On the calculation of atmospheric radiation when observing from an orbital altitude downwards.

[StarsTravel]

Hello，I would like to calculate the observation of the radiation of the various layers of the atmosphere from the high altitude of the orbit downwards。 The observer is at an altitude of 300km, and the atmospheric gas radiation is observed 1~60km downward. That is, it can be regarded as simulating 1~60km atmospheric gas upward Radiation. It stands to reason that the lower the target altitude, the higher the final observed radiation should be, but why does the LBLRTM calculate the opposite? Like what The blue is 1 km above the target and the green one is 30 km, so why does the observer observe that the radiation at 30 km is greater than 1 km? I understand, the observer looks down The upward radiation of 1 km of atmosphere is certainly not avoidable, and the upward radiation of 30 km of atmosphere is also observed, so when the observation target is located at 1 km, the amount of upward radiation should include 30 km So the actual amount of radiation represented by blue should be higher than that represented by green, so why does the LBLRTM calculation result be the opposite? In this paper, the atmospheric radiation observed by the SABER satellite is simulated by LBLRTM, and it is obvious that the lower the target altitude, the greater the amount of radiation that can be observed. Consistent with my understanding. Attached is a TAPE file with two different target heights 1km.zip 30km.zip Thank you

[kcadyper]

Hi, StarsTravel

Did you use the same emissivity files for both cases? Please send the files you used.

[kcadyper]

Hi, StarsTravel

I think what you are trying to do is calculate the radiance leaving the 1km (or 30 km) target then transmitted to the sensor at 300km. This is not what the LBLRTM runs you have set up are doing. LBLRTM requires surface temperature and emissivity at H2 (the end of the path). You provided some values, which in effect means you have inserted a surface at 288.2K with a non-zero value for emissivity. To obtain the results you actually want you will need to run LBLRTM from the TOA to the surface, then from TOA to 1km (30km), then subtract the second runs from the first, thus obtaining the radiation at TOA leaving the 1km (30km) level. Please see the tar file I uploaded, which contains the three TAPE5s you will need (note you will have to change the emissivity, as I assumed emissivity=1), a plot showing the radiances and an IDL code that calculates and plots the radiance differences. lbl_for_stars.tar.gz

[kcadyper]

Thanks for the interesting problem. Let me know if you have further questions.

[StarsTravel]

Hello, Kcadyper. Thank you very much for your answer! 'I think what you are trying to do is calculate the radiance leaving the 1km (or 30 km) target then transmitted to the sensor at 300km.' .You're right, that's exactly what I want to calculate. I have read what you said, do you mean by the picture below： If I want to calculate the radiation from 30km to TOA（3），I should calculate the radiation from TOA to the ground（1）, then calculate the radiation from 30 km to ground（2）, and finally （3）=（1）-（2）. Have I understood correctly? You provided 3 TAPE5 files, corresponding to 1km, 30km, and 300km, respectively.I calculated the following graph： The radiation corresponding to 1km is much greater than 300km. The radiation corresponding to 30km is also slightly greater than 300km. So it's not possible to subtract 1km from the radiation corresponding to 300km, instead it should be subtracted from 300km from 1km. Only in this way can it be consistent with your calculated results. So,should it be (3)=(2) - (1) ?

[kcadyper]

yes, that's what I did in the IDL code I sent.

[StarsTravel]

hello 'To obtain the results you actually want you will need to run LBLRTM from the TOA to the surface, then from TOA to 1km (30km), then subtract the second runs from the first, thus obtaining the radiation at TOA leaving the 1km (30km) level.' But the TAPE file you gave me is a bit different from what you said.

From your file, it should be 1km to ground, 30km to ground, and 300km to ground. How should I understand it？

[StarsTravel]

yes, that's what I did in the IDL code I sent.

Thank you. Let me summarize again： As you said,I am trying to do is calculate the radiance leaving the 1km (or 30 km and so on) target then transmitted to the sensor at 300km. The first step, I will need to run LBLRTM from the TOA to the surface (H1=TOA，H2=ground),this step is recorded as (1).The second step, I will need to run LBLRTM from the 30km(or other heights that interest me) to the surface (H1=30， H2=ground),this step is recorded as (2).Finally, subtract the result of the first step from the result of the second step , i.e (2)-(1). I will obtain the radiation amount from 30km to TOA.

Thank you again！！！

[kcadyper]

Yes, you are correct.

[StarsTravel]

Yes, you are correct.

Thank you very much!!!

[StarsTravel]

Hi, StarsTravel

I think what you are trying to do is calculate the radiance leaving the 1km (or 30 km) target then transmitted to the sensor at 300km. This is not what the LBLRTM runs you have set up are doing. LBLRTM requires surface temperature and emissivity at H2 (the end of the path). You provided some values, which in effect means you have inserted a surface at 288.2K with a non-zero value for emissivity. To obtain the results you actually want you will need to run LBLRTM from the TOA to the surface, then from TOA to 1km (30km), then subtract the second runs from the first, thus obtaining the radiation at TOA leaving the 1km (30km) level. Please see the tar file I uploaded, which contains the three TAPE5s you will need (note you will have to change the emissivity, as I assumed emissivity=1), a plot showing the radiances and an IDL code that calculates and plots the radiance differences. lbl_for_stars.tar.gz

As you say，'LBLRTM requires surface temperature and emissivity at H2 (the end of the path). You provided some values, which in effect means you have inserted a surface at 288.2K with a non-zero value for emissivity. ' When I set H2(the end of the path), I also need to set the corresponding temperature, emissivity, and reflectivity. I would like to ask where the reference values for emissivity and reflectance are generally obtained if H2 is ground？ If H2 is an altitudinal atmosphere, should I get temperature information for that altitude in advance? Where do I get the emissivity and reflectance reference values for that altitude ? Thank you

[max19951001]

Hi, StarsTravel, We saw this issue when we were looking at how to use LBLRTM to model atmospheric long-wave radiation. We used modtran to simulate atmospheric radiation transmission simulation before, but because it is too slow, I want to learn about LBLRTM. May I ask if your LBLRTM is operated on windows system, or under linux, and whether there are any successful cases in windows