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Lowest model level height

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Hi all, I've run WRF for three different cases by lowering the lowest model level height (z). So, the model height was set to 4 m, 10 m, and 20 m. The Figure below shows how the atmosphere was discretized in the first 500 m.

I know that WRF computes U10 and V10 in the Surface Layer parameterization, which in my case was Eta similarity together with Boulac PBL scheme.
However, when I looked at the wind speed magnitude calculated through U and V at the first layer of the model (blue) and the second layer of the model (red) and compared it with the wind speed of U10 and V10 variables, the results seemed a bit odd (as can be seen for each case above).

When the lowest model level height was set to z=20 m, the results were very similar between the wind speed at the second level (that varied approximately between 20-50 m) and wind speed interpolated at 10 m. On the other hand, when the lowest model level height was set to z=4 m, the interpolated wind speed was larger than the wind speed at levels 1 (4 m) and 2 (varied approximately between 4-10 m). So, I was wondering if there is a threshold regarding the lowest model level height when U10 and V10 are computed.

Another issue is if I want to compare my results with observations at 10 meters which approach should I follow? To use U10 and V10 to compute it or extract U and V from the layer corresponding to the 10 m, let's say in my example:
if z= 20m, I will extract U and V from the first layer.
if z= 10m, I will extract U and V from the first (0 - 10 m) or second layer (10 m- 24m) (??)
if z= 4m, I will extract U and V from the first (4 - 10 m) or second layer (10 m- 15 m) (??)

Thank you in advance.
Updating the post with some more analyses. These plots are the mean daily variation of one-month simulation.
I computed wind speed using U10/V10 (label WS10, dotted lines) and using U/V for the corresponding layer to 10 m (solid lines).

For urban built-up grid cell, in which BEP parameterization was activated, despite the interpolated WS10 being greater than layered WS10, they followed a pattern, that is interpolated WS10 was larger than their respective configurations.

However, for natural/vegetated surfaces, the behavior was different from those seen on urban surfaces. The interpolated WS10 had the same magnitude regardless of the lowest model level height (4, 10, 20), and it overestimated wind speed.
I am sorry I don't have an immediate answer to your question. U10/V10 are variables diagnosed based on similarity theory. They are greatly dependent on values in the lowest model level. Please refer to the publications I list below and hopefully you can find some helpful information.
Janjic, Z. I., 1996: The surface layer in the NCEP Eta Model. Eleventh conference on numerical weather prediction, Norfolk, VA, 19–23 August 1996. Amer Meteor Soc, Boston, MA, pp 354–355.
Jimenez, Pedro A., Jimy Dudhia, J. Fidel Gonzalez–Rouco, Jorge Navarro, Juan P. Montavez, and Elena Garcia–Bustamante, 2012: A revised scheme for the WRF surface layer formulation. Mon. Wea. Rev., 140, 898–918.
Bougeault, P., P. Lacarrere, 1989: Parameterization of Orography–Induced Turbulence in a Mesobeta––Scale Model. Mon. Wea. Rev., 117, 1872–1890. doi:10.1175/1520-0493(1989)117<1872:pOOITI>2.0.CO;2