Realistic Forests and the Modeling of Forest‐Atmosphere Exchange

Abstract

Forests cover nearly a third of the Earth's land area and exchange mass, momentum, and energy with the atmosphere. Most studies of these exchanges, particularly using numerical models, consider forests whose structure has been heavily simplified. In many landscapes, these simplifications are unrealistic. Inhomogeneous landscapes and unsteady weather conditions generate fluid dynamical features that cause observations to be inaccurately interpreted, biased, or over‐generalized. In Part I, we discuss experimental, theoretical, and numerical progress in the understanding of turbulent exchange over realistic forests. Scalar transport does not necessarily follow the flow in realistic settings, meaning scalar quantities are rarely at equilibrium around patchy forests, and significant scalar fluxes may form in the lee of forested hills. Gaps and patchiness generate significant spatial fluxes that current models and observations neglect. Atmospheric instability increases the distance over which fluxes adjust at forest edges. In deciduous forests, the effects of patchiness differ between seasons; counter intuitively, eddies reach further into leafy canopies (because they are rougher aerodynamically). Air parcel residence times are likely much lower in patchy forests than homogeneous ones, especially around edges. In Part II, we set out practical ways to make numerical models of forest‐atmosphere more realistic, including by accounting for reconfiguration and realistic canopy structure and beginning to include more chemical and physical processes in turbulence resolving models. Future challenges include: (a) customizing numerical models to real study sites, (b) connecting space and time scales, and (c) incorporating a greater range of weather conditions in numerical models.

The combined impact of canopy stability and soil NOx exchange on ozone removal in a temperate deciduous forest

“…For example, LES-MLC models can be applied to investigate how vegetated canopies affect chemical ozone flux divergence (Vila-Guerau De Arellano et al, 1993), and to test how this affects the (dis)similarity between vertical diffusivities for sensible heat and trace gases (Figure S5 in Supporting Information S1). This would require performing LES-MLC simulations that closely mimic site conditions at selected observational sites with detailed observations of in-canopy turbulence and trace gas exchange fluxes, which is an area of ongoing research (Bannister et al, 2022). The proposed developments have large potential to improve the representation of turbulent exchange in multi-layer canopy exchange models (e.g., MLC-CHEM) that can be applied in coupled 3D atmospheric chemistry model experiments used for air quality assessments and chemistry-climate studies.…”

“…Multi-layer canopy-atmosphere exchange models typically simulate vertically resolved in-canopy and canopy-surface layer turbulent exchange based on K-theory (e.g., Ashworth et al, 2015;. This formulation has strong limitations when applied in forest canopies, where the dependency between time-averaged gradients and fluxes can become invalid due to the occurrence of coherent turbulent structures, stable mixing conditions or more than one emission or deposition source (Bannister et al, 2022;Finnigan, 2000). Inferring in-canopy mixing conditions from observations requires vertical profile measurements of temperature and the sensible heat flux (e.g., Brown et al, 2020), which are not typically available at flux measurement sites.…”

The Combined Impact of Canopy Stability and Soil NOx Exchange on Ozone Removal in a Temperate Deciduous Forest

“…Multi‐layer canopy‐atmosphere exchange models typically simulate vertically resolved in‐canopy and canopy‐surface layer turbulent exchange based on K‐theory (e.g., Ashworth et al., 2015; Ganzeveld, Lelieveld, Dentener, Krol, & Roelofs, 2002). This formulation has strong limitations when applied in forest canopies, where the dependency between time‐averaged gradients and fluxes can become invalid due to the occurrence of coherent turbulent structures, stable mixing conditions or more than one emission or deposition source (Bannister et al., 2022; Finnigan, 2000). Inferring in‐canopy mixing conditions from observations requires vertical profile measurements of temperature and the sensible heat flux (e.g., Brown et al., 2020), which are not typically available at flux measurement sites.…”

Advancing process understanding of ozone dry deposition across scales

“…A4.4). This would require performing LES-MLC simulations that closely mimic site conditions at selected observational sites with detailed observations of in-canopy turbulence and trace gas exchange fluxes, which is an area of ongoing research (Bannister et al, 2022). The proposed developments have large potential to improve the representation of turbulent exchange in multi-layer canopy exchange models (e.g.…”

“…Van Pul and Jacobs (1994) derived such a parameterization from measurements over maize crop, but its applicability to other land use categories remains uncertain. Multi-layer canopy-atmosphere exchange models typically simulate vertically resolved in-canopy and canopy-surface layer turbulent exchange based on K-theory (e.g., Ashworth et al, 2015;Ganzeveld et al, 2002b), which however has strong limitations when applied for rough surfaces such as forests (Bannister et al, 2022). Inferring in-canopy mixing conditions from observations requires vertical profile measurements of temperature and the sensible heat flux (e.g., Brown et al, 2020), which are not typically available at flux measurement sites.…”