Advances in Modelling Radiative Transfer, Heat Storage and Turbulent Transport to Evaluate CO₂, Heat and Water Fluxes Over Broad-Leaved Forests: The CanVeg2 Model

Multilayer canopy models have been developed for several decades, and interest in this model class remains high despite their complexity because of their multiple advantages over big-leaf models. Nowadays, a number of scientific and technological advancements favour improving and evaluating these models. First, the advent of lidar technology has enabled detailed mapping of leaves and stems in forest canopies and enhanced the modelling of absorbed solar radiation by both elements within a canopy. Second, long-time series of eddy covariance measurements are available for model evaluation, capturing years with extreme conditions like droughts. Here we present a new version of the CanVeg model (CanVeg2) with three main modifications: (1) radiative transfer modelling using 3D ray tracing and ground lidar, (2) the addition of a stem energy balance module and (3) the use of a higher order closure model to provide profiles of horizontal wind velocity and variance in vertical wind velocity. We evaluate the CanVeg2 model at five broadleaf forest sites with contrasting canopy structures using long term eddy covariance records, and at two of the sites, soil temperature profiles and scalar vertical profiles. The modelled latent heat and CO₂ flux densities closely matched the eddy covariance measurements. The sensible heat flux density had lower coefficients of determination across sites. The modelled soil heat flux densities were notably higher than the measurements at the three sites where this flux is measured. Divergences between modelled and measured scalars in the lower canopy layers suggest the model would benefit from improved information on soil-litter moisture content. The favourable results obtained in the model evaluation suggest it may be useful towards addressing new science questions by enabling the estimation of certain canopy states nearly impossible to measure at the canopy level like vertically resolved leaf and stem temperatures and to study the interactions between highly interconnected biophysical and physiological processes.