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Questions on interpreting surface energy balance and specific humidity results in CLM

KeerZ

Member
Hi all,

I ran a CLM5 simulation using I2000Clm50SpGs compset with 25km resolution, and I got two questions while I was interpreting the results.

1) surface energy balance
It seems that surface energy balance check will be done at every time step in CLM and clm model won't keep going if errorseb is greater than 1e-5 (W/m2). However, when I was analyzing my hourly energy flux output, I found that urban and rural energy balance error terms can be pretty large negative values (-20 W/m2) for some grids. The urban(_U) and rural(_R) error terms are calculated by:

Error_U=FSA_U-FIRA_U-EFLX_LH_TOT_U-FSH_U-FGR_U+HEAT_FROM_AC+WASTEHEAT
Error_R=FSA_R-FIRA_R-EFLX_LH_TOT_R-FSH_R-FGR_R
(absorbed solar radiation - net longwave radiation - latent heat flux - sensible heat flux - heat storage flux + anthropogenic heat flux (if any))
Are there any energy flux terms that I missed?

2) 2m and forcing height specific humidity
This one might be too specific. I found that my 2m specific humidity can be lower than forcing height specific humidity at many regions, and I am not sure whether this is normal (I expect the surface specific humidity to be higher than 30m specific humidity. Please correct me if I am wrong)

Thank you! Any suggestions or opinions are appreciated!
 

oleson

Keith Oleson
CSEG and Liaisons
Staff member
Your equation for the urban energy balance error is correct. However, the HEAT_FROM_AC and WASTEHEAT variables in the history file are reported with respect to the entire gridcell, not with respect to the urban area. I believe this was done to make it easier to calculate global anthropogenic heat flux in the model (since you would then have to just multiply by the gridcell area and land fraction to get total Watts).
You need to divide the HEAT_FROM_AC and WASTEHEAT terms by the fraction of urban area in each gridcell (you can get this from the PCT_URBAN field on your surface dataset). Once you do this it should balance (it did in my test case).
I'm not sure why the rural energy isn't balancing, it isn't balancing in my test case either, I will look into it further.
What history field are you using for 2m specific humidity and in what regions is it lower than the 30m specific humidity?
 

KeerZ

Member
Your equation for the urban energy balance error is correct. However, the HEAT_FROM_AC and WASTEHEAT variables in the history file are reported with respect to the entire gridcell, not with respect to the urban area. I believe this was done to make it easier to calculate global anthropogenic heat flux in the model (since you would then have to just multiply by the gridcell area and land fraction to get total Watts).
You need to divide the HEAT_FROM_AC and WASTEHEAT terms by the fraction of urban area in each gridcell (you can get this from the PCT_URBAN field on your surface dataset). Once you do this it should balance (it did in my test case).
I'm not sure why the rural energy isn't balancing, it isn't balancing in my test case either, I will look into it further.
What history field are you using for 2m specific humidity and in what regions is it lower than the 30m specific humidity?
Hi. Thank you for replying! I will try dividing HEAT_FROM_AC and WASTEHEAT terms by Urban fraction.

I calculated 2m vapor pressure by air temperature (TSA) and relative humidity (RH2M) using Tetens' equation, and then I calculated 2m specific humidity = 0.622*VP/(101325-VP+0.622*VP) (VP: Pa).
Attached please find the figures of 2m specific humidity minus 30m specific humidity at boreal summer daytime. The 2m specific humidity is lower than forcing height specific humidity at many regions such as South Asia, South Africa and Central America in my case.

Thanks and regards,
Keer
 

Attachments

  • Specific humidity difference.png
    Specific humidity difference.png
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oleson

Keith Oleson
CSEG and Liaisons
Staff member
We are still looking at the rural energy balance.
Regarding humidity, you could look at the gridcell level specific humidity (Q2M) and compare it with the forcing specific humidity (QBOT) and see if you get similar spatial patterns for rural for example to see if your calculations are correct. I'm not sure why you would expect the 2m specific humidity to be higher than the forcing humidity everywhere...
 

KeerZ

Member
We are still looking at the rural energy balance.
Regarding humidity, you could look at the gridcell level specific humidity (Q2M) and compare it with the forcing specific humidity (QBOT) and see if you get similar spatial patterns for rural for example to see if your calculations are correct. I'm not sure why you would expect the 2m specific humidity to be higher than the forcing humidity everywhere...
OK. Thank you very much!
 
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