Analysis Nudging (Upper-Air and/or Surface)

[seti]

Dear all,

In my simulation, I need to use analysis nudging to improve wind and temperature (I am using ERA5 data, dx= 9 km).

Once I read the WRF user guide, I noticed that after running metgrid.exe, it is necessary to run OBSGRID. However, I don't have OBSGRID in my WPS folder.

So, I guess I should compile the WPS differently to have this tool. But, I have no idea how I should do this. Is there any tutorial or documentation about this, please?

Please kindly advise me on this issue.
Thanks a lot in advance.

 

[Ming Chen]

Analysis nudging does't require OBSGRID. It only needs large-scale data from other analysis or reanalysis products. Please see

https://www2.mmm.ucar.edu/wrf/users/docs/user_guide_v4/v4.0/users_guide_chap5.html#gridnudge

 

[Josh]

As I understand it, OBSGRID is required if you need to do surface grid nudging though "grid_sfdda".
But OBSGRID is not required for "grid_fdda".

"Prepare input data to WRF as usual using WPS. If nudging is desired in the nest domains, make sure all time periods for all domains are processed in WPS. For surface-analysis nudging (new in Version 3.1), OBSGRID needs to be run after METGRID, and it will output a wrfsfdda_d01 file that the WRF model reads for this option."

 

[Ming Chen]

You are right that if surface nudging is activated, then station observations need to be processed by OBSGRID.

 

[seti]

Analysis nudging does't require OBSGRID. It only needs large-scale data from other analysis or reanalysis products. Please see

https://www2.mmm.ucar.edu/wrf/users/docs/user_guide_v4/v4.0/users_guide_chap5.html#gridnudge

Dear Ming,

Thanks so much for your reply. Yes your right, it is surface nudging that requires OBSGRID.

I run the model using suggested the WRF userguid setting. However, after cheking the simulated results with observations, I noticed that nudging did not improve temperature and wind speed. Without using nudging the simulated wind speed and temperature were better. I strongly wonder why this happened. I actually wanted to apply nuding to my entire simulation time period, and also to all vertical levels of the model ( namelist attached).

Except this question, I have questions about "k_zfac_uv", "if_ramping", and "dtramp_min". What are they exactly please? Would you please explain more about them?

This was my firt time to use grid nudging and have no exprience in this regard, so I need your advise to improve the aformmentioned varibles. Would you please kindy advise me on how to sort this issue out?
I also need to know how should I set the values of "guv", "gt", and "gq" for my study area?

Thanks a million in advance.

 

[seti]

@mi

Dear Ming,

Thanks so much for your reply. Yes your right, it is surface nudging that requires OBSGRID.

I run the model using suggested the WRF userguid setting. However, after cheking the simulated results with observations, I noticed that nudging did not improve temperature and wind speed. Without using nudging the simulated wind speed and temperature were better. I strongly wonder why this happened. I actually wanted to apply nuding to my entire simulation time period, and also to all vertical levels of the model ( namelist attached).

Except this question, I have questions about "k_zfac_uv", "if_ramping", and "dtramp_min". What are they exactly please? Would you please explain more about them?

This was my firt time to use grid nudging and have no exprience in this regard, so I need your advise to improve the aformmentioned varibles. Would you please kindy advise me on how to sort this issue out?
I also need to know how should I set the values of "guv", "gt", and "gq" for my study area?

Thanks a million in advance.

@Ming Chen

 

[Ming Chen]

Please see my answers below:

Dear Ming,

Thanks so much for your reply. Yes your right, it is surface nudging that requires OBSGRID.

I run the model using suggested the WRF userguid setting. However, after cheking the simulated results with observations, I noticed that nudging did not improve temperature and wind speed. Without using nudging the simulated wind speed and temperature were better. I strongly wonder why this happened. I actually wanted to apply nuding to my entire simulation time period, and also to all vertical levels of the model ( namelist attached).

This is not an uncommon feature when running analysis nudging. WRF outputs without nudging are results of WRF physics and dynamics, which are expected to better represent real-time weather with more details compared to the large-scale forcing.

By analysis nudging, information from the large-scale forcing data are introduced into WRF to "modify" results from WRF simulation. If the large-scale forcing data is less accurate than WRF results, it is possible that the final results are worse than that without nudging.

Except this question, I have questions about "k_zfac_uv", "if_ramping", and "dtramp_min". What are they exactly please? Would you please explain more about them?

k_zfac_uv specifies the model level below which nudging is turned off.

"if_ramping" and "dtramp_min" are combined to extend nudging period, i.e., they tell the model to continue nudging over a period of dtramp_min after the time of "end_fdda_hour". To better understand this option, please see the code module_fdda_spnudging.F and module_fdda_psufddagd.F.
However, I am not sure why this option is added to WRF.

This was my firt time to use grid nudging and have no exprience in this regard, so I need your advise to improve the aformmentioned varibles. Would you please kindy advise me on how to sort this issue out?
I also need to know how should I set the values of "guv", "gt", and "gq" for my study area?

These 3 variables are nudging coefficients for u/v, temp and qvapor. They are more or less empirical. Please stay with the default values.

Thanks a million in advance.