Modeling nocturnal cooling below a forest canopy

Local nocturnal thermally driven flows, such as drainage flows, are believed to impact measurements of forest-atmosphere exchange. As observational studies are limited, numerical modeling provides an attractive option for studying these flows. While the frictional influences of a canopy on flow are moderately well understood, to model the effects of buoyancy, it is also necessary to predict the thermal structure within and above the forest canopy.

Nocturnal cooling occurs primarily due to radiative heat loss. However, much of the radiative loss occurs at the surface of the canopy elements (leaves, branches, and boles of trees), while radiative divergence in the air space within the canopy is small. The air within the canopy cools by sensible heat transfer to the canopy elements. As this sensible heat transfer is slow relative to the radiative flux, the canopy elements cool much more quickly than the canopy air space, particularly early in the night. Thus, in modeling the cooling of air within a canopy, it is not appropriate to neglect the storage change of heat in the canopy elements or to assume equal rates of cooling of the canopy air and canopy elements. Here, we present a simple parameterization of the cooling of air within the canopy, based on an approximation of the ratio of the rate of change of air temperature to the rate of change of canopy element temperature. This parameterization accounts implicitly for radiative loss from the canopy elements, heat storage in the canopy elements, and heat transfer between the canopy elements and the air.

Simulations using this parameterization are compared to data from the Morgan-Monroe State Forest (Indiana, USA) FLUXNET site. Simulated rates of cooling match the observations in their order of magnitude and vertical structure. Despite the simplicity of the model, these results are a clear improvement over previous approaches that neglect the difference in cooling rates between leaves and canopy air space.