A Simple Biosphere Model (SIB) for Use within General Circulation Models

Abstract

A simple but realistic biosphere model has been developed for calculating the transfer of energy, mass and momentum between the atmosphere and the vegetated surface of the earth. The model is designed for use in atmospheric general circulation models. The vegetation in each terrestrial model grid area is represented by two distinct layers, either or both of which may be present or absent at any given location and time. The upper vegetation layer represents the perennial canopy of trees or shrubs, while the lower layer represents the annual ground cover of grasses and other herbaceous species. The local coverage of each vegetation layer may be fractional or complete but as the individual vegetation elements are considered to be evenly spaced, their root systems are assumed to extend uniformly throughout the entire grid area. Besides the vegetation morphology, the physical and physiological properties of the vegetation layers are also prescribed. These properties determine (i) the reflection, transmission, absorption and emission of direct and diffuse radiation in the visible, near infrared and thermal wavelength intervals; (ii) the interception of rainfall and its evaporation from the leaf surfaces; (iii) the infiltration, drainage and storage of the residual rainfall in the soil; (iv) the control by the photosynthetically active radiation and the soil moisture potential, inter alia, over the stomatal functioning and thereby over the return transfer of the soil moisture to the atmosphere through the root-stem-leaf system of the vegetation; and (v) the aerodynamic transfer of water vapor, sensible heat and momentum from the vegetation and soil to a reference level within the atmospheric boundary layer. The Simple Biosphere (SiB) has seven prognostic physical-state variables: two temperatures (one for the canopy and one for the ground cover and soil surface); two interception water stores (one for the canopy and one for the ground cover); and three soil moisture stores (two of which can be reached by the vegetation root systems and one underlying recharge layer into and out of which moisture is transferred only by hydraulic diffusion and gravitational drainage).