Realistic Forests and the Modeling of Forest‐Atmosphere Exchange

• Published in: Reviews of geophysics (1985) 2022-03, Vol.60 (1), p.n/a
• Main Authors: Bannister, E. J., MacKenzie, A. R., Cai, X.‐M.
• Format: Article
• Language: English
• Subjects: Air; Air parcels; Atmosphere; atmospheric boundary layer; atmospheric modeling; Atmospheric models; Canopies; Carbon dioxide; Climate models; Climatology; Computer models; Connecting; Deciduous forests; Ecology; Eddies; Eddy flux; Exchanging; Fluid flow; Fluxes; Forest ecology; Forest management; Forests; forest‐atmosphere exchange; fragmented forests; Habitat fragmentation; Landscape; Mathematical models; Meteorology; Modelling; Momentum; Numerical models; Organic compounds; Oxygen; Patchiness; patchy landscapes; Plant cover; Pollen; Reconfiguration; scalar transport; Turbulence; Water quality; Water vapor; Water vapour; Weather; Weather conditions
• ISSN: 8755-1209; 1944-9208
• DOI: 10.1029/2021RG000746

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. Plain Language Summary Plants live by an intricate set of exchanges with the atmosphere. They draw carbon dioxide from the air—while being buffeted by the wind—and release water vapor, oxygen, pollen, and a variety of organic compounds. These exchanges are especially intricate in forests, where microbes and animals add to the quantity and variety of exchanges. Forests' patchwork structures mean that trees may experience profoundly different climates to others only meters away. These exchanges are made yet more complicated by the fragmentation of forests by human activity. Here we review recent developments in the understanding of exchanges between the air and realistic, patchy forests. We focus o