Testing the Simple Biosphere Model (SiB) Using Point Micrometeorological and Biophysical Data

Abstract

The Simple Biosphere model (SiB) of Sellers et al. (1986) was designed for use within General Circulation Models (GCMs) of the earth's atmosphere. The main objective of SiB is to provide a biophysically realistic description of those processes which control the transfer of radiation, sensible heat, latent heat and momentum between the terrestrial surface and the atmosphere. As a result, SiB is more complex and has a larger input parameter set than most equivalent formulations used in GCMs. Prior to implementing SiB in a GCM, it is essential that its components and its functioning as a whole, be thoroughly tested. Additionally, it is highly desirable that the model's response to errors or uncertainties in the input parameter set be explored. This paper discusses investigations that were directed at addressing then two issues. Micrometeorological and biophysical measurements from surface experiments conducted over arable crops in West Germany and the United States and a forested site in the United Kingdom were used to test the operation of SiB. Observed values of the downward radiative fluxes, wind speed, air temperature and water vapor pressure recorded above the surface were used as the boundary forcing for the SiB model. The predicted partitioning of the absorbed radiation into the sensible and latent heat fluxes compares well with observations and the various subcomponents of the model appear to operate realistically. The sensitivity of the model's energy balance calculations to changes in the various model parameters and the soil moisture initialization is examined. It is estimated that the model will generate uncertainties of the order of ±7% in the calculated net radiation, and up to ±25% in the calculated evapotranspiration rate, with typical values of ±15%.