DIRECT vs DIFFUSE LIGHT accounting in ORCHIDEE

This page will gather and summarise the activities around the merge of different initiative to account for Direct vs Diffuse light in ORCHIDEE for both energy related fluxes and photosynthesis activity or the emission of chemical species.

Three main developments that were done in this direction are:

• A diagnostic implementation of Direct / Diffuse light for the Chemistry (BVOC) fluxes; Diagnostic in the sense that it does not modify the photosynthesis of ORC. Key persons are Juliette and Nicolas.

Key papers are:

-Messina et al. (2016): see description in section 2.2 of the paper ​https://acp.copernicus.org/articles/16/14169/2016/acp-16-14169-2016.pdf. Palmira Messina worked at this time as a postdoctorate researcher on updating and improving the BVOC emission scheme.

-Reference papers here: Spitters1986_RadModel_Part1.pdf​​ and Spitters1986_RadModel_Part2.pdf​. In ORCHIDEE this diagnostic calculation of radiation used specifically for biogenic volatile organic compound emissions is done in ORCHIDEE/src_sechiba/chemistry.f90 at the beginning of the subroutine chemistry_bvoc.

• The work of Yuan (branch: ​https://forge.ipsl.jussieu.fr/orchidee/wiki/GroupActivities/CodeAvalaibilityPublication/ORCHIDEE_DFv1.0_site) that is described in his paper Direct_Diffuse_YuanZhang​) that is very detailed and includes the impact on photosynthesis, transpiration as well as a scheme to compute the direct vs diffuse light from standard forcing with LMDZ.

• The current Trunk of ORC with the work done in DOFOCO and the implementation of the 2-stream radiative transfert scheme of Pinty_2006​, which accounts for the effect of reflectance/transmittance at each canopy layer and thus to a certain extend of direct vs diffuse light. The article (not published) describing it is 2Stream_Radiative_model​

==> The overall objective of this task force is to possibly merge the different approaches in order to have only one schemes taking the best of existing developments.

Meeting 27 November

• Yuan summarized his approach with a few slides​
• We discuss the differences between Yuan approach and the current 2 streams RT scheme in the Trunk
  • Yuan scheme assume no reflection of light at the background which simplifies the overall computation
  • 2stream scheme also computes the diffuse and direct fraction at each layer but do not valorise for the photosynthesis; while Yuan compute two times the photosynthesis (for sunlit and shaded leaves)
  • In principle all the 2stream scheme could be use to compute like Yuan the sunlit and shaded photosynthesis
  • In addition Yuan model include a detailed computation of the fraction of incoming direct vs diffuse light depending on atmospheric conditions
• Juliette explained that the implementation for BVOCs is similar to Yuan approach (based on Spitters pappers); The scheme also compute the incoming direct vs diffuse light but with a different approach than in Yuan.