CalCOFI ADCP

 

 

Teresa K. Chereskin

I have been making direct velocity measurements on CalCOFI cruises since 1993. The program began as a 1 year pilot study funded by WOCE and has continued under the auspices of CalCOFI. Analysis of these data are funded under a grant from NASA as part of a data assimilation and modelling effort for the California Current.

Technical details of the program:

The hull-mounted ADCP is part of the ship's equipment aboard the R/V New Horizon/David Starr Jordan. The instrument is a 150 kHz narrowband unit manufactured by RD Instruments (RDI) of San Diego, CA. The ADCP transducer is mounted at a depth of 4 m. Data are acquired using AutoADCP, a modification of RDI's data acquisition system DAS version 2.48, which is configured to average the variables (east, north, vertical, and error velocity, automatic gain control, spectral width, percent good, and ship's heading) over 300 sec intervals. The AutoADCP modifications extend the DAS by various ``user-exit'' programs which include WATCHDOG, FLAG, and AGCAVE (courtesy of Dr. Charlie Flagg, Brookhaven National Laboratory) and UE4 (courtesy of Dr. Eric Firing, University of Hawaii).

The WATCHDOG timer program is useful for the unattended data collection mode used on CalCOFI. The FLAG program determines the ship's position at the end of each ensemble, and compares it with pre-defined regions that have customized configurations. On CalCOFI, FLAG is used to turn bottom tracking on/off based on regions that are shallower/deeper than 600 m. UE4 is the navigation program whose principal functions are: (1) recording GPS navigation fixes at the start and end of the ADCP averaging ensemble from a P(Y)-code receiver, (2) acquiring, screening and averaging the heading correction from the Ashtech GPS attitude sensor (New Horizon only), and (3) keeping the pc clock within 2 sec of GPS time. AGCAVE records the automatic gain control (AGC) from the 4 individual beams (normally only the average is recorded). Beam AGCs can be used to estimate the backscatter strength, a quantity directly related to biomass (Flagg and Smith, 1994).

We use an acoustic pulse and bin length of 12.56 ms, corresponding nominally to 8 vertical meters. The bin length is the time interval over which the velocity estimate is calculated. Range-gating the acoustic return as a function of time yields a vertical profile of velocity. We collect 60 depth bins. The shallowest measurement depth is 16 m. Maximum depth of good data return is about 300 m. Actual sound speed is calculated from measured temperature at the transducer and a constant salinity of 33 psu. Profiling parameters were chosen to maximize range and to minimize instrument bias; the error in the ADCP relative velocity averaged over a 10-min interval is estimated to be
less than 1 cm/s (Chereskin and Harding 1993).

Plots

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click the   symbol for the  postscript version.
Winter Spring Summer Fall
1993       nh9310-100.jpg (72507 bytes)
1994

dj9401-100.jpg (71962 bytes)

 dj9403-100.jpg (74471 bytes)   nh9410-100.jpg (70715 bytes)
1995 dj9501-100.jpg (76352 bytes) nh9504-100.jpg (74401 bytes) dj9507-100.jpg (75334 bytes) nh9510-100.jpg (75276 bytes)
1996   dj9604-100.jpg (71671 bytes) nh9608-100.jpg (71974 bytes)  
1997 dj9702-100.jpg (71611 bytes) nh9704-100.jpg (71288 bytes) dj9707-100.jpg (72879 bytes) nh9709-100.jpg (71640 bytes)
1998 dj9802-100.jpg (75570 bytes) dj9804-100.jpg (76144 bytes)    

References

Chereskin, T. K., and A. J. Harding, 1993: Modeling the performance of an acoustic Doppler current profiler. J. of Atmos. and Oc. Tech., 10, 41-63. Abstract

Flagg, C. N., and S. L. Smith, 1994. On the use of the acoustic Doppler current profiler to measure zooplankton abundance, Deep-Sea Research, 36, 455-474. Abstract

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