A Biosphere-Atmosphere Interaction Model (BAIM) for use within physical climate models was developed. BAIM has two vegetation layers and three soil layers, and predicts the temperature of each layer and moisture stored for each layer. In the presence of snow on the ground, the snow layer is divided into a maximum of three layers, and the temperature and the amount of snow and water stored in each layer are predicted. BAIM can estimate not only the energy fluxes but also the carbon dioxide flux between the land surface ecosystem and the atmosphere. The photosynthesis processes for C3 plants and C4 plants are adopted in the model. BAIM can also predict the accumulation and melting of snow on the ground, and the freezing and melting of water in the soil. Primary off-line verifications of BAIM in a snowless condition were made using point micrometeorological data observed at grassland. In general, fluxes simulated by the model agreed well with those observed. In particular, clear differences between results using the parameters for C3 plants and those using the parameters for C4 plants appeared in the net carbon dioxide fluxes. Sensitivity tests were conducted for the model to study the influence of variations in the values of parameters related to the property of vegetation. By changing the values of the parameters by ±50%, the maximum variations in the time-averaged fluxes were obtained. The values for net radiation flux, sensible heat flux, latent heat flux, and soil heat flux were about ±15W m-2, ±8W m-2, ±9W m-2, and ±1W m-2, respectively. The maximum variations in the time-averaged value of net carbon dioxide flux were about±5μmol m-2 s-1 for C3 parameters and±7μmol m-2 s-1 for C4 parameters. These maximum variation values are comparable to observation errors.
Four types of approximations of the Mie phase function were studied in calculating multiple scattering by snow particles with the doubling method. These involve the two renormalizations of Hansen and Grant, the delta-M method and direct truncation. These four approximations were compared for snow surface albedo with effective grain radii of 50, 200 and 1000μm in a wavelength region from 0.3 to 3.0μm with the delta-Eddington approximation as a reference. In the Hansen's renormalization, the maximum albedo error exceeds 0.1 for snow with an effective radius of 1000μm at small solar zenith angles. The delta-M method overestimates snow albedos at all solar zenith angles in a wavelength region smaller than 1.4μm for snow with effective radius of 1000μm. This is due to insufficient angle resolution (0.1° in a scattering angle region less than 2°) in the forward peak region of the look-up table of the Mie phase function. It has been shown that even with ten times higher resolution in the scattering angle region less than 10° a sufficient accuracy could not be obtained for an effective radius of 1000μm in a wavelength region smaller than 0.6μm. Reasonable results were obtained by the Grant's renormalization and direct truncation approximation for all cases of effective grain radii studied. It was also found that these methods save computation time and memory because sufficient accuracy is obtained even with a low angle resolution of 0.1° in the forward peak region of phase function. In direct truncation, the result was not sensitive to the choice of a truncation angle between 5° and 20°.