Realistic forests and the modeling of forest-atmosphere exchange

Abstract

Forests cover around 30% of the Earth’s land area but are becoming increasingly fragmented. In many parts of the world, edge effects dominate most of the forested area. Inhomogeneous landscapes and non-ideal weather conditions generate fluid dynamical features that cause observations to be inaccurately interpreted, biased, or over-generalized. We discuss progress towards capturing the complicated reality of forests in turbulence-resolving models. Scalar transport does not necessarily follow the flow in complex terrain, 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, driven by differential heating of the canopy, increases the distance over which fluxes adjust at forest edges. For deciduous forests, the effects of patchiness differ between seasons; eddies reach further into rougher, leafy canopies. Air parcel residence times are likely much lower in patchy forests than homogeneous ones, particularly around edges. However, the modeled probabilities of gusts are sensitive to the model setup, including any stochastic element. Eulerian parametrizations now allow researchers to investigate forest chemistry and particle deposition in the turbulent flow. The reconfiguration of plants under wind loading can be captured efficiently by modifying the velocity dependence of the aerodynamic drag. Future challenges include: (i) targeted observations in patchy landscapes; (ii) developing parametrizations of turbulent transfer applicable to larger scales; (iii) developing numerically efficient improvements to model forest structure; and (iv) simulating a greater range of weather conditions.