Ninth International Conference on Permafrost ... - IARC Research

The Sensitivity of SiBCASA-Simulated Carbon Fluxes and Biomass to NorthAmerican Interannual Climate VariationsLixin LuDepartment of Atmospheric Science, Colorado State University, Fort Collins;and CIRES and ATOC, University of Colorado, Boulder, ColoradoKevin Schaefer, Tingjun ZhangCIRES and NSIDC, University of Colorado, Boulder, ColoradoIan BakerDepartment of Atmospheric Science, Colorado State University, Fort CollinsIntroductionSimple Biosphere model (SiB2.5) (Sellers et al. 1996a,b)is coupled with Carnegie-Ames-Stanford Approach model(CASA) (Potter et al. 1993, Randerson et al. 1996) to forma new model, SiBCASA, which is capable of simulatingdiurnal to interannual variations of terrestrial carbon fluxesand biomass at plot to global scales (Shaefer et al. 2008a).While prescribing leaf biomass derived from remotelysensed Normalized Difference Vegetation Index (NDVI),SiBCASA can dynamically allocate carbon to leaf, root, andwood pools, and explicitly calculate autotrophic respiration.To improve winter-process simulations, Schaefer et al(2008b) improved the snow and soil freeze- and thaw-relatedalgorithms in SiBCASA. These modifications include,incorporating Sturm et al. (1995) snow classification system,adopting the Lawrence and Slater (2005) organic soil model,and extending the soil column depth to 15 m with an increasednumber of soil layers. These changes greatly improved theSiBCASA-simulated snow density, snow depth, as well assoil temperature, to more realistic ranges with observations.Study Sites and Experiment DesignNine eddy covariance flux tower sites (include Barrow,Bondville, Boreas old black spruce, Harvard Forest,Howland Forest, Lethbridge, Niwot Ridge, Park Falls, andWinder River) across a range of the climate-ecosystem zonesare selected for initial evaluations of SiBCASA-simulatedbiophysical and biogeochemical processes (shown in Fig.1). The meteorological forcings are derived from 32-kmgrid-spacing North American Regional Reanalysis productspanning 1979 through 2003 at 3-hourly time-step. Modeledcarbon fluxes and biomass at these sites are evaluated againsttower-observed values. In particular, the parameterizationsdescribing cold-land processes were developed andimplemented to better represent the interactions betweensnow cover, soil thermodynamics, and soil freeze-thawprocesses, as well as their influences on carbon cycle overpermafrost regions.A suite of sensitivity experiment is performed by perturbingthe atmospheric forcing variables one at a time. Maximumand minimum temperatures are increased and decreased2°C, while precipitations are increased and decreased 25%of their original values. SiBCASA-simulated carbon fluxesand biomass are also sensitive to the initial conditionsFigure 1. The distribution of nine study sites over the North Americadomain. These sites are coincident with Ameriflux eddy covarianceflux tower sites.of soil moisture and temperature, snow depth, and initialwoody pool size. Statistical analyses are being carried out tounderstand how these climate perturbations and changes ininitial conditions influence the predictions of carbon fluxesand biomass. These experiments, by artificially manipulatingthe input data to imitate possible future scenarios, will enableus to assess and quantify the sensitivity of North Americancarbon cycle to large-scale climate change.Preliminary ResultsHeat fluxesThe upper two panels in Figure 2 show SiBCASAsimulatedlatent and sensible heat fluxes from 1982 through2004 at the Barrow site. Monthly mean values are plotted tohighlight the interannual variations. From year to year, bothsensible and latent heat fluxes show changes up to 30% and40% of their averaged values, respectively. A mid-year latentheat-fluxdepression persists throughout the simulation timeperiod, indicating that soil might be drying out due to surfaceevapotranspiration and soil hydrology before it is replenishedby summer rainfalls. The latent heat fluxes share the samesign of temperature changes, up to 30% of their originalvalues when temperature increases or decreases by 2°C. Thesensible heat fluxes only vary up to 10% of their originalvalues with the same magnitude of temperature changes asin the latent heat flux experiment, and most importantly,they are in opposite sign of temperature changes. Bothheat fluxes respond to precipitation perturbations in much193