Davis Strait Freshwater Flux Project
2005-2006 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 C1 Mooring, 2005-2006. 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 "C1" 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.
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Davis Strait 2005-2006 Bottom-Anchored Mooring
Mooring site "C1"
Position = 66 degrees 41.545 minutes N, 60 degrees 46.804 minutes W
Corrected ocean depth = 463 m
Mooring Deployed 2005, 17 Sept 16:49 GMT
Mooring Recovered 2006, 04 Oct 11:00 GMT
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Nominal_depth Instrument (s/n) Filename
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90 m APL/UW ULS Mark 2 (s/n 17) ULS_17_DAVIS_ST_2005_2006_5min_V1.0.txt
90 m APL/UW ULS Mark 2 (s/n 17) ULS_17_DAVIS_ST_2005_2006_10sec_V1.0.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, 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.
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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 2005-2006
mooring at site "C1".
For further information, please 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
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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: September 2005 to October 2006
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 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
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 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.
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 greatly 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 -0.10 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
-.05m and 0 m are reset to zero. Ice drafts outside the
range -.05m < draft < 20 m are replaced by the bad
data value -999. Intervals of 100% open water near the
beginning and end of the datasets exhibit non-zero apparent
"ice draft" due to surface water waves. The drafts are assigned
the value -999 in these intervals also. This results in datasets with all
ice drafts in the interval 0 m to 20 m. The ULS is programmed
to 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 55380)
* 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 17.
8) Quality control procedures:
Occurrences of open water in the ULS time series were checked 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. Each 25-minute segment of
10-second data was plotted and inspected to identify obviously
erroneous outliers. The same procedure was applied to 24-hour
segments of the 5-minute data.
9) Data format
Space delimited ASCII, formats from header (sample 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, 2005. TIME = 366.0 days
at 24:00:00 GMT, 31 December, 2005.
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 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:
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 that use these data, please acknowledge 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 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 Grant OPP-02300381
15) Sample repository location:
AON CADIS http://aoncadis.ucar.edu/home.htm
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