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Difference Between LANDUSEF and FRC_URB2D?

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Hello, wrf team

I am wondering if there is any difference between the variable FRC_URB2D and the layer for urban land type in the variable LANDUSEF?

As far as I know, LANDUSEF is to specify the fraction of each land cover type in different grid cells. So if summing up all the layers in this variable, the values should be 1 for each grid. While the variable LU_INDEX only specifies the dominant land cover type in each grid cell. Am I correct?

Another question is that how do these variables affect the final results? which module is actually using these two variables, FRC_URB2D and LANDUSEF. If I have defined the values in the urban land type layer in LANDUSEF, does it matters to leave FRC_URB2D all zeros?

I am trying to figure out which land cover variables LANDUSEF, LU_INDEX, and FRC_URB2D dominate the final simulation results.
I am guessing that LANDUSEF should be the most important because it is actually the weight values for different properties (such as the roughness parameter, and other parameters defined in the TBL files) of different land cover types in the grid cell to help define the overall properties of the grid.

Thank you.

Hi Chang,
I first would like to apologize for the long delay in response. I've been traveling for a few weeks and am just now catching up with forum inquiries. Thank you for your patience.

I know the variables can be confusing. This is the basic definition/description of each one.
LU_INDEX: the dominant landuse category for each grid cell. So if you are using MODIS 21-cat static data, each grid cell will be assigned a category 1-21, dependent on which of those categories is dominant in that cell. This value is only used during initialization.

LANDUSEF: This is the landuse fraction by category - i.e., for each of those 21 categories, which fraction is each cell. So if you had a cell that included categories 1, 2, and 3, it would be broken into each of those, and maybe for that cell, 50% is category 1, 25% is category 2, and 25% is category 3. This value is used by the surface schemes RUC, Pleim-Xu, Noah, and CLM.

FRC_URB2D: This is the fraction of each cell that is considered to be urban, and this is only used when you are using one of the urban physics schemes (sf_urban_physics = 1, 2, or 3), and when using surface schemes Noah, NoahMP, and CLM, and PBL schemes BouLac, MYJ, and YSU.
You could look at the specific code in, say, the phys/module_sf_noahdrv.F file (or whichever scheme you're interested in) to see how it's specifically used.
Thank you for the answer, kwerner

Just to confirm:
In LU_INDEX, number 13 is for urban and built-up land cover type. Then in LANDUSEF, the number array in the 13th layer should be the same as FRC_URB2D.
Please let me know if I am correct.

Thank you

Yes, I believe that is correct. You could run a short, simple test case to determine if that's correct.