32 Years after the Simple Biosphere Model (SiB): Is the Biosphere Still Simple to Model?

1986 was a pivotal year in the atmospheric sciences with publication of the Biosphere-Atmosphere Transfer Scheme (BATS) and the Simple Biosphere model (SiB). With these land surface models, a new science of biosphere-atmosphere coupling and biotic regulation of climate was formed. A subsequent advancement in SiB2 was to incorporate leaf gas exchange theory and to analytically integrate leaf physiological processes over profiles of light and nitrogen in the canopy. More recent land surface model development has emphasized the carbon cycle and hydrological processes. However, the fundamental coupling between plants and the atmosphere in climate models still occurs with the near-instantaneous fluxes of momentum, energy, and mass over the diurnal cycle as mediated by plant physiology, the microclimate of plant canopies, and boundary layer processes. The central paradigm of land surface models has been to represent plant canopies as a homogeneous "big leaf" without vertical structure, though with separate source fluxes for sunlit and shaded big leaves and for the soil. A particular challenge is how to represent turbulent processes in plant canopies. An additional challenge, largely ignored in land surface models, is that Monin-Obukhov similarity theory fails in the roughness sublayer extending to twice the canopy height or more. Here, results are shown from a multi-layer canopy model with a roughness sublayer parameterization to test if this theory provides a tractable parameterization of canopy-induced turbulence extending from the ground through the canopy and the roughness sublayer. The Community Land Model (CLM4.5) has pronounced biases during summer months at forest sites in mid-day latent heat flux, sensible heat flux, and gross primary production, nighttime friction velocity, and the radiative temperature diurnal range. The roughness sublayer canopy model reduces biases in latent heat flux, sensible heat flux, and gross primary production compared with the CLM4.5, largely as a result of changes in stomatal conductance and the profile of leaf nitrogen and photosynthetic capacity in the canopy. Biases in friction velocity and radiative temperature are also reduced, arising from the numerical methods of the canopy model as well as implementation of the roughness sublayer theory. The profiles of wind speed and temperature within the canopy are markedly different compared with profiles obtained using Monin-Obukhov similarity theory. The multi-layer canopy with the roughness sublayer theory improves simulations compared with the CLM4.5 while also advancing the theoretical basis for surface flux parameterizations.