question about the nbq_grav option


Here is a question about the ‘nbq_grav’ cpp option. I see that in ‘step3d_fast.F’, that flag seems to control the ‘gravity’ term in the vertical momentum equation. My question is, if ‘nbq_grav’ is undefined, does it mean the nbq gravity term is not included in the w-equation? I tend to think that gravity term is important for driving non-hydrostatic dynamics. Can someone tell me the rationale for not including gravity in w-equation?


Gravity is already accounted for in the hydrostatic balance terms. This extra term is an added compressibility effect, which can be considered small or very small (it is not included in the incompressible nonhydrostatic equations for example). Because it imposes a constraint on the time step, we prefer to exclude it by default, but we left it as a possibility for people who want to try it in strong convection problems. You should not worry about it in most cases …

Thanks a lot. I have been working on an idealized simulation of a buoyancy-driven plume rising from the bottom, which can be considered a strong convection case. It is interesting to see whether ‘nbq_grav’ makes any difference.