# Questions about roughness height in Ideal case.

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#### skateboy366

##### New member
Hello,

I am trying to run ideal cases(em_les, the large eddy simulation ) these days,
but couldn't find any details about how to edit the roughness lengh (Z0) in WRF.

Actually, I want to realize the shear-driven PBL in ideal wrf cases, and the terrain
is assumed to flat in my computational domain. The roughness lengh is crucial for
the wind profile in output files. Are there any references about editing this value ?

Thank you,
Zihan, Zhao

Please take a look at the code phys/module_physics_init.F, i which you can find the following piece of code :

DO j=jts,jtf
DO i=its,itf
IF(XLAND(i,j) .LT. 1.5)THEN
IF(mminlu_loc .EQ. ' ') ALBBCK(i,j)=ALBLND
EMBCK(i,j)=0.85
ALBEDO(i,j)=ALBBCK(i,j)
EMISS(i,j)=EMBCK(i,j)
THC(i,j)=THINLD
ZNT(i,j)=ZZLND
#if ! ( NMM_CORE == 1 )
Z0(i,j)=ZZLND
#endif
MAVAIL(i,j)=XMAVA
ELSE
IF(mminlu_loc .EQ. ' ') ALBBCK(i,j)=0.08
ALBEDO(i,j)=ALBBCK(i,j)
EMBCK(i,j)=0.98
EMISS(i,j)=EMBCK(i,j)
THC(i,j)=THINLD
ZNT(i,j)=ZZWTR
#if ! ( NMM_CORE == 1 )
Z0(i,j)=ZZWTR
#endif
MAVAIL(i,j)=1.0
ENDIF

ENDDO
ENDDO

You can change the value of ZNT here, which is the roughness length used for ideal case.