Modeling the impacts of diffuse light fraction on photosynthesis in ORCHIDEE (v5453) land surface model

Aerosol- and cloud-induced changes in diffuselight have important impacts on the global land carbon cy-cle, as they alter light distribution and photosynthesis in veg-etation canopies. However, this effect remains poorly rep-resented or evaluated in current land surface models. Here,we add a light partitioning module and a new canopy lighttransmission module to the ORCHIDEE (Organising Car-bon and Hydrology In Dynamic Ecosystems) land surfacemodel (trunk version, v5453) and use the revised model, OR-CHIDEE_DF, to estimate the fraction of diffuse light andits effect on gross primary production (GPP) in a multilayercanopy. We evaluate the new parameterizations using fluxobservations from 159 eddy covariance sites over the globe.Our results show that, compared with the original model,ORCHIDEE_DF improves the GPP simulation under sunnyconditions and captures the observed higher photosynthe-sis under cloudier conditions in most plant functional types(PFTs). Our results also indicate that the larger GPP undercloudy conditions compared with sunny conditions is mainlydriven by increased diffuse light in the morning and in theafternoon as well as by a decreased vapor pressure deficit(VPD) and decreased air temperature at midday. The obser-vations show that the strongest positive effects of diffuse lighton photosynthesis are found in the range from 5 to 20◦C andat a VPD < 1 kPa. This effect is found to decrease when theVPD becomes too large or the temperature falls outside of theabovementioned range, which is likely due to the increasingstomatal resistance to leaf CO2uptake. ORCHIDEE_DF un-derestimates the diffuse light effect at low temperature in allPFTs and overestimates this effect at high temperature and ata high VPD in grasslands and croplands. The new model hasthe potential to better investigate the impact of large-scaleaerosol changes and long-term changes in cloudiness on theterrestrial carbon budget, both in the historical period and inthe context of future air quality policies and/or climate engi-neering This research has been supported by the Eu- ropean Research Council (grant no. SyG-2013-610028), the ANR CLAND Convergence Institute (grant no. 16-CONV-0003), the ANR China Trend Stream and the new 4C project. Bogdan Cho-jnicki was supported by the National Science Foundation, Poland (grant no. UMO-2017/27/B/ST10/02228), within the framework of the “Carbon dioxide uptake potential of sphagnum peatlands in the context of atmospheric optical parameters and climate changes” (KUSCO2) project.