About ERA5 pressureLevels and pressureLayers

When calculating the bright temperature, I used ERA5 data. ERA5 data has 37 pressure levels,the 37 pressure levels: 1, 2, 3, 5, 7, 10, 20, 30, 50, 70, 100, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 775, 800, 825, 850, 875, 900, 925, 950, 975, 1000. But I don’t know what pressureLevels and pressureLayers are respectively.What should the pressureLevels and pressureLayers of ERA5 be?

Hello Meng,

so, I am assuming that you want to run the CRTM with ERA5 data?

The ERA5 pressure levels are not directly related to CRTM pressure layers and levels.
In the CRTM radiative transfer solver, the 1D radiation path is discretized into N layers.
The levels are the N+1 boundaries of the layers.

The CRTM level pressure is the pressure at the layer boundaries, and the CRTM layer pressure is the average pressure of a given layer.

Let me know if you have any further questions.

Thank you. Your reply helps me a lot. I want to run the CRTM with ERA5 data. ERA5 data has 37 pressure levels, so it has 36 pressure layers, and each layer pressure is the average pressure of a given layer.Is that right?

Roughly correct, using the hypsometric equation, you can derive the layer pressure, it’s not a linear average though.

Thanks a lot. But I don’t quite understand how to use the hypsometric equation, could you please tell me more about it?

Hi, Su
May I ask how you solved it in the end? I also want to use the data of era5, if it is convenient, I hope I can refer to how you deal with it.
Thanks a lot!

The CRTM provides a function to evaluate the hypsometric equation:

USE CRTM_Hypsometric, ONLY: HypsometricEq

which computes the layer virtual temperature from the level pressure and an array of level geometric height.
So you do need to specify the level geometric height as an additional degree of freedom.
At the moment I do not know if ERA5 also provides level geometric height data.
The layer temperature is the only additional information you need to run the CRTM.
The layer pressure can be obtained from the layer density and the ideal gas equation, if desired.

Hi, @StegmannJCSDA
Sorry to bother you.
I also want to use the data of ERA5 to run CRTM mode. The ERA5 data provides geometric height, temperature, absorbed gas and other data on 37 pressure levels within the range of 1-1000hPa. In addition, it also provides the pressure, temperature and other data of the surface.
I would like to compare the results of the different modes.
When using LBLRTM, I use the surface data as the first layer and then add the other 37 pressure level data, so I set the number of atmospheric profile boundaries to 38. LBLRTM instruction says that these are data on the layer boundary. The TAPE5 files I used are as follows:

$ 111t5ref 06/12/96 CAMEX NASA Flt #93-169 09/29/93 Wallops Island *ARM Ret O3*
 HI=1 F4=1 CN=1 AE=0 EM=1 SC=3 FI=0 PL=0 TS=0 AM=1 MG=0 LA=0 OD=0 XS=0   00   00
  781.646   1400.894            -0.001                                          REJ=0
302.1         -1.000                        -1.000
    0    2   38    1    1    3    1 0  0     0.000     0.000    
  824.0000    0.0000  180.0000
     0.000     0.075     0.299     0.528     0.761     1.000     1.244     1.495
     1.752     2.016     2.286     2.565     3.146     3.764     4.422     5.127
     5.886     6.708     7.607     8.599     9.711    10.977    11.683    12.452
    13.298    14.245    15.329    16.618    18.664    20.664    23.770    26.311
    30.692    33.139    35.522    39.290    42.286    47.734
   38         INPUT FOR CAMEX
     0.000  1008.399   302.066     AA L CAC
 1.86121041e+01 3.68500000e+02 5.25739203e-05
     0.075  1000.000   299.913     AA L CAC
 1.86121041e+01 3.63000000e+02 5.25739203e-05
     0.299   975.000   298.074     AA L CAC
 1.81627210e+01 3.63000000e+02 5.53343429e-05
     0.528   950.000   296.539     AA L CAC
 1.78844956e+01 3.63000000e+02 5.56103851e-05
     0.761   925.000   295.498     AA L CAC
 1.72258779e+01 3.63000000e+02 5.39541316e-05
     1.000   900.000   294.194     AA L CAC
 1.67139130e+01 3.63000000e+02 5.28499626e-05
     1.244   875.000   293.211     AA L CAC
 1.58441758e+01 3.63000000e+02 5.20218358e-05
     1.495   850.000   291.958     AA L CAC
 1.51991301e+01 3.63000000e+02 5.14697513e-05
     1.752   825.000   290.735     AA L CAC
 1.46487113e+01 3.63000000e+02 5.03655823e-05
     2.016   800.000   289.580     AA L CAC
 1.41914112e+01 3.64200000e+02 4.98134978e-05
     2.286   775.000   288.345     AA L CAC
 1.34834067e+01 3.66300000e+02 5.03655823e-05
     2.565   750.000   287.251     AA L CAC
 1.25439246e+01 3.68000000e+02 5.14697513e-05
     3.146   700.000   284.817     AA L CAC
 9.35828120e+00 3.68500000e+02 5.69905964e-05
     3.764   650.000   281.730     AA L CAC
 6.40072230e+00 3.68500000e+02 7.49333430e-05
     4.422   600.000   278.178     AA L CAC
 4.87085946e+00 3.68500000e+02 6.14072725e-05
     5.127   550.000   274.215     AA L CAC
 2.31103475e+00 3.68500000e+02 4.56728640e-05
     5.886   500.000   269.062     AA L CAC
 1.83375374e+00 3.68500000e+02 4.65009907e-05
     6.708   450.000   263.183     AA L CAC
 1.78021985e+00 3.68500000e+02 4.89853710e-05
     7.607   400.000   257.630     AA L CAC
 7.03133073e-01 3.68500000e+02 5.67145541e-05
     8.599   350.000   250.398     AA L CAC
 3.95501708e-01 3.68500000e+02 5.97510189e-05
     9.711   300.000   242.169     AA L CAC
 1.75711162e-01 3.68500000e+02 7.49333430e-05
    10.977   250.000   232.039     AA L CAC
 9.05093384e-02 3.68500000e+02 1.13027174e-04
    11.683   225.000   226.103     AA L CAC
 6.14803984e-02 3.68500000e+02 1.25725118e-04
    12.452   200.000   219.918     AA L CAC
 3.99914428e-02 3.68500000e+02 1.38423062e-04
    13.298   175.000   213.436     AA L CAC
 2.67964700e-02 3.68500000e+02 1.43667864e-04
    14.245   150.000   206.550     AA L CAC
 1.62404918e-02 3.68500000e+02 1.42839738e-04
    15.329   125.000   199.800     AA L CAC
 6.43851206e-03 3.68500000e+02 2.09918005e-04
    16.618   100.000   196.381     AA L CAC
 3.79951751e-03 3.68500000e+02 3.69194386e-04
    18.664    70.000   197.588     AA L CAC
 3.42251829e-03 3.68500000e+02 8.54476670e-04
    20.664    50.000   207.076     AA L CAC
 2.66851985e-03 3.68500000e+02 2.42267272e-03
    23.770    30.000   212.494     AA L CAC
 2.66851985e-03 3.68500000e+02 6.78524451e-03
    26.311    20.000   214.161     AA L CAC
 2.66851985e-03 3.68500000e+02 1.12154467e-02
    30.692    10.000   227.280     AA L CAC
 3.04551907e-03 3.68500000e+02 1.62543220e-02
    33.139     7.000   238.804     AA L CAC
 3.04551907e-03 3.68500000e+02 1.59131337e-02
    35.522     5.000   245.872     AA L CAC
 3.04551907e-03 3.68500000e+02 1.36349570e-02
    39.290     3.000   252.812     AA L CAC
 3.42251829e-03 3.68500000e+02 1.04450127e-02
    42.286     2.000   258.746     AA L CAC
 3.42251829e-03 3.68500000e+02 8.09644522e-03
    47.734     1.000   269.767     AA L CAC
 3.79951751e-03 3.68500000e+02 4.61803677e-03
     0.500   781.646  1400.894    1    1       0.010   12    1    1   11
$ Transfer to ASCII plotting data
 HI=0 F4=0 CN=0 AE=0 EM=0 SC=0 FI=0 PL=1 TS=0 AM=0 MG=1 LA=0 MS=0 XS=0    0    0
# Plot title not used
781.646   1400.894     10.2000  100.0000    5    0   10    0     1.000 0  0    0
    0.0000    1.2000    7.0200    0.2000    4    0    1    0    1    0 0    3 27
781.646   1400.894     10.2000  100.0000    5    0   12    0     1.000 0  0    0
    0.0000    1.2000    7.0200    0.2000    4    0    1    1    1    0 0    3 28


Now I’m wondering, if I wrote the LBLRTM TAPE5 file this way (assuming this is correct), what my level and layer should be set accordingly in CRTM.

  • Was level set to 39 and layer set to 38, and the layer pressure, temperature and absorbed gas use the corresponding values of the 38 boundaries in TAPE5, and then the level pressure value was obtained by interpolation or some other equation?
  • Or level was set to 38 and laye to 37, the level pressure use the pressure values of the 38 boundary in TAPE5, and the layer pressure, temperature and absorbed gas values were calculated using the hypsometric equation and other equations you mentioned above?

Any reply would be appreciated.
Thank you!

Hello @kkllww.

No problem, this is actually a good question. First, I encourage you to review the CRTM user guide to get a full overview of how to enter atmospheric data into the CRTM.
That aside, a CRTM atmospheric profile consists of layers that are essentially the center of finite volume cells. The levels are the boundaries between the cells.
That means that TOA is your lowest pressure level (array index 0) and the surface is your highest pressure level (array index n_layers) and that you’ll always have one level more than the number of layers.
There is some ambiguity in computing the level and layer values.

  • First, you can simply interpolate between levels and layers.

  • Second, the CRTM provides a module Level_Layer_Conversion with conversion functions from Layer values to Level values.

  • Third, you can use MODULE CRTM_Hypsometric to compute the layer temperature from the level geometric height and pressure.

It is important to keep track which approach you used.

@kkllww your TAPE5 file looks good at first glance but I haven’t tested it.
It is important to keep in mind that LBLRTM will give you monochromatic results, whereas the CRTM only provides results at instrument channel resolution, so it is never an apples to apples comparison.

@kkllww if you are working on an ERA5 converter for the CRTM, feel free to add it to the CRTM repository, as it might be very useful for other people as well.

Hi, @StegmannJCSDA
Thank you so much for your reply and suggestions.
I convolved the monochromatic result obtained by LBLRTM with the SRF of the instrument to get the channel brightness temperature and then compared it with CRTM BT results.
Thank you for your advice. I have understood the meaning of level and layer. I will try these methods you mentioned first, and add it to github if I could work it out.
Thanks again for your help!

Best Regards

1 Like

It might not be polite to ask you this question here. please forgive me.
Can the channel average transmittance be compared? I tried to compare the channel transmittance output by RTTOV, and then convolved the monochromatic optical transmittance calculated by LBLRTM with the spectral response function. Comparing the two, I found that there is a big difference, whether it is column or Layer, I haven’t tested CRTM yet. I googled this issue and saw the discussion here and wanted to ask for advice.