Question about MEGAN and LAI

[James King]

Hi all,

I'm running CESM2.2.0 and am looking at the effects of land use on BVOC emissions under different climate scenarios. I have generated surfdata files for CLM5 which I thought were prescribing LAI, as they contain the variable 'monthly LAI'. However, when we run experiments with different CO2 levels, we see changes in the output variables 'ELAI' and 'TLAI'. Looking at the docs, I believe ELAI refers to leaves not covered by snow, and TLAI to the sum of shaded and sunlit leaves (please correct me if I'm wrong!).The ELAI/TLAI values in experiments where we use the same surfdata file but increase CO2 levels have higher ELAI/TLAI which to me implies an effect from CO2 fertilisation. I know that MEGAN uses ELAI to calculate emission activity factors My question is - what is the relationship between the prescribed monthly LAI input and the ELAI/TLAI output? From what I can see in the model code ELAI is calculated from TLAI, and TLAI from leaf carbon and specific leaf area. Furthermore, if ELAI is what drives MEGAN emissions, is there a way to prescribe it?

Thanks,

James

 

[oleson]

You are correct about the difference between ELAI and TLAI. There is a similar difference between SAI and TSAI (stem area index).
If ELAI/TLAI is changing in response to CO2 then you are running the model in "BGC" mode. In this case the MONTHLY_LAI on the surface dataset is not used. It will be used if you run the model in SP (satellite phenology) mode.
So if you want the model to use the MONTHLY_LAI on the surface dataset or prescribe the MONTHLY_LAI, then run the model in SP mode.

 

[James King]

Thanks Keith! Is SP mode valid for running with future land use and climate? Are BVOC emissions still generated by the prescribed land surface when BGC is off?

 

[oleson]

BVOC emissions will still be generated with BGC off (e.g., see the MEG_* fields on the history file). You can run with future land use and climate in SP mode. In that case, the seasonal MONTHLY_LAI and SAI be used for every year of the transient run. And the crop model will not be active (generic crop type will be used).