HR: 14:20h
AN: B52D-03    [PDF]
TI: The Effects of Shading and Diffuse Radiation in Estimating Carbon and Water Fluxes in Tropical and Midlatitude Sites
AU: * Xue, Y
EM: yxue@geog.ucla.edu
AF: UCLA, 1255 Bunch Hall, Department of Geography, Los Angeles, CA 90095 United States
AU: de Sales, F
EM: fsales@ucla.edu
AF: UCLA, 1255 Bunch Hall, Department of Geography, Los Angeles, CA 90095 United States
AU: Zhan, X
EM: xzhan@hsb.gsfc.nasa.gov
AF: UMBC-GEST, Hydrological Branch, GSFC,NASA, Greenbelt, MD 20771 United States
AU: Collatz, J
EM: jcollatz@biome.gsfc.nasa.gov
AF: GSFC/NASA, Code 923, Greenbelt, MD 20771 United States
AB: The interactions between radiation, water, and carbon are a crucial component in determining terrestrial carbon and water fluxes. In this paper, we will present evidence to demonstrate the close relationship between these processes in an integrated climate system using the Simplified Simple Biosphere Model (SSiB, Xue et al., 1991). The SSiB implemented Collatz et al' (1991, 1992) parameterizations of plant photosynthesis and stomatal conductance to consider CO2 assimilation of vegetation. Quasi-analytical solutions of these parameterizations were developed (Zhan et al., 2003) to improve the computational efficiency and to produce stable solutions for long-term simulations in GCMs and regional models. We tested this enhanced SSiB using observational data from LBA, Boreal, and AmeriFux sites. The results indicated that the model in general produced a higher than normal rate of photosynthesis which led to an overly large transpiration. We examined model performance and found that this was mainly caused by the scaling methodology. In our earlier approach described above, the sunlit and shaded leaf areas were not considered. Furthermore, only direct radiation effect was included in the scaling equation, which was adapted from SiB2. Diffuse radiation, which arises from atmospheric scattering and from scattering within the canopy, has been shown to have a crucial role in the photosynthetic process (e.g., Norman, 1982; Baldocchi, 1997). Therefore, we developed a new parameterization for shading and scaling to more realistically simulate the land/atmosphere interaction processes. The shading parameterization is based on Norman's approach (mainly relies on solar zenith angle), but we further take vegetation properties and solar radiative transfer property within canopy into consideration. The scaling method considers the effects of both direct and diffuse radiations. We have tested the new method using the observational data from the LBA experiment ( 2000) and the Boreal experiments (1996). This new method substantially improved the simulations of daily mean carbon and water fluxes and their diurnal variations, especially in the tropical area. For example, in the LBA experiment, the root-mean-square (RMS) error for the latent heat flux and the carbon flux are 15.1 w m-2 and 2.9 mmol m-2 s-1, respectively, which are much smaller than the RMS error 45.4 w m-2 and 12.7 m mol m-2 s-1 in the simulation with the old method. Experiments are also conducted to comprehensively test the effects of soil moisture, diffuse radiation, and vegetation properties in estimating the variability of carbon and water fluxes.
DE: 0315 Biosphere/atmosphere interactions
DE: 1615 Biogeochemical processes (4805)
DE: 1655 Water cycles (1836)
DE: 1818 Evapotranspiration
SC: Biogeosciences [B]
MN: 2003 Fall Meeting