Reduce NorESM3 hydrological cycle by reducing drag over ocean

[mvertens]

One option to try to reduce the hydrological cycle would be to explore reducing the drag over ocean.

One option is to model what is done in the NOAA UFS via it's implementation in CMEPS. The NOAA UFS is doing this as an option (redrag) in the University of Arizona scheme. They do not modify the drag but rather the momentum roughness length over ocean.
The UFS CMEPS implementation (in $SRCROOT/components/cmeps/ufs) functions as a CCPP host model and the physics
is in the repository https://github.com/NCAR/ccpp-physics in the routine physics/SFC_Layer/UFS/sfc_diff.f

!! This file contains the surface roughness length formulation based on
!! the surface sublayer scheme in
!! Zeng and Dickinson (1998) \cite zeng_and_dickinson_1998.

and the implementation is as follows for the roughness length over ocean (the following are the key statements in the iteration loop):

              z0s_max=.317e-2_kp 
              .............................
              z0 = 0.01_kp * z0rl_wat(i)
               ............................
              if (redrag) then
                z0rl_wat(i) = 100.0_kp * max(min(z0,z0s_max),1.0e-7_kp)
              else
                z0rl_wat(i) = 100.0_kp * max(min(z0,0.1_kp), 1.e-7_kp)
              endif

In NorESM and CESM, the Large/Yeager, Large-diurnal, UA and COARE schemes are all in the file
$SRCROOT/components/cmeps/cesm/flux_atmocn/shr_flux_mod.F90.
I am not sure how we want to implement this type of reduction in COARE - since the scheme is considerably different. It might be good to get Thomas Toniazzo involved for advice.

I think it would be good at this point to also rewrite shr_flux_mod.F90 to actually be 4 separate modules - for Large-Yeager, Large-diurnal, COARE and UA.

@MichaelSchulzMETNO @adagj - @oyvindseland and I met with Thomas Toniazzo and have come up with the following plan.

• I will enable daily output of the roughness length and surface stress for a year using the latest simulation configuration.
• Once we have that - we can determine how we want to constrain the roughness length in the COARE scheme.

[mvertens]

This discussion has been moved to NorESMhub/noresm3_dev_simulations#247

[adagj]

@oyvindseland Can you please withe a description of what you have tested, and link to the issues?

@JensBDebernard Can you please provide a description of what you suggested (this morning) to test?

[oyvindseland]

I think the story is that at the moment that 3a + c0 modifications is enough to deal with the global precipitation bias.
The work on surface stress is quite complex and with uncertain outcome

[MichaelSchulzMETNO]

What do you mean by 3a and co ? What is it, and where in the equations is it possible to vary them?

[oyvindseland]

Some details can and background information can be found in the NorESM meeting notes for the development meeting today (16th of October)

3a equal NorESM3_beta03a
A brief summary on the tuning variable c0.
c0_ocn c0_lnd is the fraction of the liquid water in the convective massflux that is converted to precipitation and is included in the PPE described in Geosci. Model Dev., 17, 7835–7853, 2024
Trude Storelvmo called for experimenting with this parameter in order to reduce the bias in precipitation.

A c0 of 0.03 --> 75 % of the liquid water is converted to precipitation and removed for every 100 meter of model height.

The GMD paper give a narrow interval for c0 compared to other publications As a result Eidhammer et al. show a relatively low impact of this parameter. A further reduction c0_ocean to the same value as over land gave better results.
According to publications c0 should depend on temperature. The high value is more typical of areas with really high SST > 28 C

The decrease in autoconversion gave an increase in low clouds. This was counteracted by tuning low clouds down. This did not have a strong impact on precipitation.

[oyvindseland]

I added one of the simulations in the simulations issues
NorESMhub/noresm3_dev_simulations#249
Inside the issue but there is a link to a discussion and description and diagnostics of the individual experiments.