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What are u_1, u_2, v_1, v_2, w_1, w_2, etc?

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I have been trying to figure this out but couldn't seem to find an answer anywhere. What are the variables u_1, u_2, v_1, v_2, w_1, w_2 in WRF? In the Registry they are listed under "Halo update communications" so I suppose they have something to do with the halo? What is the difference for example between w_1 and w_2, is w_1 the w from an older time step and w_2 the w from current time step..?

I also noticed that when I try to access (i.e. read out) the value of grid%w in the solve_em.F module, i receive the error "This is not a field name that is defined in the encompassing structure." But reading out grid%w_1 and grid%w_2 works, any idea why?

Thank you very much!
I know this can be a bit confusing. The *_1 and *_2 variables are that particular variable at the previous and current time steps. So u_2 is the U component of wind at the current time step, and u_1 is the U component of wind at the previous time step and is sometimes used to store intermediate calculations (but they still appear in the restart files, unfortunately). These are the variables used to create restart files.

grid%w is not a variable used in the solve routine. The solve routine is specific to time step integrations. grid%w represents vertical velocity in non-hydrostatic model equations, and therefore is one of the prognostic variables for the model.