Subduction Experiment Data Processing Notes WHOI-UOP 06 Aug 2000, AJP ------------------------------------------------------------------------ General ------------------------------------------------------------------------ The five subduction moorings were referred to by their relative positions: northwest (NW), northeast (NE), southwest (SW), southeast (SE) and central (C). Both 3 m diameter discus buoys and 2.3 m toroid buoys were used. This is important in that the meteorological sensors were at slightly different heights on the two different buoy types (see Brink et al., 1995, Table 7, also reproduced on the UOP Data Archive web site). The two year experiment was made up of three eight-month mooring deployments, here denoted Sub1, Sub2 and Sub3. There were several mechanical failures during the course of the experiment, resulting in moorings drifting off-station. Depl. mooring notes ----- ------- ------------------------------------------- Sub1 SW Broke free 3 Nov 1991. Upper 110m recovered 2 Feb 1992, remainder recovered 4 Feb 1992 SE Broke free 10 Oct 1991. Upper 50m recovered 30 Oct 1991, remainder recovered 8 Feb 1992 NW Broke free 3 Aug 1991. Upper 400m recovered 15 Sep 1991, remainder recovered 23 Feb 1992 Sub2 SW Broke free 4 Jun 1992. Upper 10m recovered 17 Jul 1992, remainder recovered 23 Jun 1993 Sub3 NW Broke free 13 Mar 1993. Upper 104m recovered 11 Apr 1993, remainder recovered 15 Jun 1993 SW Broke free 22 May 1993. Upper 10m recovered 25 Jun 1993, remainder recovered 21 Jun 1993 Sampling ------------------------------------------------------------------------ The sampling interval for the VAWRs was 15 min, whereas that for the IMET systems was 1 min. Numerical weather prediction (NWP) products were available only every 6 h. Data were either averaged (IMET) or interpolated (NWP) as necessary to produce files with a 15 min sample interval. The sampling interval for VMCMs provided by WHOI was 7.5 min, whereas most of those provided by SIO sampled at 15 min (two SIO VMCMs were modified to sample at 4 min during Sub3). All WHOI T-pods and most SIO T-pods sampled at 15 min. A few SIO T-pods sampled at 30 min. Data were averaged or interpolated as necessary to produce files with a 15 min sample interval. Meteorology ------------------------------------------------------------------------ The fact that two different meteorological systems were deployed on two different buoy types, along with the fact that some buoys were off station for long periods, complicated the process of producing complete two year records for each site. Detailed information is available from the references listed below. A graphical summary of the data sources for each site is provided in Moyer and Weller (1997, Fig. 2), and is also available on the web site. The VAWR was the primary supplier of meteorological data for Subduction. However, data from the IMET system were utilized on those occasions when the VAWR data were either unavailable or deemed unreliable. For example, the IMET system supplied all of the basic observables on NE (6/18/91-2/14/92) and NW (2/24/92- 10/16/92), relative humidity (2/9/92-9/12/92) and barometric pressure (11/10/92-6/19/93) on SE, barometric pressure (6/23/91- 2/11/92) and incoming longwave (9/23/91-2/11/92) on C, and incoming longwave (8/24/91-11/2/91) on SW. When neither VAWR nor IMET data were available, some variables were filled using output of the European Centre for Medium-range Weather Forecasting (ECMWF) model. These instances included: relative humidity on SE (9/12/92-10/6/92) and C (5/28/92-10/14/92), barometric pressure on SW (3/28/92-6/3/92), and winds on SE (6/29/91-10/8/91) and NW (7/3/91-8/3/91). No data were available for short periods (1-5 days) between deployments when moorings were being recovered and reset. These gaps were filled with linear interpolation. During periods when moorings were off station for extended periods due to mechanical failure, basic meteorological variables (excluding the radiative fluxes) were filled using ECMWF output. Shortwave radiation was estimated using a combination of ECMWF output and clear-sky radiation estimates. Longwave radiation was estimated from the basic meteorological variables using empirical formulas. Precipitation was measured by the IMET system on the following occasions: 9/20/91-2/15/92 and 10/1/92-6/14/93 on NE, 3/1/92- 10/16/92 and 10/16/92-6/9/93 on NW 2/5/92-9/13/92 and 10/4/9- 26/19/93 on SE and 8/9/91-2/10/92 on C. Periods when no measurements were available were filled with precipitation estimates from the National Centers for Environmental Prediction (NCEP). Wind Direction A magnetic variation (also referred to as magnetic declination) appropriate for each site was applied as a rotation to the east and north wind components. The values varied slightly for each deployment, according to the table below. Speed and direction were computed from the components. Depl. NE NW C SE SW ----- ---- ---- ---- ---- ---- Sub1 -10.6 -15.1 -13.7 -12.1 -16.4 Sub2 -10.7 -15.0 -13.6 -12.1 -16.3 Sub3 -10.6 -15.0 -13.5 -12.0 -16.2 Wind directions are in oceanographic convention (direction towards), opposite to the meteorological convention (direction from). Calibration and Errors A combination of laboratory calibrations and field comparisons resulted in the following assessment of accuracy for the field measurements during Subduction (Moyer and Weller, 1997, Table 1). Variable Accuracy --------------- -------- Wind speed 0.3-0.6 m/s Wind direction 3 deg sea temperature 0.1 C air temperature 0.2 C barometric pressure 0.3 mb relative humidity 3 % shortwave radiation 5-7 W/m^2 (3%) longwave radiation 15-20 W/m^2 (5%) Temperature ------------------------------------------------------------------------ Between 53 (Sub1) and 58 (Sub2, Sub3) Richard Brancker Research temperature loggers (T-pods) were used during Subduction. Approximately half of the T-pods were supplied by WHOI, the other half by SIO. Performance Fifteen T-pods failed during Sub1 due to the leakage of a small amount of seawater into the pressure case which damaged the electronics. T-pod preparation procedures were changed for Sub2, and mechanical modifications were made prior to Sub3. These modifications were successful in that failure due to leakage was not a problem during Sub2 or Sub3. Several instruments failed due to excessive pressure when mooring components dropped to the sea floor as a result of mechanical failure. A few instruments failed to return complete records. A summary of the performance of individual instruments can be found in Brink et al. (1995, Table 4). Calibration and Adjustments Detailed information about calibration and adjustment of Subduction T-pods is not presently available. Velocity ------------------------------------------------------------------------ Velocities were measured by VMCMs at all sites except NW, where velocities above 150 m were measured with an ADCP deployed by SIO. Of the 32 (Sub1) to 34 (Sub2 and Sub3) VMCMs used during Subduction, approximately 2/3 were provided by WHOI and 1/3 by SIO. The WHOI VMCMs were a modified version of the EG&G Sea Link instrument, whereas the others were built at SIO. Relative to the standard EG&G instruments, the WHOI VMCMs incorporated changes to the bearings, propellers, and cages as described below. Type 316 and 440 stainless steel bearings had been used in previous experiments (e.g. FASINEX) with variable results. The 316 bearings showed greater corrosion resistance, but more wear. The 440 bearings were longer wearing, but susceptible to corrosion. For Subduction, silicon nitride (SiNi) bearings, which had shown both wear and corrosion resistance in prior tests, were used. Prior deployments had shown that the commercially available (EG&G) VMCM propellers, made of injection molded Delrin 577 plastic, often returned with individual blades broken off near the hub. It was determined that the likely cause was impact damage (fish bite). After several tests, it was decided to modify the WHOI propeller material to Delrin 100ST, a material with both higher impact strength and more flexibility than Delrin 577. Instrument cages within the upper 100 m used 3/4 inch diameter rods, whereas deeper instrument cages used 1/2 inch rods. All cages had a cross-brace to support the sting, as in FASINEX. Performance The data return from Sub1 was compromised by a bad batch of cassette tapes - several instruments had incomplete records due to inability to read the tapes. Over the course of all three deployments several instruments had short records for reasons other than bad tapes, and a few were affected by problems such as fouled propellers and flooding due to excessive pressure when mooring components dropped to the sea floor as a result of mechanical failure. Performance of the SiNi bearings was excellent during all three Subduction deployments. In several cases, the bearings were in such good condition upon recovery that they were re-deployed for an additional eight months. Even after sixteen months the bearing condition was excellent. None of the instruments experienced blade breakage. A summary of the performance of individual instruments can be found in Brink et al. (1995, Table 4). Magnetic Variation A magnetic variation (also referred to as magnetic declination) appropriate for each site was applied as a rotation to the east and north velocity components. The values varied slightly for each deployment, according to the table below. Speed and direction were computed from the components. Depl. NE NW C SE SW ----- ---- ---- ---- ---- ---- Sub1 -10.6 -15.1 -13.7 -12.1 -16.4 Sub2 -10.7 -15.0 -13.6 -12.1 -16.3 Sub3 -10.6 -15.0 -13.5 -12.0 -16.2 References ------------------------------------------------------------------------ Brink, N. J., K. A. Moyer, R. P. Trask, and R. A. Weller, 1995: The Subduction Experiment: Mooring Field Program and Data Summary. Woods Hole Oceanographic Institution Tech. Rep., WHOI-95-08, 113 pp. Moyer, K.A. and R.A. Weller, 1997. Observations of surface forcing from the Subduction Experiment: A comparison with global model products and climatological datasets, J. Climate, 10, 2725-2742. Trask, R. P., and N. J. Brink, 1993: Cruise Report R/V Oceanus, Subduction 1 Mooring Deployment Cruise. Woods Hole Oceanographic Institution Tech. Rep. WHOI-93-12, 77 pp. Trask, R. P., N. J. Brink, L. Regier, and N. McPhee, 1993: Cruise Report R/V Oceanus, Subduction 2 Mooring Deployment and Recovery Cruise. Woods Hole Oceanographic Institution Tech. Rep. WHOI-93-13, 102 pp. Trask, R. P., W. Jenkins, J. Sherman, N. McPhee, W. Ostrom, and R. Payne, 1993b: Cruise Report RRS Charles Darwin, Subduction 3 Mooring Deployment and Recovery Cruise. Woods Hole Oceanographic Institution Tech. Rep. WHOI-93-18, 98 pp. Trask, R. P., N. Galbraith, P. Robbins, W. Ostrom, L. Regier, G. Pezzoli, and N. McPhee, 1993c: Cruise Report R/V Knorr, Subduction 3 Mooring Recovery Cruise. Woods Hole Oceanographic Institution Tech. Rep. WHOI-93-54, 79 pp.