1OK: Default model configuration¶

The flexibility of the ORCHIDEE model in terms of the source of the climate, the boundary files, the processes that are accounted for, and for some processes the way they are calculated, results in hundreds of possible model configurations. This section describes the default configurations for stand-alone and land-atmosphere simulations.

The stand-alone configuration requires the use of a climate forcing. By default, the 6-hourly CRUJRA climate forcing regridded to 2 degrees is used (Section 1.4). In addition, the following boundary files are used: (1) A soil properties map following a USDA classification Reynolds et al., 2000 that was adjusted to represent 13 texture classes (See 1.1);

(2) A hydrological digital elevation model based on Vörösmarty et al. (2000) to calculate the river network (See 1.6); (3) A spatially explicit time series of historical to present distribution of the 15 PFTs of ORCHIDEE (See 1.2); (4) A spatially explicit time series of historical and present forest management (see 1.10);

(5) A spatially explicit time series of nitrogen deposition, and application of nitrogen fertiliser as well as a map for biological nitrogen fixation (See 1.9); (6) A background albedo map for the visible and near infrared wavelengths (See 1.11); and

(7) A global time series for the historical and present atmospheric CO₂ concentration (See 1.3).

The model is configured to simulate 15 PFTs: (1) Bare Soil, (2) Tropical Broadeaf Evergreen, (3) Tropical Broadleaf Raingreen, (4) Temperate Needleleaf Evergreen, (5) Temperate Broadleaf Evergreen, (6) Temperate Broadleaf Summergreen, (7) Boreal Needleleaf Evergreen, (8) Boreal Broadleaf Summergreen, (9) Boreal Larix Sp., (10) Tropical C3 Grass (11) Temperate C3 Grass, (12) Boreal C3 Grass Boreal, (13) Global C4 Grass, (14) Global C3 Agriculture, and (15) Global C4 Agriculture. All PFTs are represented by a single age class (See 1.3) and the forest PFTs make use of three diameter classes (See 1.4) to represent the stand structure within the PFT. Emissions from biologic volatile organic compounds are not simulated.

For several of its process calculations, ORCHIDEE v4.2 includes two or more approaches. These approaches often represent the initial well-tested approach and a more recent, more refined or more realistic approach that is, however, less well tested. Over time, the refined or more realistic approach should become the only remaining approach. For the time being, one the approaches needs to be selected:

• For the energy budget, the following approaches are selected: (1) the background albedo is read from a map (See 1.11)

  (2) roughness length for momentum is calculated as a function of leaf area and tree height, the roughness length for heat is calculated as constant fraction of the roughness length for momentum (See 1.4.2);

  (3) mean height is used in the calculation of the roughness length (See 1.4.2);

  (4) heat conductivity in the soil is calculated as a function of soil carbon pools (See 1.7.6)

• for the water budget, the following approaches are selected: (1) infiltration along plant roots is a function of the root profile (See %s);

  (2) river routing (See 1.8) is calculated by the hybrid approach (See 1.9);

  (3) plant water stress is calculated as a linear response between field capacity and wilting point (See 1.3.1);

  (4) additional surface resistance from, for example, litter, mosses, and crusts, are not accounted for in the calculation of bare soil evapotranspiration (See 1.6.5);

  (5) the multi-layer snow scheme with 12 layers (See 1.4);

• For the carbon and nitrogen budgets, the following approaches are selected: (1) a dynamic nitrogen cycle that allows for nitrogen limitations (See ???);

  (2) the specific leaf area is calculated as a function of leaf mass (See 1.5.2);

  (3) the carrying capacity of the forest PFTs varies over time and space as a function of the environmental conditions (See 1.5.4);

  (4) the different components of the soil carbon pool are simulated as bulk pools (See 1.8).

• For the demographic calculations, the following approaches are selected: (1) the PFT distribution is read from annual maps;

  (2) a constant rate for background mortality is used in addition to self-thinning (See 1.5.4)

• For land use, the following approaches are selected: (1) for the product pools the dynamic allocation is used implying that the diameter of the wood harvest determines whether the harvest will be used for a short-lived product or a medium and long-lived products (See 1.7);

  (2) at the end of the growing season, a harvest event, removes the aboveground biomass from croplands (See 1.3); and

  (3) A parameter set tuned towards global use is selected for forest management, wind storms and bark beetle outbreak because these processes are scale dependent. Clear cuts, for example, are common at the site-level but are unlikely at the scale of a 50 x 50 km$^{2}$ pixel as that would imply a 2500 km$^{2}$ clear cut.

Features that are still under development or that have been tested and evaluated only in regional applications are by default deactivated in the stand-alone configuration:

• For the energy budget, the following processes are deactivated: (1) multi-layer energy budgets (See ???);

  (2) heat storage in lakes (See 1.9);

  (3) the calculation of soil thermal conductivity as a function of XXX (See 1.7.2 and the calculation of soil heat capacipty as a function of XXX 1.7.3)

• For the water budget, the following processes are deactivated: (1) hydraulic architecture and related functionality such as vessel mortality and water storage in the vegetation (See 1.3.3)

  (2) ice sheets are not simulated (See 1.4);

  (3) re-infiltration from ponds and floodplains (See 1.6);

• For the carbon and nitrogen cycle, the following processes are deactivated: (1) grassland density as a function of plant growth (See 1.5.4)

• For land use, the following processes are deactivated: (1) crop irrigation (See 1.2),

  (2) PFT changes after a stand replacing disturbance (See ??),

  (3) forest management changes when an opportunity arises (See

  ),

  (4) litter raking (See 1.6)

• For disturbances, the following processes are deactivated: (1) mortality from droughts (See 1.1),

  (2) mortality from wind storms (See 1.2),

  (3) mortality from insect outbreaks (See 1.3),

  (4) mortality from fire (See 1.4),

  (5) loss of biomass from herbivory (See 1.5), and

  (6) snags as a pool for coarse woody debris (1.7).

The stand-alone experiment uses the default values of global and PFT-specific parameters that are included in the model code. This stand-alone configuration is used in the TRENDY attribution experiment Sitch et al., 2024} that consists of four treatments (i.e., S0 to S3). Each treatment includes a different combination of climate forcing and boundary files but applies the same model configuration and model parameters. Likewise, the stand-alone configuration is used in the technical quality control of the code (See 1.4) and the reference simulations used to evaluate the evolution of the ORCHIDEE model.

The land-atmosphere configuration makes use of an atmospheric model for its climate conditions rather than one of the climate forcings (See 1.4). In addition, drag is calculated by the atmospheric model instead of ORCHIDEE (See

) and because land-atmosphere simulations have a higher temporal resolution than stand-alone simulations, throughfall is parametrized differently (See %s).