SBE61 CTD calibration: results from the June 2014 Tangaroa Voyage

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1 SBE61 CTD calibration: results from the June 2014 Tangaroa Voyage Phil Sutton, Matt Walkington NIWA Dave Murphy, Sea Bird Electronics Dean Roemmich, Nathalie Zilberman Scripps Institution of Oceanography

2 Outline - Data collected during the June 2014 Tangaroa voyage - The SBE-61 Deep Argo sensor - Comparisons with the shipboard SBE9: pre and post calibrations - Conclusions

3 Deep Argo Tangaroa Voyage Collaboration of N.Z/U.S./Aus. Argo and Sea Bird Electronics SIO/CSIRO/NOAA/SBE/LEARNZ/University Auckland/NIWA 9 days on Tangaroa June th Repeat CTD casts to 5600m with water samples at 177 W, 36 S. 3 SBE-61 CTDs were integrated in the shipboard system (+1 internally recording) to compare SBE-61 CTD pressure, temperature, salinity with shipboard CTD (calibrated for this voyage). Photos: LEARNZ

4 Sensor development data: Only looking at SBE61/SBE9 data in this presentation. SBE 9plus: large shipboard CTD Digiquartz pressure sensor SBE 3 temperature (dual sensors) SBE 4 conductivity (dual sensors) 24 Hz data acquisition Calibrated before and after voyage SBE 61: Deep Argo CTD Developed by SBE for use in Deep Argo floats to 6000 m. Continuous and discrete sampling modes. Pressure case is separate from the float, to eliminate possible environmental effects. Will consider 3 SBE 61s, SN 5578, 5579, 5581 Kistler strain gauge pressure sensor Integrated temperature and conductivity 4 Hz data acquisition for this test (typically 1 Hz)

5 Aspirational Accuracy Goals for Deep Argo WOCE for life Pressure ±3 dbar Kistler specified accuracy is ±7 dbar in 7000 dbar. Temperature ±0.001 C Salinity ±0.002 Only better than WOCE/GO-SHIP: ±3 dbar; ±0.002 C; ±0.002.

6 Temperature Calibrations: Deep Argo goal ±0.001 C SBE 3 thermometer (dual sensors) Stated accuracy of ⁰C SBE 61 Same calibration process as SBE 41: ±0.002⁰C All three SBE 61s showed very small changes between pre voyage and post voyage calibrations 5578 post: offset -0.04m⁰C 5579 post: offset 0.05m⁰C 5581 post: offset -0.01m⁰C

7 Salinity Calibrations: Deep Argo goal ±0.002 SBE 4 (dual sensors) Stated accuracy of SBE 61 Same calibration process as SBE 41: ±0.002 Two SBE 61s showed changes between pre voyage and post voyage calibrations 5578 post: no drift 5579 post: drifted salty ~0.006: atypical 5581 post: drifted fresh ~0.003

8 Pressure Calibrations: Deep Argo goal ±3 dbar SBE 911 (Digiquartz) ±0.015% of full scale range: ~1dbar in 6000dbar SBE 61 Kistler specified accuracy is ±7 dbar in 7000 dbar post 5581 post Two SBE 61s calibrated post voyage; 5579 not recalibrated.

9 Applying the pre and post calibrations: 5578 and 5581 (relative to the SBE 911) Using the pre and post calibrations resulted in differences of the order of the Argo targets Offsets match earlier estimates from calibrations Some inconsistencies between casts in SBE61/SBE9 comparisons But this was over ~ 5 weeks

10 Applying the pre and post calibrations: 5579 (relative to the SBE 911)???? The pressure anomalies jump around between casts. Pressure not post-calibrated data only received 18/3 after reprocessing. The pressure sensor was changed after the voyage. With a new pressure sensor and an experimental cell was added and it is now on Float 6005.

11 Status and Work In Progress: Temperature Meeting the accuracy goals. Some unexplained offsets in freshly-calibrated sensors. Conductivity Continue experiments on cell drift. Some inconsistencies between casts: i.e., offset to SBE9 changes between casts. Salinity Close to desired accuracy, refinements in pressure and conductivity may yield improvements. Will calibrate over pressure in future SBE 61s. Pressure Close to desired accuracy but some need for improvement Improve transient temperature error through mechanical design work: Onboard data processing Change in form factor to reduce heat transfer between environment and sensor. Further work on accuracy via calibration process. Could be the most critical measurement because of how it interacts with T/S gradients. Important to get right as can t correct with QC. Some similar comparison work planned for R/V Investigator voyage Overall: close to targets. Targets are indeed aspirational, especially for S, where T and P errors can account for all of S target. Stability: There are changes over this 5 week time period.

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13 Sampling strategy Did 12 casts with three general strategies: 20 minute soaks at critical depths provoked by interest in long time constant responses of pressure and oxygen 3 minutes soaks everywhere provoked by desire for better discrete samples 40 minute soaks at 4 depths provoked by interest in long time constant responses of pressure and oxygen SBE 9plus Pressure is filtered to remove digitization noise (for cast alignment of 61 and 9plus) 60 seconds of data preceding each sample bottle closure was averaged SBE 61 Temperature filtered with a 2Hz anti-aliasing filter 61 sampling is synchronous with 9plus allowing data scan alignment 60 seconds of data preceding each sample bottle closure was averaged SBE seconds of measurements were averaged at the time of each bottle closure Salinity samples Collected from most bottle closures A set was run onboard the Tangaroa, a set was analysed at SBE and a set was analysed at CSIRO

14 How the T/S/P errors interact: 1) Effects of pressure error and background gradients on T/S uncertainties: Based on ±3 dbar, effect of pressure error on T/S will dominate errors above 4500m (T) and 1300m (S). Remember: found ±4.5 dbars between SBE 61 and SBE 9. 2) Effect of T error on salinity calculation from conductivity: Assuming constant conductivity, solving for salinity with T± C results in ds of ± half of the Deep Argo target. 3) Effect of P error on salinity calculation from conductivity: Assuming constant conductivity, solving for salinity with P± 3 dbar results in ds of ± half of the Deep Argo target.