which web does the SNAP get the auxiliary data for atmospheric correction?

In processing,much dataset, such as altitude, roughness, zonal wind, meridional wind, atmospheric pressure, ozone, relative humidity, sun spectral flux, can be generated. Which web does the SNAP get the auxiliary data? are they real-time data?

I’m not sure if I understand your question correctly. But If you are talking about Sentinel-3 data (OLCI, SLSTR) then these products contain already the information you have listed.

If you have a look at the OLCI documentation you can see that products are generated in different types. Near Real Time (NRT) and Non-Time Critical (NTC).

The metrological data is retrieved from ECMWF.

Hi Marpet,

I was wondering when we do the C2RCC, do you have to change the Ozone, Air pressure at sea level, and Atmospheric aux data?
Thank you.

The influence on the result of the four parameters is quite different. So, the need to provide the values is different. Following some comments on the parameters.

Ozone: The sensitivity to ozone is quite significant. The resulting water leaving reflectance spectra varies particularly in the blue. The good news is that there are good observations from satellite, and also model predications are as good as needed for AC over ocean.
For MERIS and OLCI, ozone is taken from ECMWF data contained in the products. Since January 2022 ECMWF data is also available in S2 products in SNAP. The C2RCC processor needs an update to make use of the newly available data.

Salinity and Sea Surface Temperature: Those parameters need to be considered in combination. The impact of salinity and temperature on the pure water absorption was subject of a project some years ago. The result of this project was an update of Case2Regional with temperature and salinity as additional input neurons of the neural nets.

The influence of temperature in itself is negligible overall. The influence of salinity is relevant at low temperature, and then only the extremes matter. I.e., it makes a difference if we are in open ocean (S=36) or fresh water (S=0). In oceanic waters salinity varies between 34 and 36 (somehow) and this doesn’t matter.

The Baltic Sea in winter may be relevant. Here salinity varies between ~0 in the Neva Bay and S~25 in the Western Baltic.

Air Pressure: The air pressure has a very large impact. It is the measure of air molecules in the path of light, and thus it is determining the Rayleigh scattering, which contributes 90% or more of the signal measured at the satellite, wavelength dependent. Small errors in air pressure lead to large errors in the water reflectance and thus to errors in the IOPs and concentration values.

Hi Marpet,

Thank you so much for your reply.
I did saw there a box can be tick as use ECMWF data of source product. If i selected this box, do i need to manually change the ozne and sea level value?

I got how important is for the air pressure now. However, for the air pressure, where can i get it from? is it also from the OLCI dataset? I know it might be a silly question ,but please help with it. thank you so much.

The provided ECMWF data for ozone and air pressure by an OLCI product is sufficient.
Salinity and temperature you need to take from different source if important for your site. You might have in-situ data, or a climatology map.

I do have Salinity and temperature data, as the lake is freshwater lake,which would be nearly zero value of the Salinity.
However, for the ozone and air pressure data, would I obtain it from the ECMWF? Or air pressure can be obtain from the OLCI data profile?
Is that ECMWF only provide data information for member countries?

Ozone and pressure are provided worldwide and are contained in the product.

Yes, did find these two band inside the product.
one more silly question is that is it mean I dont need to manually add ozone or pressure data when I run the C2RCC? Since it already within the product. :smiling_face_with_tear:

Exactly. Just ensure the option ‘Use ECMWF aux data of source product’ is checked.

Thank you so much Marpet!
It mean so much for me.

Hi Marpet,

Sorry to bother you again.
Hope you can help me with this.
For S2-L1Cdata,after resampling to do the C2RCC process,there are no option as “Use ECMWF aux data of source product”, but inside the product there are air pressure and ozone file.

So how can I have these two values put into the C2RCC?
thank you

That’s currently not possible. There is already an issue for this, and it is on the development roadmap for the C2RCC.
[SIIITBX-413] C2RCC for MSI should make use of provided ECMWF data - JIRA (atlassian.net)

is that mean I cannot run S2 data?
or is there any other way for me to have sea level pressure and ozone data for S2 products?

You can use S2 data but without the data from the product. You just need to provide the values manually and there is only one value for the whole scene.
You can derive an average value from the provided data in the S2 product and use it for processing.

may i use these value? or it cannot be used?

If i cannot be used then how can i derive the value?

These values are just for certain positions, but probably good enough that you can use them.
you can get the mean by using the statistics window image

Got it. Thanks again.