Modeling the effects of three‐dimensional vegetation structure on surface radiation and energy balance in boreal forests

Abstract

Radiation and turbulent energy exchange between vegetation and the atmosphere is an important component of land surface‐atmosphere interaction. Vegetation properties strongly influence this exchange. Commonly, two‐stream models are used to compute net absorbed radiation and the partitioning of absorbed radiation between the vegetation canopy and substrate. Also, the roughness lengths for momentum and heat are often parameterized as simple functions of vegetation height. In this paper, we examine the influence of vegetation three‐dimensional (3‐D) structure on land surface radiation and energy balance over nonuniform forest canopies. To do this, parameterizations that account for the effects of canopy 3‐D structure on radiation and turbulent energy exchange were implemented in the National Center for Atmospheric Research (NCAR) land surface model (LSM) version 1.0. The NCAR LSM then was run offline for a time period encompassing both wet and dry soil moisture conditions using forcing data from the Boreal Ecosystem‐Atmosphere Study (BOREAS). Radiation and energy balance within the vegetation canopy and at the soil surface were examined at diurnal timescales. Results show that the 3‐D parameterizations increased net radiation absorbed by the soil surface and that this additional available energy was primarily dissipated through ground evaporation when soil moisture was not limiting. When the soil was dry, the available energy at the soil surface was mostly converted to sensible heat with relatively little change in the ground heat flux. In addition, modeled sensible heat from the vegetation canopy decreased, and relatively small changes in modeled canopy transpiration were observed. Comparisons of modeled heat fluxes using the 3‐D parameterizations against observations show improvement over the original treatment included in the NCAR LSM, suggesting that careful treatment of vegetation properties is important to accurate modeling of flux quantities above and underneath the canopy in soil‐vegetation‐atmosphere‐transfer (SVAT) models.