replacement of monthly albedo

[lff]

Hello, all!,

I am new to WRF and am currently trying to perform two experiments by replacing the surface albedo. The first set is a control run without changing albedo, and the second set modifies the albedo for a specific land surface category.

I have conducted a 10-day simulation and found that in the wrfout output files, the albedo differs between the two experiments only at the first time step, while from the second time step onwards, the albedo values are almost identical between the two runs.

My procedure for replacing albedo is as follows:

• I modified the ALBEDO12M variable inside the geo_em.d01.nc file.
• Then I ran metgrid.exe.
• In the namelist.input, I set usemonalb = true.
• Finally, I ran real.exe and wrf.exe to perform the simulations.

Below is the relevant part of the &physics section in my namelist.input:

&physics
sf_surface_physics = 4, ! Noah-MP
num_soil_layers = 4,
bl_pbl_physics = 1,
sf_sfclay_physics = 1,
ra_lw_physics = 4,
ra_sw_physics = 4,
cu_physics = 1,
mp_physics = 8,
radt = 25,
bldt = 0,
cudt = 5,
num_land_cat = 21,
sf_urban_physics = 0,
usemonalb = true,
/

&noah_mp
dveg = 2, !On LAI predicted; FVEG calculated
/


My questions are:

1. Is this approach of replacing albedo by modifying ALBEDO12M in geo_em.d01.nc and setting usemonalb = true correct?
2. Why does the albedo difference between the two experiments only appear at the first time step in the output?
3. For consistency, should I set usemonalb = true in both the control and modified experiments?
4. After switching to the NOAH scheme, I was able to see the albedo changes I made. I suspect that in NOAH-MP, albedo is only read during initialization and later replaced by dynamic calculations. Is that correct? If I want to keep albedo fixed (e.g., use prescribed monthly values) throughout the simulation, what is the proper way to achieve this with NOAH-MP?

Thank you very much for your time and help!