ATMWARN rectangular patterns

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dem1
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ATMWARN rectangular patterns

by dem1 » Wed Oct 17, 2018 10:18 am America/New_York

Hi,

We observed some rectangular pattern (aligned on a plate carrée grid) on the ATMWARN flag for this L2 product: V2018153003000.L2_SNPP_OC
There are other L2 products with the same issue at least on next days on the same region, for MODISA and VIIRS.
Is it normal? This cause rectangular holes on corresponding L3 products.

The same issue was raised in https://oceancolor.gsfc.nasa.gov/forum/oceancolor/topic_show.pl?pid=24572

Thanks,
Julien

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OB WebDev - norman
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ATMWARN rectangular patterns

by OB WebDev - norman » Wed Oct 17, 2018 5:01 pm America/New_York

For what it's worth, I see the same pattern that you do.

wget https://oceandata.sci.gsfc.nasa.gov/cgi/getfile/V2018153003000.L2_SNPP_OC.nc
h5dump -d /geophysical_data/l2_flags -b -o l2_flags V2018153003000.L2_SNPP_OC.nc
perl \
-e '$b=chr(0);$w=chr(255);' \
-e 'while(read STDIN,$f,4){' \
-e '  if(vec($f,22,1)==0){print $w}' \
-e '  else{print $b}' \
-e '}' \
<l2_flags \
|convert -size 3200x3248 -depth 8 GRAY:- V2018153003000.L2_SNPP_OC.ATMWARN.jpg

I've attached the output image.

I do not know the cause of the rectangular flagged regions, but it seems odd to me too.

Regards,
Norman

P.S. Note that I have not rotated my image to the north-is-up orientation.

OB.DAAC - SeanBailey
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ATMWARN rectangular patterns

by OB.DAAC - SeanBailey » Wed Oct 17, 2018 6:51 pm America/New_York

There are two types of conditions that will set ATMWARN.  The first, and most common (and the source of the clever name),  is when the derived epsilon value is outside an expected range for the aerosol model suite (epsilon is the ratio of the aerosol reflectance in the two -generally- NIR wavelengths used in the G&W  atmospheric correction approach).
The other condition is when one of the input ancillary records has a non-zero QC status set.  The former condition will never generate regular shapes (e.g. rectangles) in the flag array.
The latter will almost always do so.  So, in this case it is quite likely that some QC status was set in the source ancillary files indicating lesser confidence in the data, thus the warning.

Sean

OB.DAAC - SeanBailey
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ATMWARN rectangular patterns

by OB.DAAC - SeanBailey » Thu Oct 18, 2018 12:44 pm America/New_York

As a follow-up, in this case the "offending" ancillary input was the ozone data...

Sean

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