Abstract
Abstract. Land surface models used in climate models neglect the roughness sublayer and parameterize within-canopy turbulence in an ad hoc manner. We implemented a roughness sublayer turbulence parameterization in a multilayer canopy model (CLM-ml v0) to test if this theory provides a tractable parameterization extending from the ground through the canopy and the roughness sublayer. We compared the canopy model with the Community Land Model (CLM4.5) at seven forest, two grassland, and three cropland AmeriFlux sites over a range of canopy heights, leaf area indexes, and climates. CLM4.5 has pronounced biases during summer months at forest sites in midday latent heat flux, sensible heat flux, gross primary production, nighttime friction velocity, and the radiative temperature diurnal range. The new canopy model reduces these biases by introducing new physics. Advances in modeling stomatal conductance and canopy physiology beyond what is in CLM4.5 substantially improve model performance at the forest sites. The signature of the roughness sublayer is most evident in nighttime friction velocity and the diurnal cycle of radiative temperature, but is also seen in sensible heat flux. Within-canopy temperature profiles are markedly different compared with profiles obtained using Monin–Obukhov similarity theory, and the roughness sublayer produces cooler daytime and warmer nighttime temperatures. The herbaceous sites also show model improvements, but the improvements are related less systematically to the roughness sublayer parameterization in these canopies. The multilayer canopy with the roughness sublayer turbulence improves simulations compared with CLM4.5 while also advancing the theoretical basis for surface flux parameterizations.
Highlights
Distinct parameterizations of land surface processes, separate from the atmospheric physics, were coupled to global climate models in the mid-1980s with the Biosphere– Atmosphere Transfer Scheme (BATS; Dickinson et al, 1986) and the Simple Biosphere Model (SiB; Sellers et al, 1986)
The ML + roughness sublayer (RSL) simulation has better skill compared with CLM4.5 at most sites and for most variables (Table 5)
For over 30 years, land surface models have parameterized surface fluxes using a dual-source canopy in which vegetation is treated as a big leaf without vertical structure and in which Monin–Obukhov similarity theory (MOST) is used to parameterize turbulent fluxes above the canopy
Summary
Distinct parameterizations of land surface processes, separate from the atmospheric physics, were coupled to global climate models in the mid-1980s with the Biosphere– Atmosphere Transfer Scheme (BATS; Dickinson et al, 1986) and the Simple Biosphere Model (SiB; Sellers et al, 1986). The central paradigm of land surface models, as originally devised by Deardorff (1978) and carried forth with BATS, SiB, and subsequent models, has been to represent plant canopies as a homogeneous “big leaf” without vertical structure, though with separate fluxes for vegetation and soil. Bonan et al.: Modeling canopy-induced turbulence in the Earth system θref ga,N (a) zref uref θref qref
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