Comparison between LBLRTM and RTTOV comparison

[jianjun shi]

Dear RTTOV,

Hi.

Could you please provide a comparison of simulated results between AHI (H08) infrared channel using LBLRTM and RTTOV?
RTTOV settings should be using the “all_gass” coefficients, with atmospheric conditions set to mid-latitude summer, and observation angle at 0 degrees and the surface emissivity is 0.98, and the surface temperature is 294.2.

I have done some comparisons myself, but I am not very confident in the results. There is a significant difference.

Thank you.

[James Hocking]

Hi Jun Shi,

Please see my response to your other post. I think the best thing would be for us to look at your TAPE5 input to LBLRTM and provide advice on how to set it up consistently with RTTOV. Then you will be able to run LBLRTM simulations for any profile that you want to use for validation.

Best wishes,
James

[jianjun shi]

Dear James
Hi

Thank you for your reply.

I believe I made a significant mistake. Initially, I intended to compare the optical thickness of layers, so I directly compared the layer optical thickness calculated by LBLRTM with -log(leveltau[i] / leveltau[i-1]) calculated by RTTOV. However, upon careful examination of a series of articles on RTTOV, I realized that these two cannot be directly compared. Instead, what we should compare is the transmittance from space to the layers. Subsequently, I recalculated the total atmospheric transmittance and found that the total transmittance output by RTTOV and the convolved LBLRTM equivalent transmittance are indeed very consistent. Is my idea correct?

jun

[James Hocking]

Hi Jun,

Yes, what you say is correct. It would be best to compare level-to-space transmittances (or optical depths) because RTTOV is trained in such a way to compute these accurately. The individual layer optical depths are indeed “pseudo optical depths” computed in such a way to minimise the error that is incurred when summing them together. The error is due to the fact that they represent optical depths for channels with finite spectral bandwidth, and not at a single wavelength.

Best wishes,
James

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