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For validation of model data, I want to compare the ozone data from model to the satellite results downloaded from the following link:

The satellite data are in DU(Dobson Unit) while the model data are in ppbv. To make them comparable, I converted the units of satellite data from DU to ppbv from the formula specified in the README file.

formula for conversion

But I have problem converting values of average kernel(AKs). The readme file specifies that

Note the AKs for different units (e.g., DU or ppbv) are different, the DU/DU does not cancel out because the conversion from DU to ppbv varies with altitude (e.g. resulting from different T, P).

But, I am not sure what is the right way convert the values of Average Kernel from DU/DU to ppbv/ppbv so that I can use its values.

Mala Pokhrel
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  • potentially related Dobson unit – uhoh Feb 15 '22 at 00:31
  • If I understand your question, the averaging kernels aren't part of the conversion which is correct. Instead, the averaging kernel is a unitless multiplier (should be near 1.0) that is applied at every layer. You don't convert units of the AK because by definition it is unitless. You can do the conversion without the AKs, and compare the biases with/without (as discussed in the link you provided). – f.thorpe Feb 15 '22 at 02:32
  • Also see Figure 1 for an example of averaging kernels and how they are centered on 1.0. https://acp.copernicus.org/articles/3/1285/2003/acp-3-1285-2003.pdf – f.thorpe Feb 15 '22 at 02:37
  • @f.thorpe So that means I can use the values of AK without converting? I am just confused, as they said in the manual that the AKs for different units (e.g., DU or ppbv) are different. – Mala Pokhrel Feb 15 '22 at 03:32
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    The AK would be different for DU vs ppbv since those units are not linearly proportional with altitude. Before you use an AK, you have to convert your model data to the same layers as specified in the AK (discussed in the next page in that README). To properly interpolate your vertical resolution to the satellite retrieval layers, you have to do a thickness-weighted averaging of model layer concentrations. You can't just average the model layers that span the retrieval layers, since they likely have varying thicknesses. Then you can compare model/sat w/ or w/o the AK... your choice. – f.thorpe Feb 15 '22 at 04:28
  • If they provide AKs in ppbV, then no, you wouldn't have to convert to DU. – f.thorpe Feb 15 '22 at 04:29
  • @f.thorpe Could you please elaborate more on thickness-weighted averaging of model layer? The AK values are in DU/DU so could give coud you give me advice on how to convert as the value in ppbV ? – Mala Pokhrel Feb 15 '22 at 07:21
  • Your model layers likely get thicker as you go up vertically... so the upper model layers that are within the retrieval layer would have a larger contribution to the average. For instance if model layer 5 is 1000m thick with 100 ppbV and layer 6 is 1500m with 200 ppbV, then a simple average would be 1000/2500 * 100 + 1500/2500 * 200 = 160 ppbV. There's probably a more advanced way to do it, but at least this way you are weighting each layer by it's thickness when you average multiple model layers together to match the AK layers. – f.thorpe Feb 15 '22 at 15:42
  • If the AKs are only in DU then you could convert the model data to DU and apply the AK. After that you can convert back to ppbV if you want. FYI I have not used the specific AKs you are using so you may want to reach out to someone who works on the ozone retrievals or has direct experience with that product. – f.thorpe Feb 15 '22 at 15:44
  • @f.thorpe Ok. Thank you for your help. – Mala Pokhrel Feb 16 '22 at 02:37

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