PAR L3 product - Unexpected patterns

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julienlali
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Joined: Mon Oct 25, 2021 8:28 am America/New_York

PAR L3 product - Unexpected patterns

by julienlali » Mon Oct 25, 2021 9:15 am America/New_York

Dear OBPG,

I would like to use the PAR product from MODIS-Aqua. I wanted to create my own yearly composites from multiple images but found unexpected 'square-shaped' patterns.

Looking at the daily L3 images, for instance 25 Apr 2021,
https://oceancolor.gsfc.nasa.gov/showim ... 4km.nc.png,
I see unexpected patterns of missing data when zomming in to my area of interest (first attached image - GulfofStLawrence_PAR L3_Daily - print screen from the link above).
Looking at the worldview pseudo true-color image overlaid with MODIS-Aqua sea ice product for the same day, we see that the area is clearly ice-free:
https://worldview.earthdata.nasa.gov/?v ... 3A00%3A03Z

Does this mean the sea ice product that is being used in the standard processing chain fails in my area of interest ?

The resulting yearly product I computed using the 'mean' operator for binning is showed in the second attached image (GulfofStLawrence_PAR L3_Yearly). There is clearly an artificial pattern there (the pattern is also seen in the OBPG's annual composites but the palette adapted to global images shows little contrast).

Can anyone suggest an option to obtain a yearly PAR image in my area of interest ?

Best regards,

Julien
Attachments
GulfofStLawrence_PAR L3_Daily.png
GulfofStLawrence_PAR L3_Daily.png (80.08 KiB) Not viewed yet
GulfofStLawrence_PAR L3_Yearly.png
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Last edited by julienlali on Mon Nov 01, 2021 2:34 pm America/New_York, edited 1 time in total.

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gnwiii
Posts: 640
Joined: Fri Jan 29, 2021 5:51 pm America/New_York
Answers: 2

Re: PAR L3 product - Unexpected patterns

by gnwiii » Mon Oct 25, 2021 9:28 pm America/New_York

https://oceancolor.gsfc.nasa.gov/docs/ancillary/ describes the current sea-ice data set under "Ancillary Data/Surface Properties". I'm not sure what was used for the existing MODIS Aqua processing. The patterns you see are probably due to low resolution of the ice data. When doing an annual mean you need to consider how to handle areas with missing data due to ice.

For the Gulf of St. Lawrence, higher resolution ice maps are prepared from a combination of sources, include satellites, for navigation purposes. These maps often omit areas where no ships are active.

julienlali
Posts: 2
Joined: Mon Oct 25, 2021 8:28 am America/New_York

Re: PAR L3 product - Unexpected patterns

by julienlali » Mon Nov 01, 2021 3:21 pm America/New_York

Gniwiii, thank you for providing the link to the anc data. It seems the primary sea ice product is NSIDC0081.
Could this mask the resulting PAR product because it fails in this area?

Also, I am not sure this is what you are implying, but the square-like patterns are much larger than the NSIDC0081 spatial resolution.

Finally, when you say: "For the Gulf of St. Lawrence, higher resolution ice maps are prepared from a combination of sources, include satellites, for navigation purposes. These maps often omit areas where no ships are active.", what product are you referring to ?

Best,
Julien

gnwiii
Posts: 640
Joined: Fri Jan 29, 2021 5:51 pm America/New_York
Answers: 2

Re: PAR L3 product - Unexpected patterns

by gnwiii » Mon Nov 01, 2021 6:40 pm America/New_York

julienlali wrote: Mon Nov 01, 2021 3:21 pm America/New_York Gniwiii, thank you for providing the link to the anc data. It seems the primary sea ice product is NSIDC0081.
Could this mask the resulting PAR product because it fails in this area?

Also, I am not sure this is what you are implying, but the square-like patterns are much larger than the NSIDC0081 spatial resolution.
Depending on when the original level-2 processing was done, the ice mask may have been a lower resolution monthly climatology. You should be able to work back from level-3 to get a list of level-2 files using the NetCDF metadata, and metadata for each level-2 file should mention the ice data used.
julienlali wrote: Mon Nov 01, 2021 3:21 pm America/New_York Finally, when you say: "For the Gulf of St. Lawrence, higher resolution ice maps are prepared from a combination of sources, include satellites, for navigation purposes. These maps often omit areas where no ships are active.", what product are you referring to ?
See:
<https://www.canada.ca/en/environment-cl ... rview.html>

It has been many years since I worked with these. We got them as shapefiles which could be used to create masks. At the time there were efforts to create masks for use with numerical models, but I don't know the current status.

https://www.mdpi.com/journal/remotesens ... ar_sea_ice might be useful

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