Parameterization of three-dimensional structure in vegetation canopies for modeling radiation and heat exchange over boreal forests

Fluxes of mass and energy between ecosystems and the atmosphere exert substantial control on regional to global scale climate processes. Vegetation is one of the most important influences on these fluxes. Traditional treatments for vegetation in soil-vegetation-atmosphere-transfer (SVAT) models assume vegetation to be uniform. However, the structure of most natural vegetation is not consistent with this assumption. In this dissertation I examine how three-dimensional (3-D) structure in vegetation influences mass and energy exchange at the land-atmosphere interface. To do this, models of radiation and turbulent energy transfer are developed that include explicit treatment for vegetation 3-D structure. Results from model simulations are compared against field data from the Boreal Ecosystem-Atmosphere Study (BOREAS).; First, a parameterization is developed that extends two-stream radiative transfer models for use in non-uniform canopies. The results show that vegetation 3-D structure, and especially between-crown gaps, significantly modifies radiation interception at the surface relative to uniform canopies. Specifically, the partitioning of net solar radiation between the vegetation and substrate is sensitive to the gap structure of vegetation canopies.; Second, the effect of vegetation 3-D structure on surface roughness lengths for momentum and heat is examined. A new parameterization for surface roughness, which uses the same conceptual representation for vegetation structure as that used for radiation, is tested against data from BOREAS. The results demonstrate that site-to-site variation in canopy structure can significantly influence exchanges of momentum and sensible heat.; Finally, the parameterizations for radiation regimes and roughness lengths are implemented in a land surface model to study the simultaneous effect of vegetation 3-D structure on land surface energy and radiation balance. Comparison of modeled fluxes against observations from BOREAS shows subtle but important effects in this regard. In particular, the partitioning of net radiation between the vegetation canopy and the substrate is strongly influenced by vegetation 3-D structure. Further, variation in this partitioning propagates into the energy balances of the canopy and substrate. Therefore, physically realistic and consistent treatments for the influence of vegetation 3-D structure on radiation, heat, and momentum exchange are important for accurate simulation of surface biophysical processes in SVAT models.