ORCHIDEE retreat 2019

Monday 9H45 - 12H00: block 1

Subject: ORCHIDEE sub-grid scale processes: Multi-tiling for Energy / Water / Carbon

Leaders: Catherine Ottlé, Anne Sofie Lansø

Objectives: Define a concept document for the solution of the multi-tiling implementation and its flexibility ? Organisation:

Talks:

• The new “multi-energy” budget for ORCHIDEE for forest, lakes, glacials and urbanized areas. Present and discuss principles of the approach, first results, question on how to best implement such multi-tiling in ORC (CO/PP/VB/AL)

• Multi-tiling within the JULES model (Jan)

• ORCHIDEE-MICT crop multi-crop version (Philippe Ciais?)

• Multi-tiling for hydrology : new results about hillslope heterogeneities and perspectives for a comprehensive description of wetlands (Agnès)

• Multi-tiling for the soil carbon cycle (slides by Bertrand, presentation by CO/ASL)

Short summary of the session:

The session was initiated be several short presentations showing ideas on how multi-tiling could be implemented in ORCHIDEE and the efforts already done. CO presented different possibilities on how to share soil and water columns for the sweep of PFTs including nobio surface types. AD presented how to include low and highlands within a grid cell which needed separate water budgets, but not necessarily separate energy budgets. PC showed the importance of having individual tiles of soil and hydrology for the ORCHIDEE-MICT crop version and for the peatlands in MICT. Moreover, a short overview of the plans on how to implement multi-tiling in Hydro-JULES was presented (surface layer, sub-surface layer and open waters defined by physical and numerical constraints, and HRU in the soils to describe the lateral exchanges). ASL presented finally the coding issues which appeared in the way the multi-tiling is presently implemented in the various branches (DOFOCO, MEB and LAK). During the discussion, questions were raised as to whether we need the tiling of the surface, when we in the future likely will be able to increase the horizontal resolution of the model? First of all, for the CMIP simulations even in the future it will be highly unlike that we will have a resolution below 50 -100 km. Moreover, the nobio fraction is currently very poorly described (lakes, glaciers, urban, etc), which from an energy budget point of view is essential to improve even for higher spatial resolutions.

Topography affects the hydrology, and it was stressed it would be ideal to take that into account in the tiling making sure they would be laterally connected. AD mentioned that 2 topographic units should be enough to describe the first order effects. To accommodate open water, peatlands , crops etc. we want more flexibility than the trunk. Contrary to JULES, we do most likely not wish to decouple the PFTs and soils and want to account for their spatial correspondence. The vertical discretization of SOM affects soil thermics and hydrology, thus we cannot just distribute the soil mesh randomly.

List of actions / follow up: We decided to settle a working group in order to move forward on the multi-tiling developments in ORCHIDEE, in closed links with the atmospheric coupling with LMDZ and RegIPSL.