Canopy box framework to represent the surface energy partitioning over vegetated areas

초록

The partitioning of radiative fluxes and their conversion into heat fluxes within vegetated areas are fundamental processes in surface energy dynamics. For a simple, while physically sound, representation of the radiative transfer, heat conversion, and the energy storage, we propose to conceptualize the vegetated area as a hypothetical continuum on the ground, namely the canopy box. Complex radiant exchanges within this semi-transparent medium are modeled with varying radiant exchange parameters depending on the botanical characteristics. To more explicitly capture the function of vegetation, radiation is classified by spectral domain (short-and long-wave) and direction (upward and downward). Accordingly, the framework employs parameters for short-wave reflection and absorption, and represents long-wave exchange as bidirectional net emission from the canopy box. With the proposed idea, the surface energy balance equation is rewritten in two governing equations, one for the canopy and the other for the ground surface. The framework was evaluated at AmeriFlux sites of different vegetation types and climate where radiative fluxes measured both above and below canopy are available. Parameters of the proposed framework, derived from measured radiation data, effectively reflect not only the botanic phenology but also the ground condition.

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