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Surface pressure calculation

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Surface pressure calculation


I am trying to find out how surface pressure is calculated in CAM. I am using CESM 1.0.5, with E_2000 component set. 
I performed a run with 300 times present CO2 level (367e-6 ppm, so 300x367 e-6 = 0.11 volume mixing ratio). The partial pressure of CO2 assuming a vmr of 0.11 is 18000 Pa (with 29 gm/mole of air molecular mass & 44 gm/mole CO2 mass).

With a pCO2 of 18000 Pa, and 1e+5 Pa for air, the total surface pressure is 1.18 e+5 Pa.  When I ran the 300xCO2 model for 50 years, the global annual surface pressure reached an equilibrium value of ~ 1.03 e+5 Pa.

I was wondering what could cause difference? How is the surface pressure calculated in the model (in which routine)?

Thanks for the help

Ravi Kopparapu

Pennsylvania State University


My first though on this would be that in an incredibly warmer world the partial pressure from water vapor would be signifcantly higher than present day and could probably explain the increase in ps. This should be easy to check from your output. By the way running with 300 times the present CO2 is pushing the model beyond the limits of known performance and so there will almost certainly be some unexpected behavior.

Richard Neale CESM AMWG Co-chair eval(unescape('%64%6f%63%75%6d%65%6e%74%2e%77%72%69%74%65%28%27%3c%61%20%68%72%65%66%3d%22%6d%61%69%6c%74%6f%3a%72%6e%65%61%6c%65%40%75%63%61%72%2e%65%64%75%22%20%63%6c%61%73%73%3d%22%62%62%2d%65%6d%61%69%6c%22%3e%72%6e%65%61%6c%65%40%75%63%61%72%2e%65%64%75%3c%2f%61%3e%27%29%3b')) Atmosphere Modeling and Predictability Section, AMP, NCAR Tel (303) 497-1380 Fax (303) 497-1324


Hi Richard,

Thanks. The output '.nc' files doesn't seem to have partial pressure info (either for CO2 or H2O). The relevant ones given in the output files are volume mixing ratios (as far as I have seen). How is the ps calculated in the code?


Pennsylvania State University


Dear Ravi, I do not think CAM does not use the trace gas mixing ratios in the surface pressure calculation: they are used for radiative purposes only (except for water vapor). So increasing CO2 does not alter the atmospheric pressure, just the radiative transfer. I'm not sure exactly where the surface temperature is output in the code. As Rich noted: 300x CO2 is a way out of range for a stable climate, and the temperature will play into the surface pressure, so it is not surprising it is different and non linear. The mass of CO2 is not a factor. Hope that helps. Andrew Gettelman

Is there any way to increase the atmosphere from 1 bar to several or ~ten bars ?

I tried modifed SHR_CONST_PSTD in shr_const_mod.F90 (SHR_CONST_PSTD    = 5*101325.0_R8). However in the output file the PS are ~100000 Pa.

Thank you.

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