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A question regarding the rural emissivity calculation

KeerZ

Member
Hi all,

I ran a simulation with BSSP585 compset using cesm2.1.2. I switched to use SP mode.

Now I am trying to calculate the effective emissivity for rural, tree, grass, and bare soil land tiles (the tree and grass land tiles include all the tree and grass PFTs). I know that the emissivity of bare soil is 0.96 and I can calculate the emissivity of vegetation by : emi_veg=1-exp(-(L+S)/mu) in the Eq.2.4.20 of CLM5 Tech note (2.4. Radiative Fluxes — ctsm release-clm5.0 documentation). The L and S stand for LAI and SAI, respectively, and mu is 1.0.

The way I calculate the rural emissivity is :
1. calculate the annual L+S for each PFT and CFT.
2. calculate the emissivity for each PFT and CFT using Eq.2.4.20
3. Calculate the area-weighed mean emissivity of all PFTs (including bare soil whose emissivity is 0.96) and CFTs.

I expect the rural emissivity to be 0.96-0.98 but I got some very low rural emissivity (0.4-0.6) in some mid- to high-latitude areas (Figure attached). I wonder if I missed something here (snow cover over vegetation?). What is the best way to calculate the effective rural (and tree/grass land tile) emissivity?

Thank you very much!
 

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oleson

Keith Oleson
CSEG and Liaisons
Staff member
The equation for vegetation emissivity you have described is for the vegetation canopy only, not for the vegetation-soil (the soil beneath the vegetation) system.
As you said, the soil has an emissivity of 0.96, so I would expect the effective emissivity of the vegetation-soil system to be somewhere between the emissivity of a dense vegetated canopy and the emissivity of soil.
In addition, in snow-covered areas, snow emissivity (which I think is 0.98) would come into play.
I'm not sure what to recommend regarding calculating the emissivity of the vegetation-soil-snow system. Since the vegetation-soil-snow emissivities are quite similar, maybe just assume an average value everywhere...
 

KeerZ

Member
The equation for vegetation emissivity you have described is for the vegetation canopy only, not for the vegetation-soil (the soil beneath the vegetation) system.
As you said, the soil has an emissivity of 0.96, so I would expect the effective emissivity of the vegetation-soil system to be somewhere between the emissivity of a dense vegetated canopy and the emissivity of soil.
In addition, in snow-covered areas, snow emissivity (which I think is 0.98) would come into play.
I'm not sure what to recommend regarding calculating the emissivity of the vegetation-soil-snow system. Since the vegetation-soil-snow emissivities are quite similar, maybe just assume an average value everywhere...
I see. It seems that the best option is to assume an average emissivity (0.96) for all types of vegetated land.

I intend to use emissivity and longwave radiation to calculate the radiative temperatures. I guess the results will not be very sensitive to small emissivity differences so it is fine. Thank you very much!
 
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