Davis Strait Freshwater Flux Project 2006-2007 Upward Looking Sonar Sea-Ice Draft Data NSF Grant OPP-02300381 APL/UW Mark 2 Upward Looking Sonar (ULS) Sea-Ice Draft Data on the Davis Strait C2 Mooring, 2006-2007. Version 1.0 This archive contains the sea-ice draft data from one APL/UW ULS moored at a fixed level for one year at site "C2" in Davis Strait. Other instruments were installed on the mooring, including Aanderaa Recording Current Meters, Sea-Bird Temperature-Conductivity Recorders, and an RDI Acoustic Doppler Current Profiler (ADCP). Data from these other instruments are collected in separate archive files. ----------------------- Davis Strait 2006-2007 Bottom-Anchored Mooring Mooring site "C2" Position = 66° 45.66' N 60° 04.46' W Corrected ocean depth = 700m (approx) Mooring Deployed 2006, 16 October, 06:32:30 GMT Mooring Recovered 2007, 4 October, 21:00:00 GMT ----------------------- Nominal depth Instrument (serial number) Filename ------------- ------------------------- --------------------------------------- 83 m APL/UW ULS Mark 2 (s/n 2303) ULS_2303_DAVIS_ST_2006_2007_5min_V1.txt 83 m APL/UW ULS Mark 2 (s/n 2303) ULS_2303_DAVIS_ST_2006_2007_10s_V1.txt Each instrument is represented by 2 ASCII files, with observations sampled at 5-minute intervals and at 10-second intervals, respectively. The 5-minute sampling is continuous, except for the observation at the end of each hour, i.e. there are 11 5-minute sample per hour; the 10-second sampling occurs during each of two 25-minute intervals each day. Each file contains header information followed by the data in space-delimited columns. ------------------------------------------------------------------------------- Synopsis: As part of the Davis Strait Freshwater Flux Array (NSF Grants OPP-9910305 and OPP-0352754), oceanographic moorings are anchored to the ocean floor beneath the drifting ice near in Davis Strait. Vertically distributed instruments measure ocean properties at fixed depths, recording internally. The data are retrieved annually when the mooring is recovered, and new moorings deployed. The first Davis FW moorings were deployed in 2004. This archive contains the sea-ice draft and depth data from the APL/UW Mark 2 ULS on the 2006-2007 mooring at site "C2". For further information contact: Dr. Richard E. Moritz dickm@apl.washington.edu (206) 543-8023 Polar Science Center, Applied Physics Lab, University of Washington 1013 NE 40th, Seattle, WA 98105-6698 USA FAX (206) 616-3142 ------------------------------------------------------------------------------- Meta data ========= 1) Platform: Bottom-anchored oceanographic mooring in Davis Strait. 2) Project title: AN OBSERVATIONAL ARRAY FOR HIGH RESOLUTION, YEAR-ROUND MEASUREMENTS OF VOLUME, FRESHWATER, AND ICE FLUX VARIABILITY IN DAVIS STRAIT 3) Data collection dates: October 2006 to September, 2007 4) PI: Richard E. Moritz (dickm@apl.washington.edu) 5) Data collection method: Moored APL/UW Mark 2 Upward Looking Sonar sampling continuously at 5-minute intervals, and sampling at 10-second intervals for two 25-minute periods each day (00:00:00-00:25:00 GMT and 12:00:00- 12:25:00 GMT). The ULS measures the in-situ water pressure and the range to the sea-ice bottom or open ocean surface directly above the ULS. Depth is computed hydrostatically from the pressure measurement. Sea-ice draft is computed as the difference between depth and range. The draft of sea-ice directly above the ULS changes over time as the sea-ice moves horizontally on the surface of the ocean. 6) Data calibration method: Calibration of the pressure gauge (for estimating depth) was performed by ParosScientific Inc., Redmond WA. In general, pre-deployment calibrations were accepted. Calibration of the beam pattern and sensitivity of the acoustic transducer and lens system were performed at APL/UW using a standard hydrophone/transmitter (Model E37, serial number 003). ULS depth is calculated from the hydrostatic equation: depth = (Puls - Ps)/(rhow * g) where Puls = Pressure measured by the Paroscientific gauge Ps = Sea Level Air Pressure interpolated to the mooring position and the ULS observation time, from the NCEP analyses available at 0000 and 1200 GMT rhow = Average density of the water column above the ULS. g = Gravitational acceleration 9.8 m/s The density rhow is calculated as a function of temperature, salinity and pressure at fixed depths above the ULS, using the equation of state of seawater given in Appendix A3.1 of Gill, A.E., 1982, Atmosphere-Ocean Dynamics (Academic Press). The temperature and salinity are interpolated to the mooring position and observation time from the latest version of the monthly Polar Hydrographic Climatology (PHC) of Ermold and Steele, available at the website: http://psc.apl.washington.edu/Climatology.html. The densities are then averaged vertically to produce the average density rhow. The ULS measures the range to the bottom of the sea-ice (or to the water-air interface) by measuring the time tau for a pulse of sound to propagate up to the target and back to the ULS. The range is calculated from the equation: range = 0.5 * cbar * tau - 0.156 (meters) where cbar = Average sound speed of the water column above the ULS 0.156 = 0.5 * (cbar - clens) * tau_lens clens = Sound speed of the fluid in the lens of the ULS tau_lens = Round-trip travel time for sound to propagate between the ULS tranducer and the surface of the acoustic lens. The sound speed cbar is calculated as a function of temperature, salinity and pressure at fixed depths above the ULS, using the sound speed formula presented by Urick, 1983, Principles of Underwater Sound (McGraw Hill, New York). The temperature and salinity inputs are the same values from PHC that are used to compute rhow. Sea-ice Draft is calculated from the formula: draft = depth - range - 0.42 m where 0.42 m = Vertical distance from the pressure gauge port to the ULS tranducer, when the ULS is oriented vertically. Potentially important sources of error in the ice draft computed as described above include the following: * Systematic and random errors in density rhow * Systematic and random errors in sound speed cbar * Random errors in sea-level air pressure Ps * Errors in the local value of earth's gravitational acceleration g * Timing variations in the sonar hardware/software system * Target identification errors for first-return echos from targets not directly above the ULS, or from targets intervening between the ULS and the intended target (ice bottom or sea surface). * Non-hydrostatic (short) waves during open water intervals * Non-vertical orientation of the sonar transducer caused by mooring motion. Analysis [as described e.g. in Drucker, R., S. Martin and R. Moritz. Observations of ice thickness and frazil ice in the St. Lawrence Island polynya from satellite imagery, upward looking sonar, and salinity/temperature moorings. J. Geophys. Res., 108 (C5), 2003, 18-1 - 18-18.] yields, for a ULS deployed at approximately 40 meters depth, a net systematic error of approximately -0.2 meters, a standard deviation of random errors of approximately 0.05 meters during episodes of open water among ice floes, and an annual variation of systematic error on the order of 0.10 meters for ULS 17 at NPEO 2001-2002. Both random and systematic errors are expected to increase with the depth of the ULS. An offset correction was applied to both the 5-minute and 10-second time series, by identifying occurrence of open water based on a combined analysis of the ULS and ADCP data. The offset correction eliminates most of the net systematic error, and reduces the random errors in summer. The offset has been applied to the datasets provided here. Once the offsets are applied, summer (June-August) ice drafts between -0.20 m and +0.10 m are reset to 0 m, consistent with the range of error variability for open water episodes and with the model that ice thinner than 10 cm is rare during these months. For all other months, ice drafts between -.20m and 0 m are reset to zero. Ice drafts outside the range -.20m < draft < 21 m are replaced by the bad data flag -999. Intervals of 100% open water determined independently from Canadian sea-ice charts exhibit variations of apparent "ice draft" (these intervals usually occur in recognizable envelope patterns related to wind speed) on the order of +/- 1 meter due to surface water waves. Drafts in these intervals, at the beginning and end of the time series, are set equal to zero. When no echo is detected by the sonar, the draft is assigned the value -999. This results in datasets with all ice drafts in the interval 0 m to 21 m, and all data deemed "bad" or "missing" assigned the value -999. The 5-minute observation at the end of each hour is also assigned the value -999, because the ULS processor program writes overhead information instead of the sonar echo at that time. The ULS is programmed to look for ice drafts in a window that opens approximately 20 meters below sea level, and closes approximately 2 meters above sea level. 7) Instrumentation used APL/UW Mark 2 Upward Looking Sonar, consisting of: * Paroscientific Digiquartz pressure gauge (serial number 45454) * APL/UW sonar transceiver: frequency 300 kHz, pulse length 1 millisecond. * APL/UW Acoustic lens: -3dB beamwidth 2 degrees. * As an integrated assembly, this ULS has serial number 2303. 8) Quality control procedures: Occurrences of open water in the ULS time series were selected for consistency with the ADCP ice velocity data. The ADCP data show very distinct differences when retieving velocity from an ice surface versus an open water surface. 9) Data format Space delimited ASCII, formats from header (example below): YEAR MO DAY HR MIN SEC TIME(days) DEPTH(m) DRAFT(m) ---- -- --- -- --- --- ---------- -------- -------- 2005 12 29 14 20 0 363.59722 90.422 1.278 NOTES ----- A. Columns 1 through 6 are in GMT, as measured by the ULS clock. A ULS clock drift of +/- 10 minutes per year is typical. No correction has been made in this dataset for ULS clock drift. B. TIME is given in decimal days. TIME = 1.0 days at 00:00:00 GMT, 1 January, 2006. TIME = 366.0 days at 24:00:00 GMT, 31 December, 2006. C. DEPTH is the depth below local sea level of the pressure gauge port on the ULS. When the ULS is oriented vertically, this pressure port is 42 centimeters below the sonar transducer on the ULS. D. DRAFT is the estimated vertical distance between the detected sonar target (bottom of sea-ice) and local sea level directly above the ULS. E. The value -999.000 for DRAFT indicates missing or bad data. 10) Data collection problems: On downloading the data from ULS 2303 in 2007, it was found that no echo was detected by the sonar in approximately 25% of the observations. The frequency of occurrence of missing echos displays a definite annual cycle as follows: Percentage of undetected sonar echos ------------------------------------ Nov 10% Dec 50% Jan 35% Feb 25% Mar 25% Apr 25% May 17% Jun 10% Jul 8% Aug 2% Sep 10% Oct 10% Although the APL ULS-Mark2 is designed to detect all surface targets (sea-ice, open water) at a range of 100 meters, there is some variation from instrument to instrument, and it appears that ULS2303 at the nominal depth of 83 meters was operating near the maximum limit of its detection range during 2006-2007. The sonar target strength at 300 kHz frequency varies for different types of ice and open water, depending in a complicated manner on the distribution and refractive indices of scatterers. Target strength is relatively larger for open water, compared to ice, and relatively smaller for frazil ice than for congelation ice. There are likely to be differences between the target strengths of growing and melting congelation ice, but these differences are not well documented as a function of frequency. All other factors being equal, horizontal surfaces of congelation ice that are normal to the sonar beam have larger target strength than steeply sloping ice surfaces. This is consistent with analysis of the 25-minute segments of 10-second data, which show a higher frequency of occurrence of missing data in the midst of larger undulations in under-ice topography among the remaining ("good") data points, and fewer missing data in the midst of flatter portions of the profiles. The larger undulations tend to occur preferentially when the mean ice draft is larger. For these reasons, it is likely that larger ice drafts are more likely to produce undetected sonar echos than smaller ice drafts. This would introduce a bias in statistics estimated from the data. For example, by linearly interpolating the ice drafts to the times of missing/bad data in the 25-minute segments during days 400-500 (4 February, 2007 to 15 May, 2007), we estimate mean drafts as follows: Mean draft for all "good" data 1.22m Mean draft for all interpolated data 1.28m Mean draft of data interpolated to the times of "bad" (-999) points 1.46m When estimating statistics from the datasets, users are cautioned to analyze biases that may be caused by draft-dependence of the probability of occurrence of the missing/bad data flag. The author of these datasets is conducting ongoing investigation of the bias, and will update this documentation when more complete information is available. 11) Other related data sets: Data from the Recording Current Meters and Sea-Bird Temperaure-Conductivity Recorders on this same mooring are provided together with this archive. Data from this mooring's ADCP and from Davis Strait Freshwater Flux moorings covering subsequent years are being processed and will be submitted when available. Information on the project is available at: http://iop.apl.washington.edu 12) Conditions for use or citation: In reports and publications using these data, please cite their source: R. E. Moritz, Davis Strait Freshwater Flux Array, NSF Grant OPP-02300381 13) Data qualifications or warnings: Estimated accuracy: Depth .01% of the depth value. Sea-ice draft, error standard deviation of 10 cm for level and gently undulating ice. Ice drafts for rough ice are biased high because the range is determined by the first return, which comes from the ice nearest the sonar, and within the beam footprint. This ice is sometimes not directly above the ULS as required for unbiased sampling. This dataset has not been adjusted to account for this rough ice bias. The bias associated with first-return ranging is common to many ULS sea-ice draft datasets acquired from moorings and from submarines. Additional bias on the order of 10cm (see item (10) above). 14) Grant numbers: NSF Grant OPP-02300381 15) Sample repository location: AON CADIS http://aoncadis.ucar.edu/home.htm -------------------------------------------------------------------------------