How to use the L2P data?

How to use the L2P data?

Postby creamnotcream » Sat Aug 08, 2020 7:40 pm

I use the formula:
SST = sea_surface_temperature * factor_scale + add_offset
to caculate the SST of a specific region.
But computed results are all around 274K. What does that mean? It must't be SST, and what formula should I use to get SST?
Here is a image I draw with the data, you can see the colorbar is around 274K.
The nc file name is 20160101-MODIS_A-JPL-L2P-A2016001050500.L2_LAC_GHRSST_D-v01.nc

[img]E:\2016-01-01%20050509%20UTC.jpg[/img]
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Re: How to use the L2P data?

Postby creamnotcream » Sun Aug 09, 2020 12:15 am

Here is the figure I draw.
Image
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Re: How to use the L2P data?

Postby yiboj » Mon Aug 10, 2020 8:15 pm

Hi,
Thanks for your inquiry.
The SST calculation formula is right and the results are in Kelvin which show the almost uniform feature around 274K in the region you are interested (just guess since I could not see the image you attached). You may get SST in Celsius by subtracting 273.15 from Kelvin.
Hope this helps an please let us know if you have any further questions.
Regards,
PODAAC DE
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Re: How to use the L2P data?

Postby creamnotcream » Sat Aug 15, 2020 8:27 am

yiboj wrote:Hi,
Thanks for your inquiry.
The SST calculation formula is right and the results are in Kelvin which show the almost uniform feature around 274K in the region you are interested (just guess since I could not see the image you attached). You may get SST in Celsius by subtracting 273.15 from Kelvin.
Hope this helps an please let us know if you have any further questions.
Regards,
PODAAC DE


Thank you for your reply!
I know the unit of it is Kelvin, but the region is in tropical, the SST here cannot be about 274K (1℃). So I guess if I use the data in a wrong way.
And here is my image again. I am sorry that I did not upload it successfully last week. Maybe I input the URL in a wrong way, if you still cannot see it, just tell me.
Image

Image
https://ibb.co/68D5qtY
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Re: How to use the L2P data?

Postby creamnotcream » Sat Aug 15, 2020 8:43 am

I also use the data in summer to draw a figure. The SST is still all around 274K.

Image

https://ibb.co/yQS6T0P
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Re: How to use the L2P data?

Postby yiboj » Sat Aug 15, 2020 8:25 pm

Hi,
When you read the nc file, the nc reader should automatically convert the data into kelvin already so you don't need to convert again. Please try and let us know if you have further questions.
Regards,
PODAAC DE
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Re: How to use the L2P data?

Postby creamnotcream » Sat Aug 15, 2020 11:32 pm

yiboj wrote:Hi,
When you read the nc file, the nc reader should automatically convert the data into kelvin already so you don't need to convert again. Please try and let us know if you have further questions.
Regards,
PODAAC DE


Hello!
I didn't "convert". I just read the variable sea_surface_temperature, and the attributes (scale_factor and add_offset) of it, and then use the formula
(SST = sea_surface_temperatur * scale_factor + add_offset) to calculate SST.

All available elements of the matrix sea_surface_temperature are between 200K to 300K. The scale_factor is 0.005. The add_offset is 273.15K.
So the result SST can only be about 274K. And I use MATLAB to read it.
Image

I think the scale_factor should be about 0.5, then the result can be reasonable. Do I miss some steps before getting SST?
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Re: How to use the L2P data?

Postby yiboj » Sun Aug 16, 2020 7:17 pm

Hi,
The values from ncread function should be the correct SST since ncread does inernal conversion already, so you don't need to do SST = sea_surface_temperatur * scale_factor + add_offset again.
Please try and let us know if you have any further questions.
Regards,
PODAAC DE
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Re: How to use the L2P data?

Postby creamnotcream » Mon Aug 17, 2020 6:52 am

yiboj wrote:Hi,
The values from ncread function should be the correct SST since ncread does inernal conversion already, so you don't need to do SST = sea_surface_temperatur * scale_factor + add_offset again.
Please try and let us know if you have any further questions.
Regards,
PODAAC DE


Oh, that means when I use the function ncread of matlab to read the variable sea_surface_temperature, I get the correct SST, and I don't need to convert it.
Thank you very much!! You really help me a lot! :D :D
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Re: How to use the L2P data?

Postby creamnotcream » Wed Aug 26, 2020 7:40 am

yiboj wrote:Hi,
The values from ncread function should be the correct SST since ncread does inernal conversion already, so you don't need to do SST = sea_surface_temperatur * scale_factor + add_offset again.
Please try and let us know if you have any further questions.
Regards,
PODAAC DE


Hello!
I still have one problem about "quality_level" and "proximity_confidence".
The data in 2018 has a variable "quality_level" which ranges from 0 to 5 (the bigger this value is, the better the quality is), and I can use this level to choose the good part of the data (for example only use the data when its quality_level is 5). The data in 2016 doesn't have "quality_level", but it has a variable "proximity_confidence". What does this variable mean? Can I use "proximity_confidence" as I use "quality_level"?

Image
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