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The value of g

This post was from a previous version of the WRF&MPAS-A Support Forum. New replies have been disabled and if you have follow up questions related to this post, then please start a new thread from the forum home page.

Aviv Ofir

New member
I collected all appearances of the value of gravitational acceleration g using the simple command: grep gravity */*.[fF]

Surprisingly, I found no less than five distinct values:
chem/module_aer_drydep.F: real, parameter :: gravity = 9.80616
phys/module_gfs_physcons.F: real(kind=kind_phys),parameter:: con_g =9.80665e+0! gravity (m/s2)
phys/module_ra_rrtmg_lw.F: grav = 9.8066_rb ! Acceleration of gravity
phys/module_mp_nssl_2mom.F:! g=9.806 ! g: gravity constant
share/module_model_constants.F: REAL , PARAMETER :: g = 9.81 ! acceleration due to gravity (m {s}^-2)

The difference is small (0.04%) but the value of such a constant should be, ah.., constant.

That is - unless one accounts for the change of gravity with altitude: already at an altitude of ~32km the local gravity is 1% weaker than on the ground due to the increased distance from the center of the Earth. Question: Is this effect taken into account in WRF? from what I saw, and I am a new user, ELEVATION = GEOPOTENTIAL/9.81 -- this is not correct due to the fact that Earth is a sphere - gravity decays with distance.
I've moved this topic to the WRF section of the forum, since the question seems to be about WRF rather than about MPAS-Atmosphere. That being said, I wouldn't be entirely surprised if it were the case that multiple differing definitions of 'g' were found throughout the MPAS-A code as well.
I don't think WRF considers the changes of g with height. g is specified to be a constant. However, different developers might use slightly different values as shown in your post.