Best Practice for Using MERRA-2 Lowest Model Level Height in Noah-MP

[mbechtold]

Hi! I have a question regarding the use of the lowest model level from MERRA-2 forcing in Noah-MP.

In MERRA-2, the height of the lowest model level (HLML), provided in the hourly tavg1_2d_flx_Nx files, has a mean value of about 60 m, with moderate spatial (~5 m) and temporal (~1.5 m) variability.

In Noah-MP, however, only a single reference height can be specified (Noah-MP v4.0.1 reference height ...). Given this limitation, what is the recommended best practice when running Noah-MP with MERRA-2 forcing together with lowest model level? Should a fixed value of ~60 m be used as the reference height?

Additionally, is there a reason why HLML from the MERRA-2 flx_Nx files is not directly used in Noah-MP, instead of a fixed reference height, as is done in Noah v3.6 via LIS_FORC_Forc_Hgt?

Any guidance or clarification would be appreciated! Thank you!

[dmocko]

Hi @mbechtold

Thanks for the excellent questions. No, there is no reason why HLML can't be used in Noah-MP instead of a fixed reference height.
We just haven't added this capability into the code; however, it can certainly be done.

If you want to use the HLML forcing from MERRA-2 right away, you could just use a mean value as the Noah-MP reference height.

But if you do want to use the HLML height as the Noah-MP reference height (varying spatially/temporally), you are welcome to
migrate the capability over to the LSM. I can answer any questions about it, as I added this to Noah-3.6 as you noted.

[mbechtold]

Hi @dmocko
thanks so much for the prompt response! This helps us to decide how to proceed. If we find the time to implement the HLML instead of using a fixed 60 m height, we let you know!