Source of QSFC?

[Chetan]

Hi everyone,
The module_sf_sfclay.F module used the QSFC ( ground saturated mixing ratio) to calculate the Q2 (2m mixing ratio). Does anyone know what is the source of this variable? Does WRF itself calculate it or its come from the reanalysis data?
One more question, which module in wrf/wps gives the information about, how and what variables are read by the model?

Thank you for your time and consideration.

Best,
Chetan

 

[Ming Chen]

Chetan,
QSFC is specific humidity at the surface. WRF calculates this variable in sfclay scheme. Depending on which scheme you choose, its calculation is slightly different, but its physical meaning is same.

Please clarify what you mean by asking "which module in wrf/wps gives the information about, how and what variables are read by the model?"

 

[Chetan]

Thank you @Ming Chen for your response.
In module_sf_sfclya.F, inside subroutine SFCLAY1D there is an equation Q2(I)=QSFC(I)+(QX(I)-QSFC(I))*PSIQ2/PSIQ
to calculate 2m mixing ratio, which used the QSFC.

However, from the following code
DO 60 I=its,ite
E1=SVP1*EXP(SVP2*(TGDSA(I)-SVPT0)/(TGDSA(I)-SVP3))
! for land points QSFC can come from previous time step
if(xland(i).gt.1.5.or.qsfc(i).le.0.0)QSFC(I)=EP2*E1/(PSFC(I)-E1)
! QGH CHANGED TO USE LOWEST-LEVEL AIR TEMP CONSISTENT WITH MYJSFC CHANGE
! Q2SAT = QGH IN LSM
E1=SVP1*EXP(SVP2*(T1D(I)-SVPT0)/(T1D(I)-SVP3))
PL=P1D(I)/1000.
QGH(I)=EP2*E1/(PL-E1)
CPM(I)=CP*(1.+0.8*QX(I))
60 CONTINUE

It seems that QSFC is only calculated for water bodies and under negative conditions.
My confusion is regarding how the equation takes QSFC into account for land conditions.

Regarding my second question: how does the model (WPS/WRF) handle the reanalysis data? How the real program takes met_em* data and how the WRF model takes initial condition data? Is there a specific module or subroutine for this?

Note: I am a beginner in Fortran programming and am still learning the language.


Thank you very much for your time and consideration.

 

[Ming Chen]

QSFC is specific humidity at the lowest model level over land.

Reanalysis data is used to derive initial and boundary conditions for WRF. Please take a look at the code "module_initialize_real.F", which reads met-em data and produce input data for WRF.

You may also need to read the WRF Tech Note to better understand how the system works.

 

[Chetan]

Thank you Ming for your response.

 

[Christian_]

QSFC is specific humidity at the lowest model level over land.

Hello Ming,
hello everyone,

I have a follow-up question on this, because I recently got confused about the QSFC variable, too. At the moment, I am doing idealized WRF LES simulations to make some tests using the sfclayrev scheme and isfflx = 1 and no other scheme activated, especially no land-surface model.
From a slide of the WRF tutorial 2021, I understood that you can set the surface vapor mixing ratio in the input_sounding file, see the attached file with the red ellipse mark. So far I thought that this input will set the value of the variable QSFC (like you set surface skin temperature TSK with the value in the 2nd column). However, when tracking QSFC during my simulation, I realized that this is not the case. (The value I used in the input_sounding was 12 g/kg, but QSFC showed 2.27804165E-02 in every timestep).

So my question is two-fold concerning the vapor mixing ratio:
- Which concrete variable in the WRF code do I actually set in the input_sounding file in the first line (third column)? I understood that the values from line 2 on will be used to set the QV variable (the values will be interpolated to the z-level accordingly).
- Where and how does the WRF code compute QSFC, especially when there is no land-surface model activated? I could not find a calculation that could explain the value of 2.27804165E-02.

Regards,
Christian