Posted by Design Studio
26 June 2024
Challenge 2: Soil Carbon
Introduction
The energy balance is a fundamental component of the Earth system. The typical spatial resolutions employed in the land surface models, however tend to miss the significant sub-scale heterogeneity in surface and subsurface energy balance and prevents the incorporation of new landforms and processes which require an independent energy budget. This article implements the representation of a tiling energy budget in the land surface model, ORCHIDEE-MICT and assesses its impacts on the hydrology, energy, and carbon cycle.
Current energy budget in ORCHIDEE-MICT
ORCHIDEE-MICT is a branch of the ORCHIDEE model specifically developed to enhance the representation of hydrologic and biogeochemical interactions in high-latitude regions. For the moment, there are three major modules in the model: carbon, water, and energy. The carbon cycle module operates for each PFT sub-grid and the water cycle operates for each soil tile (divided into bare soil, tree PFTs, grass and crop PFTs, and peatland PFT categories), while the energy module is solved only at the total grid-cell level (Figure (a)).
Implementation of tiling energy budgets in ORCHIDEE-MICT-teb
To represent the sub-grid energy budget in MICT, we calculate PFT-specific surface properties including roughness height and albedo of different PFTs to start the separation of surface energy budgets for each PFT, and then add the PFT-specific calculation for energy budget at the surface as well as in snow and soil layers (Figure (b)). Owing to using distinct input variables from the energy budget module, some processes in the hydrology cycle and carbon cycle are also modified correspondingly.
Impacts of tiling energy budget on energy, hydrology and carbon processes
With the specific values of surface properties for each vegetation type, the new version presents warmer surface and soil temperatures (~0.5 °C, 3%), wetter soil moisture (~10 kg m-2, 2%), and increased soil organic carbon storage (~170 PgC, 9%) across the Northern Hemisphere. Despite reproducing the absolute values and spatial gradients of surface and soil temperatures from satellite and in-situ observations, the considerable uncertainties in simulated soil organic carbon and hydrologic processes prevent an obvious improvement of temperature bias existing in the original ORCHIDEE-MICT. However, the separation of sub-grid energy budgets in the new version improves permafrost simulation greatly by accounting for the presence of discontinuous permafrost types (~ 3 million km2).
Outlook
Based on the tiling energy budget version, next step we will introduce new landforms for permafrost regions, such as landforms with or without ground ice, and landforms with the formation of thermokarst lakes. The investigation of the response of soil carbon over permafrost regions after including the new processes will be also conducted.
Schematic representation of energy budgets at the surface, snow layers, and soil layers in one grid cell of ORCHIDEE-MICT (MICT) (a) and the new tiling energy budget version (MICT-teb) (b). SWin, SWout, LWin, LWout, H, and, LE represent incoming ShortWave radiation, outward ShortWave radiation, incoming LongWave radiation, outward LongWave radiation, sensible heat flux, latent heat flux, respectively. PFT indicates Plant Function Type. There are 3 layers for snow, and 32 layers for soil for each PFT in the model. In MICT, SWin, SWout, LWin, LWout, H, and heat fluxes in snow and soil layers are calculated as grid-cell mean but LE is calculated for each PFT, while in MICT-teb, all of the heat fluxes are calculated for each PFT. The red and blue arrows distinguish the grid-cell mean and PFT-specific
Read the publication in full https://doi.org/10.5194/gmd-2023-207
