Understanding GPP differences between SP prescribed-LAI and transient BGC simulations in CLM5 (version ctsm5.4.003)

[Makduma Badhan]

I'm investigating prescribed LAI in the CLM single-point (SP) configuration. My goal was to determine whether prescribing a more realistic seasonal LAI could improve simulated GPP.

I first prescribed MODIS high-quality LAI, but GPP remained substantially lower than observations. I then prescribed the LAI directly from a transient CLM simulation while using the same meteorological forcing. Even with the same prescribed LAI, the SP simulation still produces considerably lower GPP than the transient simulation.


This makes me wonder whether factors other than LAI are responsible for the discrepancy.


While reading the documentation, I saw that Vcmax is a prognostic variable in CLM5 and is computed by the LUNA model. This led me to two questions:

1. Do both the SP configuration (with prescribed LAI) and the transient BGC configuration use LUNA to calculate prognostic VCMAX25, or is VCMAX treated differently between these configurations?
2. Besides LAI, what variables or model components would you recommend comparing first to diagnose why the SP simulation produces substantially lower GPP than the transient simulation under the same meteorological forcing?

Any suggestions or guidance would be greatly appreciated.


Thank you!

 

[oleson]

1. You can look at "use_luna" in the lnd_in from your SP and BGC cases. I believe it will be true in both. Which means that LUNA will calculate VCMAX25. I'm not sure though if that means that VCMAX25 will be the same in both simulations. Have you checked the Vcmx25Z history variable from both simulations?
2. If I understand correctly, you are asking why the SP transient simulation has much lower GPP than a BGC transient simulation for the same site and atmospheric forcing, even when the SP transient LAI is prescribed from the LAI output of the BGC transient? If so, I might start diagnosing by looking at the energy budget terms. For example, is the net radiation similar between the two simulations, and, for example, the absorbed solar radiation? I would think that the absorbed solar would be one of the primary drivers of photosynthesis.

 

[oleson]

I assume you are comparing the history variable FPSN in the SP simulation to the GPP variable in the BGC simulation...

 

[aswann2]

My group has also noticed this behavior. Vcmax is treated differently in SP mode compared to BGC mode, which allows the FPSN per leaf area to change between the runs resulting in different total gpp.