WRF-CTSM: Bottom layer specific humidty sent from the atmosphere model is less than zero

[negin513]

Brief summary of bug

Iris Mužić (@irismuz) has reported the following error for her WRF-CTSM simulation after 11 days:

ERROR: Bottom layer specific humidty sent from the atmosphere model is less than zero.

This error is produced from lnd_import_export_utils.F90 , where it checks specific humidity forc_q_not_downscaled_grc coming from the atmosphere with zero:

CTSM/src/cpl/utils/lnd_import_export_utils.F90

Lines 129 to 134 in d301320

	if ( wateratm2lndbulk_inst%forc_q_not_downscaled_grc(g) < 0.0_r8 )then
	call shr_sys_abort( subname//&
	' ERROR: Bottom layer specific humidty sent from the atmosphere model is less than zero' )
	end if
	end do

She has checked the 2m specific humidity (Q2M) variable output from all produced ctsm_lilac.clm*.nc files, however, none of the time steps exhibit values less than zero.

General bug information

CTSM version you are using: @irismuz can you please fill this

[negin513]

Here CTSM is complaining about negative specific humidity value coming from the atmosphere model (WRF).

My first thought with this is WRF advection schemes:

Although it is physically impossible to have negative mass, the basic RK time integration in WRF advection scheme produces negative mass or negative mixing ratio at sharp gradients. To address this issues, WRF introduced positive definite and monotonic advection schemes, which get rid of the negative mixing ratios caused by advection.

Therefore, the user should always use advection options moist_adv_opt, momentum_adv_opt, and scalar_adv_opt of 1 (positive definite) or 2 (monotonic advection).

For this particular issue, @irismuz has confirmed they are using monotonic advection option (option 2); therefore the advection option is not what causing this issue.

[irismuz]

Thank you for following up on this issue. I am using ctsm5.1.dev042.

[chrystali2002]

I have this same issue, may I ask if this was solved yet so that I could have access to the solution?

[chrystali2002]

@irismuz, @negin513, may I know how you are able to solve this issue?

[irismuz]

Hi @chrystali2002,
The solution we applied is available under https://bb.cgd.ucar.edu/cesm/threads/bottom-layer-specific-humidty-sent-from-the-atmosphere-model-is-less-than-zero.7228/.