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Upper boundary condition?

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Hi all!

This probably is a really simple question. I only seem to find discussions on what happens at the bottom boundary, but what does WRF do at the top of the domain? I keep finding mentions that WRF uses w = 0 at a constant pressure level, but neither the WRF4 technote is very enlightening nor other documentation I found.

I assume if there's w = 0 at the top then there's no need to apply boundary conditions on, for instance, the microphysics species advected in the wind field to prevent mass in/outflow from the domain.

Is there a place in the WRF documentation where this is brought up but I just missed it?
kwerner said:
If you haven't already seen this Dynamics presentation, take a look and see if you're able to find the answer you're looking for. If not, let me know.

Hadn't seen that one thanks, yes that helps. I'm still trying to understand what's happening to the microphysics species (e.g. water vapor, cloud water, cloud ice, ...) at the top though. I assume that the model top is expected to be high enough up so that mixing ratios of e.g. water vapor or rather cloud ice are low enough to be essentially neglectable?

What does WRF do in case of a downdraft then? It must impose some kind of boundary condition in that case (e.g. zero value?)?
I reached out to one of our dynamics/physics specialists to make sure I got a correct reply for you. They said that the top surface is a constant pressure surface as the pressure-velocity, Omega, is zero there, so there is no flux across the top surface.