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WRF SCM Boundary Conditions

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I am trying to understand how the WRF SCM works (not many people seem to use it). I am confused by the use of the multiple grids (2x2) in the namelist (s_we=1, e_we=3; s_sn=1, e_sn=3). If I understood correctly this is mainly for numerical computing reason. However, I am wondering how does the Boundary Conditions in WRF SCM work? Specifically:

  • Does the forcing data replace boundary conditions in the SCM setup? i.e. Forcing data is used the same way as BCs?
  • In the namelist (as well as the README file for SCM), the Boundary Condition option is set to "Periodic" in X and Y. Is this periodic BC per grid cell? or per domain? That is, does the forcing/BC go out one end of the GRID (e.g. at j=1) and then enter from the other end of the GRID (e.g. at j=2), or does it go out from one end of the DOMAIN (e.g. at j=1) and come back in the other end of the DOMAIN (i.e. at j=3)

Sorry if the questions are a bit trivial, I appreciate any insights you can provide me!
Your questions are certainly not trivial, and you're right - not many people use it, perhaps because we do not have a lot of documentation on it, and it's one of the lesser-understood applications among our current support group. We are hoping to receive some documentation in the future that will hopefully make this an easier concept to interpret.

First, the domain is actually 3x3, as the start/end points are in the center of the grid cells. And you are correct, that the necessity of declaring a size larger than 1x1 is for numerical computation purposes. This is essentially the smallest size domain that we could use without having to restructure some of the source code. While this creates a 9-grid column, instead of a single column, for each variable, at each time, the values are the same in all 9 grids.

As for the boundary conditions, there really are no official boundary conditions. There are initial conditions that are given by the input_sounding and input_soil files, and then as the model runs, the values in the outer cells are passed back to the opposite side (periodic conditions), though it doesn't really matter since the values in each cell are the same. So to answer your question regarding the way in which the data is periodic, I suppose your second thought (goes out from one end of the DOMAIN (e.g. at j=1) and come back in the other end of the DOMAIN (i.e. at j=3)) would be the correct way to interpret it. Again, this is for computational purposes.

Hopefully this helps. I know this can be a bit confusing.