Using MUR SST Instead of ERA5 SST in WPS/WRF: Questions on Metgrid Priority and SST Update

[Prolay Saha]

Dear WRF Support Team,

I am using ERA5 data to force WRF. For WPS preprocessing, I run ungrib.exe separately for pressure-level and surface-level ERA5 files:

For pressure-level data:


&ungrib
prefix = 'PLV',
/


For surface-level data:


&ungrib
prefix = 'SFC',
/


Then I run metgrid.exe with:


&metgrid
fg_name = 'PLV','SFC',
/


The ERA5 surface files already contain SST, so the resulting met_em* files include ERA5 SST information.

I would like to replace the ERA5 SST with SST from the Multi-scale Ultra-high Resolution (MUR) SST product. My understanding is that I could convert the MUR SST data into WPS intermediate format and then run metgrid.exe using:


&metgrid
fg_name = 'PLV','SFC','SST',
/


where SST refers to the MUR SST dataset.

I would appreciate clarification on the following points:

1. If both the ERA5 surface files (SFC) and the external MUR SST dataset (SST) provide SST information, will metgrid.exe automatically use the MUR SST and override the ERA5 SST, or do I need to remove the SST field from the ERA5 surface files before running ungrib.exe/metgrid.exe?

2. If the met_em* files contain time-varying SST fields (e.g., from MUR or ERA5), but I run WRF with:


&physics
sst_update = 0,
/


is it correct that real.exe will use only the initial SST field and that WRF will keep this SST fixed throughout the simulation?

3. If the met_em* files do not contain any SST field at all, and I run with:


&physics
sst_update = 0,
/


how does real.exe/wrf.exe handle the ocean surface temperature? Will the model fail during initialization, or will it derive SST from another field?

Thank you very much for your time and assistance.