Using sky view factor to model radiation and thermal structure within and above a forest canopy in sloping terrain

Nocturnal thermotopographic winds (e.g. drainage flows), which may arise from the interaction of local thermal gradients and sloping terrain, have received increased attention in the last decade, as they are suspected to affect the accuracy of measurements of ecosystem-atmosphere exchange of carbon dioxide. These nocturnal flows result from strong cooling of air within and above the canopy, driven by strong net radiative loss. Uncertainty remains about whether thermotopographic flows will occur in a given forest, their diel patterns, drivers that influence them, and their effects on measurements of forest-atmosphere exchange. Numerical modeling provides one option for addressing these questions.

One requirement of such a model is the parameterization of the transmission of longwave radiation through the forest canopy. Fully three-dimensional simulations of the radiation transfer through a forest in sloping terrain are computationally intensive and, when used in a flow model, may become prohibitive in terms of computing time. However, neglecting the heterogeneity of the terrain removes spatial variability in cooling and flow. To address this, the concept of view factor was used to adjust radiative fluxes obtained through computationally-efficient one-dimensional calculations, which assume horizontal homogeneity, to account for the heterogeneous terrain in which slope flows develop. While sky view factor has been used for determining radiative fluxes in urban areas or in unvegetated sloping terrain, the use of this concept below a forest canopy is believed to be a significant innovation.

The effects of this correction on the rate of cooling and on flow, over shallow simple slopes, are shown. While the effects of this correction are not straightforward in these simulations, the physical rationale for the correction justifies its use. Further, this type of correction is likely to be more important in steeper terrain, in valleys, or in other more complex terrain.