North Pole Environmental Observatory2001-2002 Upward Looking Sonar Sea-Ice Draft DataNSF Grants OPP-9910305 and OPP-0352754APL/UW Mark 2 Upward Looking Sonar (ULS) Sea-Ice Draft Data on the NPEO 2001-2002 Mooring.  Version 1.0This archive contains the sea-ice draft data from one APL/UW ULS mooredat a fixed level for one year near the North Pole.  Other instruments were installed on the mooring, including four AanderaaRecording Current Meters, Sea-Bird Temperature-ConductivityRecorders, and an RDI Acoustic Doppler Current Profiler (ADCP).Position  =  89 degrees  33.412 minutes North,  66 degrees  38.820 minutes EastCorrected ocean depth  =  4293mYear/day/time at depth    =  2001/100/0813 GMTYear/day/time of release  =  2002/109/1645 GMTNominal_depth  Instrument (s/n)                 Filename-------------  -------------------------        ------------------------------     48 m      APL/UW ULS Mark 2 (s/n 22)       Mooring_NP2001_ULS_22_5min_ver1.txt        48 m      APL/UW ULS Mark 2 (s/n 22)       Mooring_NP2001_ULS_22_10sec_ver1.txtEach instrument is represented by 2 ASCII files, with observationssampled at 5-minute intervals and at 10-second intervals, respectively.The 5-minute sampling is continuous, the 10-second sampling occursduring each of two 25-minute intervals each day.  Each file containsheader information followed by the data in space-delimited columns.-------------------------------------------------------------------------------Synopsis:As part of the North Pole Environmental Observatory (NSF Grants OPP-9910305 andOPP-0352754), an oceanographic mooring is anchored to the ocean floor beneaththe drifting ice near the North Pole.  Vertically distributed instrumentsmeasure ocean properties at fixed depths,  recording internally.  The data areretrieved annually when the mooring is recovered, and a new mooring deployed.The first NPEO mooring was deployed in April 2001.  This archive contains thesea-ice draft and depth data from the APL/UW Mark 2 ULS on the 2001-2002 mooring.For further information, please contactDr. Richard E. Moritz  dickm@apl.washington.edu     (206) 543-8023Polar Science Center, Applied Physics Lab, University of Washington1013 NE 40th, Seattle, WA  98105-6698   USA      FAX (206) 616-3142------------------------------------------------------------------------------- Meta data ========= 1) Platform:  Bottom-anchored oceanographic mooring near the North Pole. 2) Project title:  North Pole Environmental Observatory 3) Data collection dates: April 2001 to April 2002 4) PI: Richard E. Moritz (dickm@apl.washington.edu) 5) Data collection method:Moored APL/UW Mark 2 Upward Looking Sonar sampling continously 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 andthe range to the sea-ice bottom or open ocean surface directlyabove the ULS.  Depth is computed hydrostatically from the pressure measurement.  Sea-ice draft is computed as the differencebetween 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 byParosScientific Inc., Redmond WA.  In general, pre-deployment calibrationswere accepted.  Calibration of the beam pattern and sensitivity of theacoustic 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 gaugePs     =  Sea Level Air Pressure interpolated to the NPEO mooring          position and the ULS observation time, from the NCEP          analyses available at 0000 and 1200 GMTrhow   =  Average density of the water column above the ULS.g      =  Gravitational acceleration 9.8 m/sThe density rhow is calculated as a function of temperature, salinityand 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-OceanDynamics (Academic Press).  The temperature and salinity are interpolatedto 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 soundto 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)  wherecbar   =  Average sound speed of the water column above the ULS0.156  =  0.5 * (cbar - clens) * tau_lensclens  =  Sound speed of the fluid in the lens of the ULStau_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, salinityand 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 temperatureand 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   where0.42 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 intervalsExtensive analysis, as reported in Moritz (2005) yields a net systematic error of approximately +0.3 meters, a  standard deviation of random errors of approximately 0.05 meters during the summer, and an annual variation of systematic error on the order of 0.10 meters for ULS 22 at NPEO 2001-2002.  The summertime random errors show some persistenceover intervals of at least several hours. An offset correction was applied to both the 5-minute and10-second time series, by identifying occurrence of open waterbased on a combined analysis of the ULS and ADCP data. The offset correction eliminates most of the net systematicerror, and greatly reduces the random errors in summer. The offset has been applied to the datasets provided here.Once the offsets are applied, summer (June-Auguest) ice drafts between -0.10 m and +0.10 m are reset to 0 m, consistentwith the range of error variability for open water episodes andwith the model that ice thinner than 10 cm is rare duringthese months.  For all other months, ice drafts between -.05m and 0 m are reset to zero.  Ice drafts outside the range -.05m < draft < 20 m are replaced by the baddata value -999.   This results in datasets with all ice drafts in the interval 0 m to 20 m.  The ULS is programmedto look for ice drafts in a window that opens approximately 20 meters below sea level. 7) Instrumentation used:APL/UW Mark 2 Upward Looking Sonar, consisting of:* Paroscientific Digiquartz pressure gauge (serial number 80070)* 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 22. 8) Quality control procedures:Occurrences of open water in the ULS time series were checked forconsistency with the ADCP ice velocity data.  The ADCP data showvery distinct differences when retieving velocity from an icesurface versus an open water surface.  Each 25-minute segment of 10-second data was plotted and inspected to identify obviouslyerroneous outliers.  The same procedure was applied to 24-hoursegments of the 5-minute data.   9) Data formatSpace delimited ASCII, formats from header: YEAR  MO DAY  HR MIN SEC   TIME(days)  DEPTH(m) DRAFT(m) ----  -- ---  -- --- ---   ----------  -------- -------- 2001   4   2   0   0   0    92.00000    -0.405  -999.000NOTES-----A.  Columns 1 through 6 are in GMT, as measured by theULS clock.  The ULS clock was set to GMT minus 3 minutes (to avoid overlap with ADCP sampling) on 2 April, 2001.  The ULS clock drifted by approximately -16 minutesover 370 days.  No correction has been made to the times and dates contained in this dataset,for the initial ULS clock setting or for the clock drift.B.  TIME is given in decimal days.  TIME = 1.0 daysat 00:00:00 GMT, 1 January, 2001.  TIME = 366.0 daysat 24:00:00 GMT, 31 December, 2001.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 is42 centimeters below the sonar transducer on the ULS.D.  DRAFT is the vertical distance between the bottomof the sea-ice and local sea level, directly above the ULS.E.  The value -999.000 for DRAFT indicates missing or bad data.  F.  See the README.txt file for information on data acquisition,data reduction and error statistics. 10) Data collection problems: 11) Other related data sets:Data from the Recording Current Meters and Sea-Bird Temperaure-ConductivityRecorders on thissame mooring are provided together with this archive.   Data from thismooring's ADCP andfrom North Pole Environmental Observatory moorings covering subsequent yearsare beingprocessed and will be submitted when available.  Other parts of the Observatoryarerepresented by data sets from drifting buoys and aircraft CTD and chemistrysurveys.These data and general information about the North Pole EnvironmentalObservatory areavailable at at the NPEO website:   (http://psc.apl.washington.edu/northpole/Mooring.html).NPEO data is permanently archived at the   Arctic System Science Data Coordination Center   (http://nsidc.org/arcss/) 12) Conditions for use or citation:In reports and publications that use these data, please acknowledge theirsource: R. E. Moritz, North Pole Environmental Observatory,  NSF Grants OPP-9910305and OPP-0352754.Reference:  Moritz, R.E., "Variability of sea-ice draft near the North Pole on time scales of 1 week to 2 years",in prep., to be submitted to J. Geophys. Res (Oceans) in 2005. 13) Data qualifications or warnings:        Estimated accuracy:  Depth .01% of the depth value.        Sea-ice draft, error standard deviation of 5 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. 14) Grant numbers:NSF Grants OPP-9910305 and OPP-0352754 15) Sample repository location:The Arctic System Science Data Coordination Center   (http://nsidc.org/arcss/)-------------------------------------------------------------------------------