Intro to AWAC Ice measurements

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Intro to AWAC Ice measurements

Posted by Torstein Pedersen at September 15. 2011

Here is a brief introduction on how to ice thickness is estimated.  You will find a link to two references below that give a more detailed description of the methods and challenges.

 

AST distance is perfomed in different manners.  The boundary interface (water-air or water-ice) is detected either with a max peak (standard) or a leading edge.  The max peak is better suited for wave measurements whereas the leading edge is better suited for ice measurements.  Note that the leading edge data may be used for wave processing however the max peak may not be well suited for ice measurements.

 

The AWAC alternates between pairs of max peak and leading edge when the ice option is enabled in the AWAC deployment configuration.  Only max peak is utilized when not enabled.            

 

The Quickwave/Storm/Prolog software estimates a mean value of the AST distance.  This is found in the WAP file.  The mean value is really best suited for fast ice that does not have substantial variations during the burst.

 

In order to resolve dynamic ice thickness as it passes the AST's field of view, the user needs to use the AST data found in the WAD file.  The AST data is reported as pairs of max peak and leading edge; the first pair is max peak and the second pair is leading edge.

 

Note that the leading edge detector is only perfomed if (a) the AWAC firmware is version 3.31 or later (mars 2010), (b) the ice mode is enabled.

 

The ice feature in  firmware is available for AWACs delivered after the end of 2008 (midlife firmware). This is indicated by a firmware version 3.xx and a hardware serial munber>1000.

 

The option for a temperture compensated pressure sensor is available for AWACs after spring 2011.  This is an option that does not come standard with the AWAC.   The error for the pressure estimate is reduced from 0.5% to 0.1% (full scale).  For a 100 meter sensor this translates to a reduction in uncertainty of 50 cm to 10 cm, which can be important when estimating ice thickness.

A discussion on estimating ice thickness can be found in the Nortek bibliography (N4000-735 and N4000-738).

 

Corrections for atmospheric pressure is simply to correct for the variations during the deployment, where the zero datum is established perferrably at the begining or end when the instrument is in air, otherwise this can be done at similar points in time for a known depth.

 

It is worth pointing out that any pressure sensor bias can be identified and removed if the AWAC has any measurements in the air, typically at the beginning and end of the deployment.

 

Uncertainty from the speed of sound and density variations can be reduced by identifying ice free periods and setting the AST and pressure distance equal.  This does not provide a unique solution for estimating the integrated density or speed of sound but it does allow the user to "tie" these values together and begin making limits on these values - bounding the solution.

 
Happy ice hunting,
Torstein

Re: Intro to AWAC Ice measurements

Posted by Torstein Pedersen at September 26. 2011

Correction to references listed.   It should read as follows:

"A discussion on estimating ice thickness can be found in the Nortek bibliography (N4000-735 and N4000-739)."

 

-Torstein

Re: Intro to AWAC Ice measurements

Posted by John G at October 19. 2019
That was so interesting for me knowing about ice thickness. Here I must say that God is great how wonderfully he has created every single thing f this world. We should be thankful to him for all the blessings we have today. Thanks for bringing college paper.org updates on your blog these are pretty helpful and understandable as well.
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