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TAO Sampling

TAO Sampling

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The TAO Project Office provides near-real-time daily-averaged surface and subsurface data from ATLAS moorings as our standard product for climate research. Daily averages are computed starting at 0000GMT and are assigned an observation "time stamp" of 12000GMT. Higher resolution data are recorded onboard the moorings and are made available after mooring recoveries, which are scheduled on an annual basis. High resolution data from older ATLAS moorings prior to 1991 (Mangum et al., 1995) and Current Meter moorings through spring 1999 (Freitag et al., 1991) consisted of a variety of sampling and averaging periods.

The tables below describe the present sampling schemes for Next Generation and Standard ATLAS moorings. Observation times in data files are assigned to the center of the averaging interval unless otherwise noted.

Basic measurements on ATLAS moorings are winds, air temperature, relative humidity, SST, and 10 subsurface temperatures. NextGeneration ATLAS moorings have the flexibility for additional sensors (rainfall, shortwave and longwave radiation, barometric pressure, salinity and ocean currents) which are deployed at selected mooring sites often in collaboration with other research programs. Basic data from ATLAS moorings are also available on the GTS network.

Next Generation ATLAS Sampling

Measurement Sample rate Sample period Sample time Data recorded in memory Transmitted data
Wind velocity components, air temperature, relative humidity 2-hz 2 min 2359-0001, 0009-0011,... 10 min Daily mean and 2-min mean from top of most recent hour
Rain rate [1] 1-hz 1 min 0000-0001, 0001-0002,... 1 min Daily mean, standard deviation, and percent time raining
Shortwave radiation 1-hz 2 min 2359-0001, 0001-0003,... 2 min Daily mean and standard deviation
Longwave radiation (thermopile, case and dome temperatures) 1- hz 2 min 2359-0001, 0001-0003,... 2 min Daily mean
Barometric Pressure 2-hz 2 min 2359-0001, 0059-0101,... Hourly Daily mean and most recent 2-min average
Sea surface and subsurface temperature and conductivity [2] 1 per 10 min instantaneous 0000, 0010,... 10 min Daily mean
Current velocity 1-hz 2-3 min 2359-0001, 0009-0011,... 10-min Daily mean
[1] To reduce instrumental noise, internally recorded 1-minute rain accumulation values are smoothed with a 16-minute Hanning filter upon recovery. These smoothed data are then differenced at 10-minute intervals and converted to rain rates in mm hr-1. The resultant rain rate values are centered at times coincident with other 10-minute data (0000, 0010, 0020...). For further information see Serra et al (2001).

[2] To minimize spiking in the salinity record due to sensor response time mismatches, the internally recorded 10-minute conductivity and temperature values are smoothed with a 13-point Hanning filter and subsampled at hourly intervals. Salinities are calculated from the smoothed hourly conductivity and temperature values using the method of Fofonoff and Millard (1983). These hourly data constitute the high-resolution salinity time series in the data base. High-resolution temperatures are offered at their original 10-minute sampling increment.

Standard ATLAS Sampling

Measurement Sample rate Sample period Sample time Data recorded in memory Transmitted data
Wind velocity components 2- hz 6 min 2357-0003, 0057-0103,... Hourly Daily mean and most recent 6-min mean
Air temperature, relative humidity, sea surface temperature 1 per 10 min 60 min 0000, 0010,... Hourly (Time stamp at end of average) Daily mean and most recent hourly mean
Subsurface temperature 1 per 10 min 1 day 0000, 0010,... Daily Daily mean


Subsurface ADCP Mooring Sampling


In addition to ATLAS moorings, there are presently 5 subsurface upward-looking ADCP (Acoustic Doppler Current Profiler) moorings deployed along the equator within the TAO/TRITON Array. Four of these sites (165E, 170W, 140W, and 110W) are maintained by PMEL while the site at 147E is maintained by the Japan Agency for Marine-earth Science and Technology (JAMSTEC). Prior to August 1996, the 170W site was maintain by the University of South Florida (USF).

Data from these moorings are available only after mooring recoveries, which are scheduled on an annual basis. The sampling scheme for TAO ADCP moorings maintained by PMEL are listed below.

Measurement Sample rate Sample period Sample time Data recorded in memory Transmitted data
Water velocity profile.

Narrowband 150kHz ADCP

0.333 hz 15-min 0000-0015, 0100-0115,... Hourly (Time stamp at beginning of average) None

ADCP data from moorings maintained by PMEL have been corrected for variations in sound velocity. The ADCP nominal bin width and pulse length were set to 8m, resulting in a velocity measurement that represents a weighted average over a nominal 16m depth, sampled at approximately 8m-depth intervals. The data available here have been remapped by linear interpolation to 5m depth intervals. Daily average data are computed as an average of 0000 to 2300 hr, requiring a minimum of 6 good hourly values.

In contrast to the present upward-looking subsurface moorings, some downward-looking surface moorings were deployed in the past. ADCP data from these moorings (named PROTEUS) at 170W, 140W, and 110W were at times significantly biased due to the presence of fish near the moorings. Corrections to the 140W and 110W ADCP data have been made using coincident mechanical current meter data (Plimpton et al., 2000). Corrections to the 170W data have been made by analysis of the ADCP echo intensity (Plimpton et al., 1997). Corrections were made to daily mean velocity profiles only. Hourly data have not been corrected, but erroneous hourly values have been edited out of the original time series.

ADCP data from PROTEUS moorings at 0, 156E and 0, 165E did not appear to have significant bias due to fish.

PROTEUS moorings were deployed for the following time periods: May 1991 to August 1995 at 110W; May 1990 to September 1995 at 140W; April 1993 to March 1994 at 170W; March 1991 to January 1996 at 165E; August 1991 to March 1993 and April to December 1994 at 156E.

For details on the JAMSTEC data see Kutsuwada and Inaba (1995). For the USF data see Weisberg and Hayes (1995), and Plimpton et al. (1997). Sampling schemes from earlier generation TAO ADCP moorings can be found in Plimpton et al. (2000).

References:
Fofonoff, P., and R. C. Millard Jr.: Algorithms for computation of fundamental properties of seawater, Tech. Pap. Mar. Sci., 44, 53 pp., Unesco, Paris, 1983.

Freitag, H.P., M.J. McPhaden, C.S. Coho, and A.J. Shepherd (1991): Equatorial wind, current and temperature data: 108W to 140W; April 1983 to October 1987. NOAA Data Report ERL PMEL-35 (PB92-119817), 116 pp.

Kutsuwada, K. and H. Inaba, (1995): "Long-Range Measurement of Surface Oceanic Current in the Western Equatorial Pacific by Acoustic Doppler Current Profiler", J. of Meteor. Soc. of Japan, 73, No. 1, pp. 1-11, 1995.

Mangum, L.J., H.P. Freitag, and M.J. McPhaden (1995): TOGA TAO array sampling schemes and sensor evaluations. Proceedings, Oceans '94 OSATES, 13-16 September 1994, Parc de Penfeld, Brest, France, II-402-II-406.

Plimpton, P.E., H.P. Freitag, and M.J. McPhaden (2000): Correcting moored ADCP data for fish-bias errors at 0, 110W and 0, 140W from 1993 to 1995. NOAA Tech. Memo. OAR PMEL-117, 35 pp.

Plimpton, P.E., H.P Freitag, M.J McPhaden, and R.H. Weisberg, (1997): "Using echo intensity to correct moored ADCP data for fish-bias error at 0, 170W", NOAA Technical Memorandum ERL PMEL-111, 17pp.

Serra, Y.L., P.A'Hearn, H.P. Freitag, and M.J. McPhaden, 2001: ATLAS self-siphoning rain gauge error estimates. J. Atmos. Ocean. Tech., 18, 1989-2002.

Weisberg, R.H. and S.P. Hayes (1995): "Upper ocean variability on the equator in the west-central Pacific at 170W", J. Geophys. Res., 100, pp. 20485-20498.

 

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