sf_urban_physics and height of the first layer

Hi all,
I have some doubts about the concepts of the height of the first layer when we are using the sf_urban_physics.
Reading this file https://ral.ucar.edu/sites/default/files/public/product-tool/Multi_layer_UCM.pdf, the author says: "to run WRF-BEP it is necessary to have a very high vertical resolution (5-10m)" (page 8)

However, when I set my namelist to run with sf_urban_physics =1 and height of the first layer = 10 meters, I got the error "ZDC + Z0C + 2m is larger than the 1st WRF level", which made me increase the height of the first layer to 20 meters, as suggested by others in this forum.

So is someone knows to explain the reason of that?
Thank you
Yasmin

Yasmin,
The first model level must be above the urban canopy. SO please obey the rule that ZDC + Z0C + 2m must be smaller than the height of the 1st model level.