Spinup of the Carbon variables

Author: P. Peylin, F. Maignan
Last revision: 2020/02/28, F. Maignan

Principle

The carbon stocks and fluxes take a relatively long time to be in equilibrium when starting from zero.

• Vegetation C stocks/fluxes: The spinup needs to bring the vegetation above and below ground carbon stocks at equilibrium. This is relatively short for the LAI: few decades or even just one decade are enough. For the other C pools it is linked to the turnover of the Sap/Heart/wood C pools: usually 100 to 200 years are sufficient.
• Soil C stocks/fluxes: These pools take a much longer time to be in equilibrium as the passive C pool has a turnover on the order of one thousand years. Usually we need several thousand years to reach the equilibrium. Using the in-built iterative pseudo-analytical spinup procedure reduces this step as we then only need to have the input of C to the soil in equilibrium.

Standard protocol

A standard protocol is provided by the TRENDY model intercomparison project. The protocol comprises:

• 1) A long spin up simulation equivalent to several thousands of years should be done, recycling the meteorological forcing (usually cycling over the first 10 years of available forcing files) and choosing the atmospheric CO2 concentration of pre-industrial conditions. This brings the above and below ground C pools into equilibrium. It should be done either with:
  • The iterative pseudo-analytical spinup procedure: in this case a relatively shorter simulation should first be done (around 200 to 300 years) in order to bring the above ground C pools and fluxes into equilibrium. This provides an equilibrium for the input of C to the soil; the iterative pseudo-analytical spinup configuration then allows to bring the soil carbon pools to equilibrium in a few matrix inversions, given that the soil carbon dynamic follows first order kinetic equations.
  • The forcesoil configuration for 2-3 thousands years
  • A simple model simulation for 2-3 thousands years (cycling 10 years of meteorological forcing files)

This first step leads to an equilibrium of all C reservoirs with the chosen climate and atmospheric CO2 concentration.

• 2) A first transient simulation without real climate is usually done still cycling the meteorological forcing files, but starting increasing the atmospheric CO2 from pre-industrial time and changing the yearly PFT maps, to the start of the period that is investigated (usually the beginning of the twentieth century) in order to have to take into account rising CO2 and land use change on the gross and net carbon fluxes and the C pools.

• 3) A second transient simulation with real climate is then done, following up the first transient simulation, using besides this time real years of the forcing files (no more cycling).