 failed to operate during the deployment. The conductivity cell constant was calculated by comparing PCM profiles with concurrent colocated CTD casts from R/V Wecoma at deployment and recovery.
The calibration of the thermistor on PCM S/N 6 drifted during the deployment. Fortunately, the R/V Wecoma performed CTD measurements near PCM N almost monthly during the COARE Intensive Observing Period (IOP). A two-parameter fit was made in T-S space from the observed PCM profile to the Wecoma CTD profile, using thermistor calibration drift and conductivity cell constant as free parameters. From these fits, an exponential curve representing the thermistor drift and a linear conductivity cell constant drift were determined and applied to the PCM S/N 6 observations of temperature and conductivity. Further information on the fitting process, quality of fit, and error estimates are available upon request.
Data Processing PCM profile data are vertically averaged and filtered. The instrument samples at 1 Hz during its profile. These samples are averaged onboard the instrument into 5-m-wide depth bins, approximately centered on even 5-m-multiple depths. The instrument typically takes about 30 seconds to traverse a depth bin, so each bin's measurement is the average of about 30 samples. This averaging reduces surface wave contamination of current measurements.
Special attention was paid to the temperature and conductivity observations. If the PCM occupied a depth bin for less than 10 seconds, temperature and conductivity observations in that bin were rejected. Temperature and conductivity observations during the two stationary 1-min averaging periods that initiate and conclude each profile were rejected. Velocity observations were retained in all cases.
Data channels are disabled during the PCM's buoyancy adjustment maneuvers. This may result in gaps in a profile if the buoyancy adjustment maneuver spans a bin.
The discrete sampling interval and variation of ascent rate means that the width of the depth bins varies slightly from bin to bin, and the center of the bins varies slightly from profile to profile. A filter is applied to the profile data to calculate values at the centers of fixed depth bins. The filter gain is triangular, with gain of unity at the center of a given depth bin and decreasing linearly to zero 10 meters above and 10 meters below the bin center. This action creates profiles of regularly spaced samples at depths that are the same for each profile.
Gaps in the data have been filled in two ways. First, internal gaps in profiles were filled using cubic splines. Second, one- and two-point gaps in the timeseries at a given depth were filled by linear interpolation.
For more information, please contact:
Neil M. Bogue
School of Oceanography WB-10
University of Washington
Seattle WA 98195
USA
email : bogue@ocean.washington.edu
Phone: (206) 543-4485
FAX: (206) 685-3354
or
Charlie Eriksen
School of Oceanography WB-10
University of Washington
Seattle WA 98195
USA
email : charlie@ocean.washington.edu
Phone: (206) 543-6528
FAX: (206) 685-3354
References Eriksen, C., Dahlen, and Shillingford, 1982: "An Upper Ocean Moored Current and Density Profiler Applied to Winter Conditions Near Bermuda", JGR, 87, 7879-7902.
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