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Let's Discuss Altimetry

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Old 04-16-2018, 07:18 AM
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Originally Posted by mr buck View Post
By the time you see an altimetry chart it is already a week old.
It still remains a useful tool knowing this. If you reflect on the recent hx the site shows you can see where an up or down dwelling has moved based on wind and current.
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Old 04-16-2018, 07:19 AM
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The SeaView Mixed Layer Depth (MLD) is the same as altimetry and shows the day before, so unsure why other services would not be the same.
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Old 04-16-2018, 10:41 AM
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Originally Posted by aftergolf View Post
The SeaView Mixed Layer Depth (MLD) is the same as altimetry and shows the day before, so unsure why other services would not be the same.
Seaview refers to altimetry as sea surface height. Mixed layer depth is for lack of better terms initial thermocline depth.
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Old 04-16-2018, 08:44 PM
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Hey guys, I wanted to give a few comments in regards to altimetry and how it is acquired/updated. My statements are in reference to how RipCharts handles altimetry, which are in alignment with the best practices defined by leading oceanography and marine science institutions.

How is Altimetry obtained?
Unlike ssts and chlorophyll, altimetry is measured as a series of point readings across the earths surface as satellites orbit. An analogy is like a sewing machine stitching lines across the globe each time the satellite orbits the earth. This is unlike sst and chlorophyll, which have a wide swath to measure ocean properties as they orbit. Several altimetry satellites are acquiring sea surface height (aka altimetry) measurements as they orbit each day. In order to get enough data points to build an altimetry model, we have to aggregate 3-5 days of altimetry track lines in order to build an accurate altimetry model. Obviously newer tracks and readings will replace older ones within the same proximity. (trackline example attached below)

How do up/downwellings hold their shape and not equalize?
Upwellings and downwellings are constantly changing and trying to equalize on the world scale. However, with the corolias effect ( https://en.wikipedia.org/wiki/Coriolis_force ) altimetry upwellings and downwellings will not equalize due forces from the earths rotation and ocean currents/eddies.

Suggestions on using altimetry
Altimetry is a great dataset for finding water that contains the "chain of life". Combine the zero line with an upwelling eddy current, a temp/color break, bottom structure (canyon, shelf,etc) , or a FAD (buoy, oil rig, etc) and it can be very beneficial. I will say in areas where there are substantial upwellings and eddies, altimetry can have a larger influence than areas where there are mild changes in altimetry. For example our customers in Australia that fish the EAC and the massive upwellings/eddies it produces, will live or die by altimetry. When an upwelling is spinning off of their coast, they will have great marlin and tuna bite. I've attached a recent report from the sunshine coast Australia.

New to altimetry?
For those of you hearing about altimetry for the first time, or wanting to learn more, below is a video tutorial that would serve as a good starting point.

Hopefully this is helpful. Let me know if you have any questions about altimetry, RipCharts, or would like to test drive the RipCharts service.
Chris

Attachemnts below include:
1. example altimetry track lines.
2. recent report detailing the benefits of altimetry



Nice example showing altimetry tracklines, from the University of Tasmania, IMOS

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Old 04-17-2018, 06:50 AM
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Originally Posted by aftergolf View Post
How do you know that it is not the day before shot?
Good explanation by Southbound above. An SST chart is simply an infrared image capture by a satellite that captures a wide swath of ocean on every pass. There are less satellites in the sky that can measure altimetry, which is measured by the Satellite sending a narrow-beam pulse and waiting for the return (not unlike sonar). Since its a narrower beam you need multiple passes to build the altimetry map along the coast which takes several days.

Originally Posted by wet_rat View Post
It still remains a useful tool knowing this. If you reflect on the recent hx the site shows you can see where an up or down dwelling has moved based on wind and current.
For sure. By looking at Altimetry it can help you groudtruth the SST map, but you just have to be careful about drawing too many conclusions since you are looking at data with different timestamps.
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Old 04-17-2018, 01:24 PM
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Thanks Chris.
Ken
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