# Why roughness(ZNT) of Urban is lower than Croplands in UCM?

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#### duxgmail+foxmailcom

##### New member
Hi,

I am trying to figure out the differences of roughness length with/without Urban Canopy Model. I output ZNT but find that the initial ZNT(t = 0) in urban is greater than croplands, but the following output(t = 1,2,3,4h~) shows that urban roughness is lower than cropland when I use UCM(sf_urban_physics = 1).

The initial roughness in urban is about 0.8 and the following roughness is only about 0.10, lower than cropland which is about 0.15. My question is why urban roughness is lower than cropland when using UCM ? When I don't use UCM, urban roughness is greatr than cropland.

The land surface model I use is Noah. When I use Noah-MP and without UCM, there is the same problem that urban roughness is lower than cropland.

Looking forward to any reply! Thanks for any help!

Would you please upload your namelist.input for me to take a look? Which version of WRF are you running? Thanks.

Dear Chen,
Thank you for your reply! And sorry for my delay in response.
I am using WRF4.0 with Noah-MP surface physics scheme and a single-layer urban canopy model.

&physics
mp_physics = 6, 6, 6,
ra_lw_physics = 4, 4, 4,
ra_sw_physics = 4, 4, 4,
sf_sfclay_physics = 1, 1, 1,
sf_surface_physics = 4, 4, 4,
sf_urban_physics = 1, 1, 1,
bl_pbl_physics = 11, 11, 11,
bldt = 0, 0, 0,
cu_physics = 1, 0, 0,
cudt = 0, 0, 0,
isfflx = 1,
ifsnow = 1,
icloud = 1,
surface_input_source = 3,
num_soil_layers = 4,
num_land_cat = 21,
sst_update = 1,
isftcflx = 2,

Hi,

This is my namelist.wps.
Looking forward to any help! Thanks!

&share
wrf_core = 'ARW',
max_dom = 3,
start_date = '2018-08-15_06:00:00','2018-08-15_06:00:00','2018-08-15_06:00:00',
end_date = '2018-08-17_12:00:00','2018-08-17_12:00:00','2018-08-17_12:00:00',
interval_seconds = 21600,
io_form_geogrid = 2,
debug_level = 0
/

&geogrid
parent_id = 1, 1, 2,
parent_grid_ratio = 1, 3, 3,
i_parent_start = 1, 38, 92,
j_parent_start = 1, 40, 77,
e_we = 210, 406, 730,
e_sn = 205, 385, 703,

geog_data_res = 'default','default', !‘usgs_lakes+default’, ‘usgs_lakes+default’,
dx = 18000,
dy = 18000,
map_proj = 'lambert',
ref_lat = 29.75,
ref_lon = 122,
truelat1 = 30,
truelat2 = 60,
stand_lon = 122,
geog_data_path = './WRFV4.0DATA/geog' ! should use WPSV4.0DATA
/

&ungrib
out_format = 'WPS',
prefix = 'SST',

/

&metgrid
fg_name = 'FILE','SST',
io_form_metgrid = 2,
/

Ming Chen said:
Would you please upload your namelist.input for me to take a look? Which version of WRF are you running? Thanks.
Hi!
I have tried to modify module_sf_urban.F to change the calculation of Z0C to find whether the calculation of surface roughness in the Urban Canopy Model would influence the output of ZNT from wrfout files. But I find that the change in Z0C will not impact the output of ZNT that ZNT in wrfout files kept about 0.12 no matter how Z0C change.

Hope for any suggestions!

Hello,

I encountered the same issue using WRF v4.3 with BEP model. Urban girds initially had ZNT of 0.8 (default in LANDUSE.TBL), then had the ZNT drop to 0.14, which is an unreasonable value for built-up area. Have you come to a solution?

Many thanks