# The density problem of module_mp_fast_sbm.F

#### Yuting_Deng

##### New member
Recently I have some questions adding variables to the fast_sbm file, the density (rho_phy) unit in this file is [kg m-3], and rhocgs unit should be [g m-3] because there is a statement 'rhocgs(I,K,J)=rho_phy(I,K,J)*0.001' in this file.

Now the problems are coming, an example: QG(I,K,J)=QG(I,K,J)+(1./RHOCGS(I,K,J))*COL*chem_new(I,K,J,KR)*XG(KRR)*XG(KRR)*3, I extrapolate from the entire file that the chen_new's unit is [g-1 cm-3], XG's unit is g, and the unit of QG is [kg kg-1], if the unit of rhocgs is [g cm-3] then the resulting unit correspond.

However, the next statement "QNG(I,K,J)=QNG(I,K,J)+COL*chen_new(I,K,J,KR)*XG(KRR)*3/rhocgs(I,K,J)*1000", the unit of QNG is [kg-1,] now we still assume that the unit of rhocgs is [g cm-3], if we want to do that, it should multiply by 10^-3 instead of 10^3. Interstingly, even assuming that the unit of rhocgs is [g m-3], the result is still incorrect.

I have been troubled by this question for a long time and expecting an explanation.

screenshots of module_mp_fast_sbm.F:

screenshots of module_microphysics_driver.F:

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I will contact the developer and hopefully he can get back to you ....

I will contact the developer and hopefully he can get back to you ....
greately appreciate

greately appreciate
Hi Yuting,

The units of the FSBM scheme are CGS, except at the start/end where we adjust the units from/to the main WRF (MKS as pointed above at the microphysics driver).
Specifically for your question, 'rhocgs' is in [g/cm3], and 'QG' and 'QNG' are mass and number mixing ratios in [kg/kg] and [#/kg], respectively (that is, at the units the microphysics driver is expecting).

As a side note: you are likely using quite an old version of the FSBM scheme (I can see it by the 'FAST_KHAIN_LYNN' flag), so I would strongly recommend at least comparing your results with the updated version in the main WRF github repo.

Good luck,
Jacob

Hi Yuting,

The units of the FSBM scheme are CGS, except at the start/end where we adjust the units from/to the main WRF (MKS as pointed above at the microphysics driver).
Specifically for your question, 'rhocgs' is in [g/cm3], and 'QG' and 'QNG' are mass and number mixing ratios in [kg/kg] and [#/kg], respectively (that is, at the units the microphysics driver is expecting).

As a side note: you are likely using quite an old version of the FSBM scheme (I can see it by the 'FAST_KHAIN_LYNN' flag), so I would strongly recommend at least comparing your results with the updated version in the main WRF github repo.

Good luck,
Jacob
Thank you for answering my question! I'll use rhocgs for calculation and check the updated version.
Send you a flower

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